13 September 2026

A Level Business Studies: Income Statements — Turning Sales into Profit

A business can be busy, have plenty of customers and take large amounts of money through the tills — and still make very little profit.

It can even make a loss.

That is one of the reasons why students studying A Level Business need to understand the income statement.

An income statement does much more than tell us whether a business made a profit. It shows us where the money came from, where it went and what was left over.

Once students learn to read one properly, they can begin asking much more interesting business questions:

Are sales increasing?
Are the products actually profitable?
Are costs rising faster than revenue?
Is the business spending too much on administration?
Could management increase prices?
Are suppliers becoming too expensive?
Is the business performing better or worse than last year?

Those questions move income statements away from being a simple calculation exercise and towards what Business Studies is really about:

making decisions.

What Is an Income Statement?

An income statement is a financial document showing the:

revenue, costs, expenses and profit of a business over a particular period of time.

For many businesses, the accounting period will be one financial year, although businesses can also prepare monthly, quarterly or half-yearly management accounts.

At A Level, students will often encounter a simplified structure such as:

Revenue

minus Cost of sales

= Gross profit

minus Expenses

= Operating profit or net profit

There may be additional items in more detailed company accounts, including:

interest;
taxation;
depreciation;
exceptional costs;
finance expenses.

However, the essential principle remains the same.

The income statement asks:

How much did the business earn, how much did it cost to earn it, and how much profit remained?

Why Do Businesses Produce Income Statements?

The obvious answer is to calculate profit.

But the income statement is much more useful than that.

It provides managers and other stakeholders with a breakdown of the financial performance of the business during a particular trading period.

That information can then be used to identify strengths, weaknesses and possible areas for improvement.

Imagine that a business reports:

Year 1 operating profit: £480,000

Year 2 operating profit: £350,000

We immediately know that profit has fallen by £130,000.

But that alone does not tell us why.

Perhaps revenue has fallen.

Perhaps the cost of raw materials has increased.

Perhaps wages have risen.

Perhaps the business has dramatically increased its marketing expenditure.

Perhaps energy costs have increased.

Perhaps the business deliberately accepted lower short-term profits while opening new shops.

The income statement helps management investigate what actually happened.

The Basic Structure of an Income Statement

A simplified income statement might look like this:

Revenue: £500,000

Cost of sales: £300,000

Gross profit: £200,000

Expenses: £140,000

Operating profit: £60,000

The calculation follows a logical sequence.

First:

Gross profit = Revenue - Cost of sales

Therefore:

£500,000 - £300,000 = £200,000

Then:

Operating profit = Gross profit - Operating expenses

Therefore:

£200,000 - £140,000 = £60,000

The business therefore generated £500,000 of sales but ultimately retained only £60,000 as operating profit.

That distinction is enormously important.

Revenue is not profit.

Revenue — How Much Has the Business Sold?

Revenue is the value of goods or services sold by the business during a particular period.

The basic formula is:

Revenue = Selling price x Quantity sold

Suppose a business sells 10,000 products for £25 each.

Revenue = £25 x 10,000

Revenue = £250,000

That does not mean the business has made £250,000 profit.

It simply tells us the value of its sales.

The costs must still be deducted.

Revenue Is Not Necessarily the Same as Cash Received

This is another useful distinction.

A business may make sales on credit.

Imagine a company supplies £20,000 of equipment to another business in March but allows the customer 30 days to pay.

The £20,000 forms part of the company's revenue even though the cash may not arrive until April.

This is why students should avoid automatically thinking:

Revenue = cash in the bank.

It does not necessarily.

That distinction becomes particularly important when students later study cash flow.

A profitable business can still suffer serious cash-flow problems.

How Can Revenue Be Increased?

Students are often asked to recommend ways of improving profitability.

One possibility is to increase revenue.

Management might attempt this by:

increasing the selling price;
selling a greater quantity;
entering new markets;
introducing new products;
improving advertising;
improving product quality;
expanding distribution;
selling online;
opening additional locations;
increasing customer loyalty;
improving customer service.

However, good evaluation is required.

Simply saying:

"Increase the price."

is rarely enough at A Level.

Increasing price may increase revenue if customers continue buying.

But if demand is price sensitive, higher prices could reduce quantity demanded so much that total revenue actually falls.

Likewise, advertising may increase sales, but the additional sales must be worth more than the cost of the campaign.

Business decisions involve trade-offs.

Cost of Sales — What Did the Products Sold Actually Cost?

Cost of sales, sometimes called cost of goods sold, represents the direct cost associated with the goods sold during the accounting period.

For a retailer, the traditional calculation is:

Cost of sales = Opening inventory + Purchases - Closing inventory

For example:

Opening inventory = £30,000

Purchases during the year = £170,000

Closing inventory = £40,000

Therefore:

Cost of sales = £30,000 + £170,000 - £40,000

Cost of sales = £160,000

Why subtract closing inventory?

Because those goods have not yet been sold.

They remain assets owned by the business and may generate revenue in the following accounting period.

An Important Distinction: Cost of Sales Is Not the Same as All Business Costs

This is a common source of confusion.

For a clothes retailer, the cost of the garments sold would normally form part of cost of sales.

However, items such as:

head-office salaries;
advertising;
administration;
office rent;
accountancy fees;

would normally appear separately as operating expenses.

Depending on the type of business and its accounting treatment, certain directly attributable production costs can form part of cost of sales.

The important A Level principle is:

Cost of sales relates closely to producing or obtaining the goods that generated the revenue, while operating expenses cover the wider costs of running the business.

Keeping the two separate allows us to calculate gross profit.

How Can a Business Reduce Its Cost of Sales?

There are several possibilities.

Negotiate Better Supplier Prices

A business purchasing large quantities may be able to obtain bulk discounts.

For example, a retailer might negotiate a reduction from £12 to £11 per unit.

That sounds small.

But if it purchases 100,000 units:

Saving per unit = £1

Total saving = £1 x 100,000

Total saving = £100,000

Small changes in unit cost can therefore have major effects on profit.

Build Stronger Supplier Relationships

Businesses do not always choose suppliers purely because they offer the lowest price.

A reliable supplier may provide:

better quality;
faster delivery;
more flexible payment terms;
fewer defective products;
greater consistency;
priority during shortages.

A slightly more expensive supplier might therefore reduce other costs within the business.

This is where evaluation becomes important.

Cheapest does not automatically mean best.

Reduce Waste

Waste can significantly increase costs.

A food retailer may suffer from products reaching their expiry dates.

A manufacturer may waste raw materials.

A restaurant may throw away unsold food.

A fashion retailer may be left with large amounts of unsold seasonal stock.

Better stock control, forecasting and lean production methods can all reduce wastage.

Shop Around for Alternative Suppliers

Competition between suppliers may enable a business to obtain better prices.

However, switching supplier carries risks.

A cheaper supplier may have:

poorer quality;
unreliable delivery;
longer lead times;
worse payment terms.

Again, an A Level answer should evaluate the consequence rather than simply state that cheaper suppliers are always preferable.

Gross Profit — Profit from the Core Trading Activity

Gross profit is calculated by deducting cost of sales from revenue.

Gross profit = Revenue - Cost of sales

Suppose:

Revenue = £800,000

Cost of sales = £500,000

Then:

Gross profit = £800,000 - £500,000

Gross profit = £300,000

Gross profit tells us how effectively the business is generating profit from its core sales before wider operating expenses are deducted.

A business can improve gross profit by:

increasing revenue;
reducing cost of sales;
or doing both.

Why Gross Profit Matters

Imagine two shops each generate £1 million in revenue.

Business A:

Revenue = £1,000,000

Cost of sales = £600,000

Gross profit = £400,000

Business B:

Revenue = £1,000,000

Cost of sales = £850,000

Gross profit = £150,000

The businesses have identical revenue.

But their underlying trading performance is dramatically different.

Business A retains 40p from every £1 of revenue before operating expenses.

Business B retains only 15p.

This is why analysing sales revenue alone can be misleading.

Expenses — The Cost of Running the Business

Once gross profit has been calculated, the business must deduct its operating expenses.

Examples might include:

administration salaries;
office rent;
marketing;
insurance;
telephone and internet;
professional fees;
stationery;
some utility costs;
management salaries;
IT systems.

The exact classification of individual costs can depend upon the type of business and the accounting approach being used.

But the principle is straightforward.

The higher the operating expenses, everything else being equal, the lower the operating profit.

Expenses Are Not Automatically Bad

Students sometimes write as though every business should reduce every expense.

That can be dangerous.

Consider advertising.

Reducing advertising expenditure from £500,000 to £100,000 certainly reduces expenses.

But what if that causes revenue to fall by £2 million?

Profit could become worse rather than better.

The same applies to:

staff training;
maintenance;
research and development;
customer service;
IT systems;
quality control.

Cutting costs can increase short-term profit but damage long-term competitiveness.

The better question is therefore not:

"How can we cut expenses?"

It is:

"Which expenses create value, and which expenses can be reduced without damaging the business?"

That is a much stronger Business Studies argument.

Operating Profit or Net Profit

In a simplified A Level income statement, profit after operating expenses may be referred to as operating profit or sometimes simply net profit, depending upon the terminology used by the course or question.

The basic calculation is:

Operating profit = Gross profit - Operating expenses

For example:

Gross profit = £300,000

Operating expenses = £220,000

Operating profit = £80,000

More detailed company accounts may then deduct finance costs, interest and taxation before reaching the final profit for the year.

Students should therefore always look carefully at the terminology used in the examination question.

A Complete Worked Example

Consider a fictional business called GreenBean Coffee Ltd.

During the year it sells 200,000 cups of coffee at an average selling price of £3.50.

Revenue = Price x Quantity

Revenue = £3.50 x 200,000

Revenue = £700,000

Its cost of sales is £250,000.

Therefore:

Gross profit = Revenue - Cost of sales

Gross profit = £700,000 - £250,000

Gross profit = £450,000

Operating expenses are:

Staff and administration = £180,000

Rent = £80,000

Marketing = £40,000

Insurance and other expenses = £30,000

Total expenses = £330,000

Therefore:

Operating profit = Gross profit - Expenses

Operating profit = £450,000 - £330,000

Operating profit = £120,000

The income statement would therefore show:

Revenue: £700,000

Cost of sales: £250,000

Gross profit: £450,000

Operating expenses: £330,000

Operating profit: £120,000

Now the Interesting Part: Interpretation

Calculation is only the beginning.

Suppose last year's operating profit was £150,000.

This year it has fallen to £120,000.

Management now needs to investigate why.

Perhaps coffee bean prices increased.

Perhaps staff wages increased.

Perhaps rent increased.

Perhaps the business deliberately spent more on marketing.

Perhaps new competitors forced the business to discount its prices.

Simply writing:

"Profit fell by £30,000."

is observation.

A stronger answer asks:

Why did it fall, what are the consequences, and what should management do about it?

Comparing Two Years

Consider this simplified data:

Year 1

Revenue: £600,000

Cost of sales: £220,000

Gross profit: £380,000

Expenses: £250,000

Operating profit: £130,000

Year 2

Revenue: £700,000

Cost of sales: £250,000

Gross profit: £450,000

Expenses: £330,000

Operating profit: £120,000

At first glance, Year 2 appears better.

Revenue increased by £100,000.

Gross profit increased by £70,000.

Yet operating profit actually fell by £10,000.

Why?

Because expenses increased by £80,000.

That is exactly the sort of observation students should make.

Sales growth does not automatically produce greater profit.

What Might Management Do?

Management might investigate the £80,000 increase in operating expenses.

But it should not immediately assume that the increase is a problem.

Suppose £60,000 of the increase resulted from opening a second shop.

Short-term profits may have fallen, but the investment could generate considerably greater revenue and profit in future years.

This demonstrates an important A Level principle:

Financial information needs context.

A number by itself rarely tells the complete story.

Who Uses an Income Statement?

Income statements are useful to several stakeholder groups.

Managers

Managers can use them to:

monitor financial performance;
identify rising costs;
compare actual performance with budgets;
make pricing decisions;
assess departments or product ranges;
plan future investment.

Owners and Shareholders

Owners want to know whether their investment is generating satisfactory returns.

Increasing profit may support:

higher dividends;
expansion;
increased business value;
greater retained profit.

However, shareholders may also accept lower short-term profits if management is investing successfully for future growth.

Banks and Other Lenders

A bank considering lending money may examine profitability to assess whether the business appears capable of meeting future repayments.

A business with consistently declining profits might be considered a greater lending risk.

Employees

Employees may be interested in profitability because a successful business may offer:

greater job security;
opportunities for promotion;
higher wages;
bonuses;
investment in training.

However, employees and owners do not always have identical interests.

Management may attempt to increase profit by limiting wage increases, which could create conflict.

Suppliers

Suppliers may want confidence that the business will continue trading and pay its bills.

Strong financial performance may also improve the company's ability to negotiate credit terms.

Government

Government is interested in business performance for several reasons, including:

taxation;
employment;
economic activity;
regulation.

Income Statements and Business Decisions

The real value of an income statement comes from the decisions it supports.

Suppose a restaurant discovers:

Revenue has increased by 5%.

Cost of ingredients has increased by 18%.

Gross profit has fallen.

Management might consider:

changing suppliers;
renegotiating contracts;
reducing food waste;
changing portion sizes;
altering the menu;
increasing prices.

But each option has consequences.

Increasing prices could upset customers.

Reducing portion sizes could damage reviews.

Using cheaper ingredients could reduce quality.

Changing suppliers could create reliability problems.

There is rarely one perfect answer.

That is why Business Studies is about judgement, not merely calculation.

A Common Exam Mistake: Confusing Revenue and Profit

Imagine somebody says:

"The company made £4 million last year."

What do they mean?

Revenue?

Gross profit?

Operating profit?

Profit after tax?

The numbers can be dramatically different.

A company might have:

Revenue = £4,000,000

Cost of sales = £2,700,000

Gross profit = £1,300,000

Expenses = £1,150,000

Operating profit = £150,000

The company did not "make £4 million profit".

It sold £4 million worth of goods or services but generated only £150,000 of operating profit.

Precise terminology matters.

Another Common Mistake: Assuming Higher Revenue Means Better Performance

Imagine revenue rises from £10 million to £12 million.

That sounds positive.

But suppose profit falls from £1 million to £500,000.

Revenue has increased by 20%, while profit has halved.

Perhaps the company increased sales by using heavy discounts.

Perhaps input costs rose.

Perhaps marketing expenditure became excessive.

Perhaps expansion created large additional expenses.

The examiner wants students to look beyond the headline figure.

Income Statements Lead Naturally to Profit Margins

Absolute profit figures are useful, but they become even more useful when expressed relative to revenue.

For example:

Gross profit margin = Gross profit / Revenue x 100

Operating profit margin = Operating profit / Revenue x 100

Using GreenBean Coffee:

Gross profit = £450,000

Revenue = £700,000

Gross profit margin = £450,000 / £700,000 x 100

Gross profit margin = 64.3%

Operating profit = £120,000

Operating profit margin = £120,000 / £700,000 x 100

Operating profit margin = 17.1%

Margins allow students to compare businesses of different sizes and investigate changes over time.

That makes them enormously useful in examination questions.

A Simple Student Exercise

Take the following information:

Revenue = £900,000

Cost of sales = £540,000

Operating expenses = £270,000

First calculate gross profit.

Gross profit = £900,000 - £540,000

Gross profit = £360,000

Then calculate operating profit.

Operating profit = £360,000 - £270,000

Operating profit = £90,000

Now ask the more important questions.

What happens if revenue increases by £50,000 but cost of sales increases by £70,000?

What happens if the business reduces expenses by £20,000?

Would reducing advertising by £20,000 necessarily be a good decision?

Could increasing wages actually increase profit?

Those questions turn a financial statement into a business discussion.

How to Approach Income Statement Questions in an Exam

When answering an income statement question, I encourage students to follow a simple sequence.

1. Calculate Carefully

Write down the formula before substituting numbers.

For example:

Gross profit = Revenue - Cost of sales

This reduces careless mistakes.

2. Identify the Change

Has revenue risen?

Has gross profit fallen?

Have expenses increased?

3. Explain Why It Matters

Do not simply repeat the figures.

Explain the consequence.

For example:

Higher cost of sales reduces gross profit, which may reduce the funds available to cover operating expenses.

4. Consider the Cause

Could supplier prices have increased?

Has the business discounted its products?

Has it expanded?

5. Consider the Context

A fall in profit is not necessarily evidence of poor management.

The business may be investing for future growth.

6. Reach a Judgement

Strong answers frequently finish with:

"It depends..."

But they must then explain precisely what it depends upon.

From Calculation to Business Thinking

One of the things I find particularly important when teaching financial accounts is preventing students from treating them as merely another set of maths exercises.

The arithmetic is usually not difficult.

Revenue minus cost of sales gives gross profit.

Gross profit minus expenses gives operating profit.

The difficult — and far more valuable — part is understanding what those figures are telling us about the business.

A student who merely calculates that profit has fallen has demonstrated numerical competence.

A student who notices that revenue increased while the profit margin fell, identifies rising costs as a possible cause, evaluates whether those costs might be investment expenditure, and recommends an appropriate management response is thinking like a business analyst.

That is the level we should be aiming for at A Level.

Final Thought: Profit Is a Story, Not Just a Number

An income statement appears to be a collection of figures.

In reality, it tells a story.

Revenue tells us something about the business's ability to sell.

Cost of sales tells us something about production, purchasing and sourcing.

Gross profit tells us how successfully the core trading activity is working.

Expenses tell us something about how the organisation is being operated.

Profit tells us what remained after all those competing demands.

But even profit is not the end of the story.

A fall in profit might indicate a business in trouble.

Or it might indicate a business investing heavily in its future.

A rise in profit might indicate excellent management.

Or it might have resulted from cost cuts that will create serious problems next year.

That is why understanding an income statement is such an important part of A Level Business Studies.

The calculations give us the numbers.

Business analysis gives those numbers meaning.

12 September 2026

Now Put Linux to Work — Build Your Own Web Server on a Raspberry Pi

Learning Linux commands is useful.

But there comes a point when simply moving files around, listing directories and installing packages stops feeling like a project.

So now that we have discovered Linux, it is time to make Linux actually do something.

One of my favourite first projects is surprisingly ambitious:

Build your own web server and host a website from your own Raspberry Pi.

You can start with a blank Raspberry Pi, type a handful of commands into the terminal and, a short while later, open a browser on another computer and see a web page being delivered by the Pi.

Then comes the really interesting possibility.

With some additional configuration, that little computer sitting on your desk can serve a website to somebody hundreds or thousands of miles away.

At that point Linux suddenly stops being an operating system you are studying.

It becomes part of the Internet.

What Exactly Is a Web Server?

When you visit a website, your browser is not normally downloading pages from some mysterious thing called "the Internet".

It is communicating with another computer.

Your browser sends a request.

The remote computer receives that request and sends something back — perhaps an HTML page, an image, some JavaScript or data.

The program performing that job is the web server.

Popular web-server packages include Apache and Nginx.

For this project I am going to use Apache, partly because it is extremely well established and partly because it makes a very approachable first Linux server project.

Raspberry Pi OS is based on Debian Linux, which means software can be installed using Debian's package-management system. The current Raspberry Pi OS is based on Debian, making standard Linux server tools readily available.

Why a Raspberry Pi Is Such a Good Server Project

A Raspberry Pi is not going to replace the enormous server infrastructure used by Amazon, Google or Microsoft.

That isn't the point.

For a small personal website, school project, development server, experimental dashboard or home intranet, it is remarkably capable.

More importantly, it allows students to see several areas of computing come together at once.

Suddenly we are dealing with:

Linux commands and file systems;
HTML and CSS;
IP addresses;
client-server computing;
TCP/IP networking;
ports;
DNS;
permissions;
services and processes;
cybersecurity;
remote access.

That is why I like this project so much.

We are no longer learning these ideas as isolated examination topics.

We are building something in which they all have a purpose.

Step 1 — Start With Your Raspberry Pi

You need relatively little equipment:

A Raspberry Pi running Raspberry Pi OS, a microSD card or SSD, a network connection and access to a terminal.

You can use the full Raspberry Pi OS desktop or Raspberry Pi OS Lite.

For a dedicated server, Lite is particularly interesting because it has no graphical desktop. You are interacting with the machine almost entirely through the command line.

That initially feels like a disadvantage.

It quickly becomes part of the attraction.

You begin to realise that a server doesn't actually need a screen, keyboard or mouse permanently attached to it.

It just needs to be running.

Step 2 — Update Linux

Open the terminal and begin with:

sudo apt update
sudo apt upgrade -y

There are several useful Linux ideas contained in those two commands.

sudo means we are running the command with elevated administrator privileges.

apt is the package-management system.

update downloads current information about available packages.

upgrade updates software already installed on the machine.

This is a good habit before installing new server software.

It is also an opportunity to discuss something that is sometimes missed when students study operating systems:

software maintenance is part of running a computer system.

A server that works perfectly but is never updated eventually becomes a security problem.

Step 3 — Install Apache

Now comes the wonderfully satisfying part.

Type:

sudo apt install apache2 -y

That is essentially it.

Linux downloads Apache, installs the necessary files and configures the web-server software.

You can check it with:

systemctl status apache2

You should see that Apache is running.

The systemctl command gives us another useful Linux concept.

Apache isn't simply a program we have opened in a window.

It is running as a service.

The server can start automatically when the Raspberry Pi starts and continue running quietly in the background.

That is how much of Linux server administration works.

Step 4 — Find the Raspberry Pi's Address

Every device on your home network needs an IP address.

Try:

hostname -I

You might see something resembling:

192.168.1.74

The exact address will be different on different networks.

Now go to another computer, tablet or phone connected to the same network and enter:

http://192.168.1.74

using your own Pi's address.

If everything has worked, something rather wonderful happens.

A web page appears.

It isn't coming from Google.

It isn't coming from a web-hosting company.

It is coming from the Raspberry Pi sitting beside you.

The Raspberry Pi Foundation has long used essentially this kind of Apache experiment as an introduction to hosting local HTML pages on a Pi.

Pause Here — Because Something Important Has Happened

This is a moment worth thinking about.

We installed some software.

The software opened a service listening for web requests.

Another machine found the Raspberry Pi using its IP address.

A browser sent an HTTP request across the network.

Apache received it.

Apache located an HTML document.

Apache sent the document back.

The browser interpreted the HTML and displayed the result.

We have just constructed a genuine client-server system.

Suddenly those network diagrams found in computing textbooks make considerably more sense.

Step 5 — Replace the Default Page With Your Own Website

The standard Apache website files are normally stored in:

/var/www/html

Go there:

cd /var/www/html

Have a look:

ls

You should find an HTML file.

We can replace it with our own page.

For example:

sudo nano index.html

Then create something simple:

<!DOCTYPE html>
<html>
<head>
    <title>My Raspberry Pi Server</title>
</head>

<body>
    <h1>Hello from my Raspberry Pi!</h1>

    <p>This webpage is being served by a computer in my house.</p>

    <p>I built this server using Linux and Apache.</p>
</body>
</html>

Save the file.

Refresh the browser.

Your page appears.

There is something particularly satisfying about this because we can immediately see the result of what we have done.

Now Make It Look Like a Proper Website

The next stage is obvious.

Add some CSS.

Create several pages.

Add photographs.

Build navigation.

Perhaps create:

index.html
about.html
projects.html
contact.html

You could create folders for:

/images
/css
/javascript

Now our Linux project has naturally become a web-development project as well.

A student who has learned some HTML and CSS can suddenly host what they have created on a real server rather than simply double-clicking an HTML file on their computer.

That difference matters.

Try Talking to the Server Without a Browser

There is another lovely Linux experiment we can perform.

On the Raspberry Pi type:

curl http://localhost

Instead of beautifully formatted text and pictures, you should see the HTML returned directly to the terminal.

Why?

Because curl is acting as the client.

The browser isn't essential.

This is an excellent demonstration of the difference between the data returned by the server and the way a browser interprets that data.

Look Behind the Scenes

Once a student has a working server, there is plenty more to investigate.

For example:

sudo systemctl restart apache2

restarts the server.

You can look at Apache's log files:

cd /var/log/apache2

and examine requests arriving at the server.

One particularly interesting command is:

sudo tail -f /var/log/apache2/access.log

Leave that running and visit your website from another device.

Requests begin appearing.

Refresh the browser.

Another request appears.

Visit another page.

Another appears.

This is the Internet becoming visible.

A website visit that seems almost instantaneous from the user's side is actually producing identifiable network activity on the server.

A Great Networking Investigation

There is an excellent experiment you can perform before going anywhere near the public Internet.

Try accessing the Raspberry Pi from:

the Raspberry Pi itself;
another computer connected by Ethernet;
a laptop connected by Wi-Fi;
a phone connected to your home Wi-Fi;
the same phone after switching Wi-Fi off.

The first four may work.

The fifth probably will not.

Why?

Because something fundamental has changed.

The phone is no longer part of your local network.

And that introduces one of the most important distinctions in networking:

a private IP address is not the same as a publicly reachable Internet address.

So How Do We Put the Website on the Internet?

This is where the project becomes even more interesting.

Inside your house, your Raspberry Pi might have an address such as:

192.168.1.74

That is a private address.

Millions of networks can use addresses like that.

Someone elsewhere on the Internet cannot simply type that address and arrive at your Raspberry Pi.

Traditionally, we solve this using technologies including:

NAT, port forwarding, DNS and a public IP address.

A home router can be configured so that requests arriving on particular ports are forwarded to your Raspberry Pi.

Web traffic commonly uses:

Port 80 = HTTP
Port 443 = HTTPS

You might also register a domain name so that people visit something meaningful such as:

www.myexperimentalsite.co.uk

rather than remembering an IP address.

Suddenly DNS — another subject that can seem rather abstract when taught from a diagram — has a very obvious purpose.

But Don't Simply Open Everything on Your Router

This is also where the project gives us an important lesson in cybersecurity.

Putting a server onto the public Internet means computers anywhere in the world can attempt to communicate with it.

That means we need to think about security before simply opening ports.

At a minimum, a public server should be kept updated, unnecessary services should remain closed, administrator accounts should use strong authentication, remote SSH access should preferably use keys rather than passwords, and a public website should use HTTPS.

Raspberry Pi's own current security guidance recommends key-based authentication for improving SSH security and also discusses tools such as Fail2Ban for systems operating as servers.

This is another reason this project is so valuable.

Cybersecurity stops being a theoretical discussion about "hackers".

The student now owns a machine that may potentially be reachable from the Internet.

The question becomes:

What exactly am I exposing?

A Modern Alternative — Use a Secure Tunnel

There is another approach that I think is particularly interesting for an educational project.

Instead of opening incoming ports on your home router, services such as Cloudflare Tunnel can create an outbound connection from the Raspberry Pi to a public service.

Your website can then be associated with a public hostname without directly exposing your home's public IP address or opening inbound ports.

Cloudflare describes its Tunnel system as using an outbound-only connection, allowing a public hostname to be mapped to a local service such as a web server running on localhost.

That gives us another excellent computing discussion.

There are now at least two possible architectures:

Internet
   |
Router
   |
Port forwarding
   |
Raspberry Pi

or:

Internet
   |
Tunnel provider
   |
Encrypted outbound tunnel
   |
Raspberry Pi

Neither should simply be memorised.

Ask instead:

What are the advantages, disadvantages and security implications of each?

That is much closer to real computing.

What About HTTPS?

When browsing modern websites you will normally see:

https://

rather than:

http://

The S matters.

HTTPS encrypts the communication between the browser and web server and uses certificates to establish the site's identity.

This provides another natural extension to the project.

Instead of merely asking students to define encryption or digital certificates, let them investigate what has to happen to turn their own HTTP website into an HTTPS website.

Tools such as Certbot can obtain certificates and configure supported web servers, with automated renewal available on typical installations.

Now suddenly public-key cryptography, certificates and certificate authorities have a reason to exist.

A Website Is Only the Beginning

Once the basic server works, there are dozens of directions in which this project could develop.

You could create a personal portfolio.

You could host revision material.

You could build a household information page.

You could display data from a Raspberry Pi sensor.

You could connect a weather station.

You could create a database-backed application.

You could write a Python application and put a web interface in front of it.

You could make a dashboard displaying temperature, pressure, humidity or electricity generation.

You could create a small API that another computer queries.

You could even have several Raspberry Pis sending measurements back to one central server.

The simple HTML page has become the starting point for much more substantial computing projects.

My Favourite Extension — Build a Live Science Dashboard

For somebody interested in both computing and science, this is where things become especially interesting.

Imagine connecting a temperature sensor to the Raspberry Pi.

A Python program records:

Time
Temperature
Humidity
Pressure

The readings are saved.

The web server then displays them.

Now somebody on another computer can open a browser and see the latest measurement.

Add a graph and the project becomes better still.

You now have:

Sensor -> Raspberry Pi -> Python -> Data -> Web server -> Network -> Browser

That is a complete system.

Each individual element is understandable, but together they form something genuinely useful.

Turn It Into a Proper Student Investigation

Rather than providing every instruction, I would be tempted to give students a challenge:

Can you build a website on a Raspberry Pi that I can view from another computer without anybody telling you exactly how to do it?

Once they succeed, introduce the next challenge:

Can you work out how the second computer actually found the Raspberry Pi?

Then:

Can somebody outside our network see it?

Then:

Why not?

Then:

How could we make that possible safely?

That sequence transforms the exercise from following instructions into problem solving.

And that is where some of the best computing education happens.

Useful Questions to Ask Along the Way

A project like this can generate considerably more learning if students have to explain what is happening.

What does sudo actually do?

Why do we need apt update?

What is a Linux service?

Why does Apache continue running after we close the terminal?

Where are the website files stored?

Why can another computer access the Raspberry Pi?

What does an IP address identify?

Why does localhost work?

What is port 80?

What is the difference between a private and public IP address?

What does DNS do?

Why is HTTPS preferable to HTTP?

Why might directly exposing a home computer to the Internet be risky?

Those questions move the exercise far beyond simply copying commands.

From Command Line to Internet Server

This is exactly why I think students should experience Linux rather than merely learn definitions about it.

At the beginning we type:

sudo apt install apache2

It looks like just another Linux command.

But follow what happens next.

Software is downloaded.

A service starts.

A network port begins listening.

A second computer sends a request.

Linux receives it.

Apache processes it.

A file is retrieved from the filesystem.

It is transmitted across the network.

A browser interprets it.

A website appears.

Then perhaps we introduce DNS.

Then HTTPS.

Then server logs.

Then scripting.

Then databases.

Then security.

One small Raspberry Pi has become a laboratory for understanding an enormous proportion of modern computing.

Conclusion — Don't Just Learn Linux. Build Something With It.

There is a danger when teaching computing that operating systems, networks, programming and cybersecurity become separate chapters.

Students learn a definition of an IP address.

Then they learn some Linux commands.

Then perhaps some HTML.

Then they learn that port 80 is used for HTTP.

Then they memorise what DNS does.

A web-server project pulls those separate ideas back together.

The IP address now has a purpose because we need to find our Raspberry Pi.

HTML has a purpose because we need something for Apache to serve.

Ports matter because requests must reach the correct service.

DNS matters because people prefer names to numbers.

Linux permissions matter because server files must be controlled.

HTTPS matters because communication across a public network should be protected.

Cybersecurity matters because putting a computer on the Internet has consequences.

And perhaps most importantly, the student finishes with something that actually works.

There is a considerable difference between being told how a web server works and typing an address into a browser on another computer and seeing a page that is being delivered by a Raspberry Pi sitting on your own desk.

That is when Linux starts to feel less like another topic in Computer Science.

It starts to feel like a tool.

And once students realise that, there is an enormous amount they can build next.

11 September 2026

Making Glue from Milk — The Chemistry of Casein, Coagulation and Precipitation

Milk does not immediately suggest itself as a building material.

We pour it over cereal, add it to tea and coffee, turn it into yoghurt and cheese, and perhaps occasionally forget about it at the back of the fridge.

But hidden inside milk is a substance that can be separated, treated and turned into something quite unexpected:

glue.

The key ingredient is casein, the main family of proteins found in cow's milk.

By adding acid to milk, we can make those proteins come out of suspension as solid curds. Separate those curds, remove as much liquid as possible, and then treat the casein appropriately, and we can produce a surprisingly effective adhesive.

It is an excellent experiment because what initially looks like a simple kitchen activity opens the door to some serious chemistry.

We encounter:

proteins;
acids;
pH;
electrical charge on molecules;
colloids;
precipitation;
coagulation;
filtration;
neutralisation;
polymers;
intermolecular forces;
and the science of adhesives.

It also provides a wonderful reminder that chemistry is not simply about producing coloured solutions in test tubes.

Sometimes chemistry produces useful materials.

Milk Is Much More Complicated Than It Looks

At first glance, milk appears to be a simple white liquid.

Chemically, however, it is a remarkably complicated mixture containing:

water;
proteins;
fats;
lactose;
calcium compounds;
vitamins;
minerals;
and many other dissolved or suspended substances.

Approximately 80% of the protein in cow's milk is casein.

But casein is not simply floating around as individual protein molecules.

Much of it is organised into tiny structures called casein micelles.

These microscopic particles remain dispersed through the water in milk, contributing to its familiar white appearance.

Under normal conditions, the micelles repel one another sufficiently to remain dispersed.

Change the chemistry of their surroundings, however, and that stability can disappear.

That is exactly what we are going to do.

The Key Idea — Make the Casein Precipitate

One of the easiest ways of separating casein from milk is to make the milk more acidic.

Ordinary white vinegar works very well because it contains dilute ethanoic acid, also known as acetic acid.

Milk normally has a pH somewhere around 6.5 to 6.8.

Casein proteins contain groups that can gain or lose H+ ions depending upon the pH.

As the pH falls towards approximately 4.6, casein reaches what chemists call its isoelectric point.

This is extremely important.

At the isoelectric point, the overall electrical charge on the protein is approximately zero.

The protein particles therefore repel each other much less strongly.

Instead of remaining distributed through the liquid, they begin sticking together.

The casein coagulates and precipitates.

Suddenly our smooth white milk begins separating into:

solid curds and liquid whey.

Anyone who has made cheese will recognise what is happening.

Precipitation, Coagulation and Cheese Chemistry

This gives us an opportunity to introduce several useful scientific words.

Precipitation

A substance that was previously dispersed or dissolved comes out of the liquid as a solid.

Coagulation

Small particles come together to form larger clumps or masses.

In this experiment the casein particles lose much of the electrostatic repulsion that normally keeps them apart.

They aggregate.

We see this as curds forming in the milk.

The remaining liquid is often called whey.

This is very similar to some of the chemistry used in food production.

But our objective is not cheese.

We are going to turn our casein into an adhesive.

What You Will Need

For a straightforward investigation you need:

about 100 mL of skimmed or semi-skimmed milk;
approximately 10-15 mL of white vinegar;
a small saucepan, beaker or heat-resistant container;
a thermometer if available;
a spoon or stirring rod;
filter paper, muslin, cheesecloth or a fine kitchen sieve;
paper towel;
sodium bicarbonate;
a small container for making the glue;
two wooden lolly sticks or pieces of wood for testing it.

Skimmed milk is particularly useful because the lower fat content generally gives a cleaner casein preparation.

Whole milk will still work, but the additional fat can make the separated material feel greasier.

Stage One — Warm the Milk

Measure approximately 100 mL of milk.

Warm it gently to around 40-50 degrees C.

It does not need to boil.

In fact, vigorous boiling is unnecessary and can complicate the experiment.

The warming simply helps the acid interact with the milk and allows the casein to coagulate relatively quickly.

Already there is a useful scientific point here.

Students often assume that whenever heat is used in an experiment, heat must be causing the chemical change.

Here it is mainly helping the process occur efficiently.

The acid is the crucial ingredient.

Stage Two — Add the Vinegar

Add approximately 10 mL of white vinegar while stirring gently.

The transformation can be remarkably rapid.

Within moments the previously smooth milk begins looking lumpy.

White solids appear.

The liquid surrounding them becomes more transparent and slightly yellowish.

Those solids contain the precipitated casein.

Add a little more vinegar if necessary until separation appears reasonably complete.

It is worth stopping at this stage and simply looking.

This is one of those practical experiments where the change is visually obvious enough that very little explanation is initially required.

Something fundamental has happened to the milk.

What Has the Acid Actually Done?

A very simplified explanation would be:

acid makes the casein precipitate.

But the underlying chemistry is more interesting.

Casein molecules contain acidic and basic groups.

The electrical charge carried by the proteins therefore depends upon the surrounding pH.

At normal milk pH, casein micelles carry sufficient charge to help keep them dispersed.

Adding ethanoic acid increases the concentration of H+ ions.

As the pH approaches the isoelectric point of casein, the overall charge decreases.

Repulsion between neighbouring protein particles becomes weaker.

They begin aggregating.

At the same time, acidification affects the calcium phosphate associated with the casein micelles, further destabilising their structure.

The result is the spectacular coagulation that we can actually see.

Is This Just Protein Denaturation?

Students may immediately think of cooking an egg.

Heating egg white causes proteins to unfold and form a solid network.

That is commonly described as protein denaturation.

Casein precipitation is slightly different.

Caseins do not have the same tightly folded structures as many other proteins.

Here, reducing the electrical charge and destabilising the casein micelles is especially important.

So although the words coagulation and denaturation are sometimes used rather loosely in everyday explanations, the chemistry deserves a little more care.

What we are primarily observing is acid-induced casein precipitation and aggregation.

Stage Three — Separate the Casein

Pour the mixture through filter paper, muslin or a fine sieve.

The liquid whey passes through.

The solid casein remains behind.

Press the casein gently with paper towel to remove as much liquid as possible.

You can rinse the curds with a little clean water and filter them again if you want to remove some of the remaining acid and soluble material.

Eventually you should have something resembling a soft white paste or crumbly putty.

At this stage it still does not look particularly promising as glue.

But we have now separated a natural polymer from milk.

Stage Four — Turn the Casein into Glue

Transfer the casein to a small container.

Add a small amount of sodium bicarbonate.

Start with perhaps a quarter of a teaspoon for casein obtained from around 100 mL of milk.

Mix thoroughly.

You may notice some gentle fizzing.

This occurs because sodium bicarbonate reacts with remaining acid.

A simplified ionic equation is:

H+ + HCO3- -> CO2 + H2O

The carbon dioxide produces the bubbles.

The bicarbonate also raises the pH.

This helps transform our acidic casein curds into a smoother and more usable adhesive paste.

Add a few drops of water if necessary.

The objective is not to produce a thin liquid.

You want something resembling a thick glue.

Why Should Protein Work as Glue?

This is perhaps the most interesting question in the entire experiment.

Why should something extracted from milk stick pieces of wood together?

Proteins are enormous molecules.

Casein molecules contain many different chemical groups capable of interacting with other substances.

When casein glue is spread across a surface, it can make close contact with microscopic irregularities in the material.

At the molecular level there can be interactions including:

hydrogen bonding;
electrostatic interactions;
attraction between polar groups;
and mechanical interlocking with pores and rough surfaces.

As water leaves the adhesive during drying, the casein molecules become increasingly concentrated.

Eventually they form a solid protein-rich layer between the two surfaces.

We have effectively created a natural polymer adhesive.

Test the Glue

A scientific investigation should not finish with:

"It looks like glue."

We should test whether it actually works.

Take two wooden lolly sticks.

Overlap them by perhaps 2 cm.

Spread approximately the same amount of casein glue over the overlapping region.

Clamp them together or place a small weight on top while the adhesive dries.

Leave them for several hours, preferably overnight.

Then try pulling them apart.

The result can be surprisingly convincing.

Turn It into a Proper Investigation

This experiment becomes much more interesting when students begin changing variables.

For example:

Does milk type matter?

Compare:

skimmed milk;
semi-skimmed milk;
whole milk.

Keep everything else constant.

Which produces the most casein?

Which produces the strongest glue?

How Much Casein Can You Obtain?

You can also turn the practical into a quantitative experiment.

Weigh a clean dry filter paper before filtration.

Collect the casein and allow it to dry thoroughly.

Weigh everything again.

Calculate:

Mass of casein = final mass - mass of filter paper

You could then calculate percentage yield relative to the mass of milk used:

Percentage yield = mass of dry casein / mass of milk x 100

Students should be careful with their interpretation.

Milk contains plenty of water, so we are not expecting an enormous percentage yield.

Does the Amount of Acid Matter?

Try adding different quantities of vinegar to identical volumes of milk.

For example:

2 mL;
5 mL;
10 mL;
15 mL;
20 mL.

Measure the mass of casein obtained.

At first, increasing the acid concentration should promote more complete precipitation.

Eventually, however, adding more acid should produce little additional benefit.

This is a useful demonstration of an important experimental principle:

more reagent does not necessarily mean more product indefinitely.

Measure the pH

If you have a pH meter or suitable pH probe, the experiment becomes considerably more informative.

Measure the starting pH of the milk.

Then add vinegar gradually while monitoring the pH.

Watch carefully as coagulation becomes extensive near the casein isoelectric region.

This converts what looks like an elementary kitchen experiment into a very good piece of analytical chemistry.

You can plot:

mass of casein precipitated against pH

or perhaps:

turbidity against pH

if suitable sensors are available.

Which Acid Works Best?

Another extension would be to compare different food-safe acids.

You might investigate:

white vinegar;
lemon juice;
citric acid solution.

Use solutions of comparable acidity where possible.

Students could investigate whether the type of acid matters or whether the principal factor is simply the pH achieved.

This is where experimental design becomes important.

If one sample receives much more acid than another, we cannot confidently say that differences arose because the acids themselves were different.

How Strong Is Our Milk Glue?

The most enjoyable extension may be engineering rather than chemistry.

Prepare several identical wooden joints.

Glue each pair using a different adhesive.

For example:

casein glue;
PVA glue;
flour paste;
starch adhesive.

Allow each sample to dry for the same length of time.

Then gradually add mass until the joint fails.

A simple results table might contain:

Adhesive	Overlap area	Drying time	Maximum load before failure
Casein	4 cm2	24 h	...
PVA	4 cm2	24 h	...
Flour paste	4 cm2	24 h	...

Now students are not simply making glue.

They are carrying out materials testing.

Be Careful About What You Call "Strong"

Suppose one glue supports a heavier mass than another.

Is that enough to declare it better?

Perhaps not.

We might also ask:

How long did it take to dry?
Was the joint waterproof?
Did the glue remain flexible?
Did it become brittle?
How easily could it be applied?
How long could it be stored?
Did it stick better to wood than plastic?
What happened in humid conditions?

Real engineering decisions rarely depend upon a single measurement.

The "best" material depends upon what we want the material to do.

A Glue with a Long History

Casein adhesives are not simply a classroom curiosity.

Before modern synthetic glues became widespread, casein-based adhesives were important materials for woodworking and plywood manufacture.

They offered a way of creating useful adhesives from naturally occurring proteins.

Modern synthetic adhesives such as PVA, epoxies and polyurethane products have largely replaced casein glue for many applications because they can offer better consistency, durability and water resistance.

Nevertheless, making casein glue gives students a glimpse of an earlier form of materials technology.

It also challenges an assumption we make increasingly often:

that useful manufactured materials must begin with petroleum or sophisticated industrial chemicals.

Nature already produces extraordinarily complicated polymers.

Sometimes chemistry is about learning how to separate and use them.

Milk, Cheese and Glue Are Connected by Chemistry

One of my favourite features of this experiment is the way it links apparently unrelated objects.

Milk.

Cheese.

Protein.

Glue.

They appear to belong to completely different worlds.

But at molecular level the connections become obvious.

Cheese making relies upon manipulating milk proteins.

Our glue-making experiment does something related, but instead of preparing food we deliberately recover the protein as a functional material.

This is one of the strengths of practical science.

The divisions between "food science", "chemistry" and "materials science" start disappearing.

There is simply matter — and what happens when we alter its conditions.

A Good Experiment for Discussing Colloids

Milk also gives us an opportunity to discuss something that receives surprisingly little attention in school science:

colloids.

Not every mixture is simply a solution or a suspension.

Milk contains extremely small particles dispersed through another substance.

Its proteins and fats interact with water in complicated ways.

Colloids are everywhere.

Examples include:

milk;
mayonnaise;
fog;
smoke;
paint;
shaving foam;
gelatin;
many cosmetics;
and numerous medicines.

Changing the conditions surrounding a colloid can cause it to become unstable.

Our precipitating casein is a beautiful example.

Common Problems

Nothing seems to happen

The milk may not be sufficiently warm, or you may not have added enough acid.

Add vinegar gradually while stirring.

The mixture is very greasy

You may be using milk with a high fat content.

Try skimmed milk.

The casein glue is too runny

Too much water has probably been added.

Add water only a few drops at a time.

The glue is too crumbly

Mix the casein more thoroughly with a small quantity of sodium bicarbonate and a few drops of water.

The joint seems weak

Allow considerably longer for drying.

Casein adhesive needs water to evaporate before maximum strength develops.

Also check that your surfaces are clean and make reasonably close contact.

Safety

Although this is a comparatively low-risk practical, normal laboratory precautions still apply.

Do not eat or drink laboratory materials.
Milk proteins can cause allergic reactions in people with milk allergies.
Take care when heating liquids.
Do not boil the milk unnecessarily.
Wash hands after the experiment.
Clean surfaces and equipment afterwards.
Once milk has been used experimentally, treat it as laboratory material rather than food.
Do not store homemade casein glue for long periods. Make a fresh batch when required.

Young students should carry out heating and chemical handling with appropriate adult supervision.

What Students Are Really Learning

At first sight, this might appear to be a novelty experiment.

"Make glue from milk."

But scientifically it contains far more depth than that description suggests.

Students encounter:

Biology:
Proteins and their properties.

Chemistry:
Acids, pH, neutralisation, precipitation and molecular charge.

Physics:
Intermolecular forces and material behaviour.

Materials science:
Polymers, adhesives and mechanical testing.

Experimental science:
Variables, controls, measurement, repeatability and data interpretation.

That combination makes it particularly valuable.

A good practical does not merely illustrate something students already know.

It gives them something new to think about.

From Breakfast to Materials Science

Perhaps the most memorable moment comes at the very beginning.

You start with an ordinary glass of milk.

Add a little acid.

Suddenly the liquid separates.

Filter it.

Treat the solid.

Spread it between two pieces of wood.

The following day those pieces may be firmly attached.

Nothing magical has occurred.

We have simply changed the conditions surrounding a naturally occurring polymer.

But that is precisely why the experiment is so satisfying.

Science allows us to look at an everyday substance and ask a completely different question.

Not:

"Can I drink this?"

but:

"What is actually inside it, how can I separate those substances, and what else could they do?"

That change of perspective is at the heart of good science.

Milk is not merely milk.

It is water, sugars, minerals, fats and proteins assembled into an extraordinarily complicated material.

And hidden among those proteins is casein — waiting for a little chemistry to turn breakfast into glue.