Chemiluminescence — Creating Light From a Chemical Reaction

We are used to making light with electricity, flames or something that is already extremely hot. Chemiluminescence is different. The chemistry itself produces the light.

There are some practical demonstrations that immediately change the atmosphere in a laboratory.

Chemiluminescence is one of them.

Turn down the room lights, start the reaction, and suddenly a solution begins to glow blue. There is no electrical connection to it. There is no bulb hidden underneath it. Nothing is burning.

The light is being produced by a chemical reaction.

That makes chemiluminescence spectacular to watch, but it also makes it an excellent piece of science. Behind that glow are several important ideas: chemical energy, electron excitation, photon emission, reaction rates, catalysts and activation energy.

And, importantly, it gives us something that can be investigated rather than simply admired.

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What Actually Is Chemiluminescence?

Chemiluminescence is the production of light as a result of a chemical reaction.

Normally, when an exothermic reaction releases chemical energy, much of that energy eventually appears as thermal energy.

We notice the mixture getting warmer.

Chemiluminescent reactions take a rather more interesting route.

Part of the energy released by the reaction is used to produce molecules in an electronically excited state.

Those excited molecules are unstable.

They eventually return to a lower-energy state and release the excess energy as a photon of light.

In simplified form:

chemical reactants -> excited product -> product + light

We can represent the final stage as:

excited molecule -> ground-state molecule + photon

The energy of the photon is related to its frequency:

E = hf

where:

• E is photon energy;
• h is Planck's constant;
• f is frequency.

So the colour we observe is directly connected to the amount of energy being released when the molecule returns to its lower-energy state.

That is a remarkable connection.

A bottle glowing blue in a dark laboratory is ultimately demonstrating quantum behaviour.

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Light Without Becoming Red Hot

This is perhaps the first surprising feature.

If I want a piece of metal to glow, I normally have to make it extremely hot.

The filament in a traditional incandescent lamp works because electricity heats the filament to a very high temperature.

A flame emits light because energetic particles and hot gases are involved.

Chemiluminescence does not require the material to reach anything approaching those temperatures.

For that reason it is sometimes described as a form of cold light.

That does not necessarily mean absolutely no heat is generated. The chemical reaction can still release thermal energy.

The important point is that the visible light is not being generated simply because the material has become incandescent.

The mechanism is different.

And that immediately raises a good question for students:

If it isn't hot enough to glow, where is the light coming from?

That question takes us straight into energy levels.

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The Famous Example: Luminol

Probably the best-known chemiluminescent substance is luminol.

Under suitable chemical conditions, luminol undergoes an oxidation reaction that ultimately produces an electronically excited product.

As that product returns to its lower-energy state, blue light is emitted.

It is a wonderfully striking demonstration.

In a sufficiently dark laboratory, the glow can appear almost unreal.

For teaching purposes I prefer to concentrate on the science rather than merely trying to create the brightest possible reaction.

A good chemiluminescence demonstration should lead to questions such as:

• Why is light produced?
• Why is it blue?
• Why does the brightness change?
• Why does the reaction eventually stop?
• What determines how quickly the light fades?
• Can we measure it?

The moment students start asking those questions, the demonstration has become an experiment.

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Why Luminol Appears in Crime Programmes

Luminol is also famous because of forensic science.

The iron associated with haemoglobin in blood can help catalyse the oxidation chemistry involved in the luminol reaction.

Investigators can therefore spray an appropriate luminol reagent over a suspected area in darkness and look for chemiluminescence.

Very small traces of blood may sometimes become visible.

Television crime programmes have understandably made this look enormously dramatic.

But the real chemistry is more interesting than the television version.

A positive glow does not automatically prove:

"There is definitely human blood here."

Other materials can interfere with or catalyse similar reactions.

In forensic science, luminol is therefore useful as a presumptive test, rather than being the final word on what a sample contains.

That distinction is also a useful lesson in experimental science.

A result can provide evidence without providing complete proof.

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Glow Sticks Are Chemiluminescence Too

Students may have encountered chemiluminescence long before entering a laboratory.

A glow stick is essentially a small chemical reactor.

Inside are substances kept separate until the stick is bent or activated.

Once mixed, a sequence of chemical reactions transfers energy to a fluorescent dye.

The dye becomes electronically excited.

When it relaxes:

excited dye -> dye + light

Different fluorescent dyes can therefore produce different colours.

This makes glow sticks particularly interesting because they show how chemistry can be used to control the colour of emitted light.

And they provide us with an extremely simple investigation.

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A Very Accessible Experiment: Hot and Cold Glow Sticks

Take identical glow sticks and activate them at approximately the same time.

Keep one at room temperature.

Place another, still sealed, in cold water.

Place another in comfortably warm water.

Do not use very hot water and do not cut the glow sticks open.

Now compare their brightness.

The warm glow stick will normally appear brighter.

The cold one will normally be dimmer.

But wait.

Leave them for longer and another difference becomes apparent.

The warmer glow stick tends to use up its reactants more quickly.

It may initially be bright but fade sooner.

The colder glow stick generally reacts more slowly.

It is dimmer, but the glow can persist for longer.

This is a lovely demonstration of reaction kinetics.

Higher temperature -> faster reaction -> brighter light initially -> shorter useful lifetime.

Lower temperature -> slower reaction -> dimmer light -> longer useful lifetime.

Suddenly the familiar statement that "increasing temperature increases the rate of reaction" becomes something students can actually see.

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From Demonstration to Investigation

The next stage is to stop relying on our eyes.

Can we measure the light?

A light sensor or data logger can transform the experiment.

Place the chemiluminescent source in a darkened enclosure with a light sensor positioned at a fixed distance.

Record light intensity against time.

Instead of simply saying:

"It gets dimmer."

we can produce a graph.

That immediately opens the door to much more interesting questions.

For example:

How quickly does intensity decrease?

Record intensity every few seconds or use continuous data logging.

Plot:

light intensity against time

and compare different conditions.

How does temperature affect maximum brightness?

Repeat the experiment at several controlled temperatures.

How does temperature affect duration?

Define a threshold intensity and measure how long the glow remains above it.

Is the relationship linear?

Probably not.

And that gives students another important lesson.

Natural systems do not have to produce nice straight-line graphs simply because straight lines are convenient to analyse.

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Measuring Chemiluminescence With a Camera

A camera can also be turned into a surprisingly useful scientific instrument.

Keep:

• exposure time constant;
• aperture constant;
• ISO constant;
• camera position fixed;
• background lighting constant.

Photograph the chemiluminescent reaction at regular intervals.

You can then compare image brightness.

Even simple image-analysis software can allow students to extract approximate intensity values from the photographs.

This brings together chemistry, physics and computing.

The result is no longer simply:

"The reaction glowed."

It becomes:

"The recorded light intensity decreased with time, and the rate depended upon temperature."

That is a much stronger scientific statement.

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The Luminol Demonstration in the Laboratory

For a more advanced laboratory demonstration, luminol provides the classic blue chemiluminescence experiment.

Because luminol demonstrations involve oxidising reagents and alkaline conditions, this is something I would carry out using a recognised educational procedure or commercial demonstration system, with appropriate eye protection, gloves and laboratory controls rather than improvising concentrations simply to obtain a stronger glow.

But once the reaction is running, there are several excellent investigations that can be built around it.

Rather than merely turning off the lights and watching, students might investigate:

• light intensity against time;
• the effect of temperature;
• the effect of changing concentrations using an approved procedure;
• the role of a catalyst;
• how long measurable light emission continues;
• the colour or wavelength of the emitted light.

That last possibility takes us into spectroscopy.

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Can We Measure the Colour?

To our eyes, luminol appears blue.

But "blue" is not a particularly scientific measurement.

If suitable equipment is available, the emitted light can be examined using a spectrometer.

Now we can investigate the wavelength distribution of the emitted radiation.

This creates a beautiful connection between chemistry and physics.

The reaction determines the electronic state of the product.

The electronic transition determines the energy released.

The energy determines the photon frequency.

The frequency determines the wavelength.

And the wavelength determines the colour we perceive.

So one glowing solution can connect:

chemical reactions -> molecular energy -> photons -> spectroscopy -> human vision

That is exactly the sort of connection that makes practical science so powerful.

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Chemiluminescence, Fluorescence and Phosphorescence Are Not the Same Thing

These terms are easily confused.

They all involve molecules releasing light, but the source of the excitation is different.

Fluorescence

A molecule absorbs electromagnetic radiation, often ultraviolet light.

It becomes excited and then rapidly emits light.

Turn off the exciting radiation and the fluorescence normally disappears almost immediately.

Phosphorescence

Energy is again absorbed first, but the return to the lower-energy state can be much slower.

The material can therefore continue glowing after the original light source has been removed.

That is why some "glow in the dark" materials remain visible.

Chemiluminescence

The initial energy comes from a chemical reaction.

No external ultraviolet lamp is required to excite the molecules.

The chemistry itself supplies the energy.

That difference is crucial.

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Biology Has Its Own Version: Bioluminescence

Nature discovered the same principle long before chemists did.

Fireflies, some fungi, marine organisms and many deep-sea creatures can produce light through biochemical reactions.

This is called bioluminescence.

Bioluminescence is essentially chemiluminescence occurring within a biological system.

Luciferin molecules undergo enzyme-controlled reactions involving luciferase.

The result is light.

Organisms use it for remarkably different purposes:

• attracting mates;
• communication;
• camouflage;
• attracting prey;
• startling predators;
• signalling.

Think about the evolutionary significance.

In the deep ocean, sunlight may be virtually absent.

An organism capable of producing light has suddenly gained an entirely new method of communication.

Chemistry has become biology.

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Why Does the Reaction Eventually Go Dark?

This is another deceptively useful question.

Students sometimes think of the glow as if it were a property of the substance.

But chemiluminescence exists only while the necessary chemical reaction continues.

Reactants are being consumed.

Eventually one or more reactants becomes sufficiently depleted that the reaction rate falls.

Fewer excited molecules are being produced each second.

So fewer photons are emitted.

The glow becomes dimmer.

Eventually it disappears.

This reinforces a very basic but important chemical idea:

A reaction cannot continue indefinitely if its reactants are being consumed.

A glow stick is therefore also a tiny practical demonstration of limiting reactants.

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Brightness and Duration Present an Interesting Trade-Off

This leads to one of my favourite aspects of the glow-stick investigation.

Suppose your goal is simply:

Make the glow stick as bright as possible.

Warm it.

But suppose your goal changes:

Make it remain visible for as long as possible.

Now cooling it may be advantageous.

Neither condition is universally "best".

The best condition depends upon what you are trying to achieve.

That is a very useful scientific and engineering lesson.

Optimization almost always requires deciding what we actually want to optimise.

Maximum brightness?

Maximum duration?

Total light output?

Minimum chemical use?

Performance at a particular temperature?

The science gives us the data.

The engineering problem determines how we use it.

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A Possible Student Investigation

A very manageable investigation would be:

Question

How does temperature affect the intensity and duration of chemiluminescence?

Use identical sealed glow sticks from the same batch.

Test them under several temperature conditions.

Measure:

• temperature;
• initial light intensity;
• maximum light intensity;
• intensity at regular time intervals;
• time taken to fall below a chosen brightness level.

Then plot graphs of:

light intensity against time

for each temperature.

Students could then discuss:

• reaction rate;
• collision theory;
• energy transfer;
• repeatability;
• control variables;
• uncertainty;
• limitations of the measuring equipment.

Suddenly a relatively inexpensive glow stick has become an experiment touching several areas of GCSE and A-level science.

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What Would We Need to Control?

This is where the practical becomes particularly valuable.

If we want to compare the results scientifically, we need to think about controls.

Keep constant, as far as possible:

• glow-stick type;
• manufacturing batch;
• activation method;
• time between activation and first measurement;
• distance from sensor;
• sensor orientation;
• ambient lighting;
• temperature throughout the experiment.

That last point is particularly interesting.

Putting a glow stick into water at 10 C does not necessarily mean that the reacting chemicals instantly become 10 C.

There will be a period of thermal equilibration.

That provides another opportunity to discuss the difference between the condition we think we have created and the condition actually experienced by the experimental system.

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What About Experimental Error?

Imagine two glow sticks apparently behaving differently.

Is that because of temperature?

Perhaps.

But there could also be manufacturing variation.

One stick might contain slightly different quantities of reactants.

They might not have been activated in exactly the same way.

One could be positioned slightly closer to the sensor.

External light might interfere.

That means a better experiment would use repeats.

For each temperature:

repeat the measurement several times

and calculate a mean.

Students can then begin thinking about spread and uncertainty rather than treating every individual measurement as perfectly reliable.

Again, chemiluminescence has become much more than a colourful demonstration.

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Why I Like Experiments Like This

One reason I enjoy practical science is that a relatively simple observation can lead surprisingly far.

You begin with:

"Look — it glows."

Five minutes later you can be discussing:

• activation energy;
• molecular collisions;
• electron energy levels;
• photons;
• spectroscopy;
• catalysts;
• rate equations;
• forensic science;
• biological evolution;
• experimental uncertainty.

That is what good practical science should do.

The experiment is not an interruption to the theory.

It creates reasons to want to understand the theory.

A student who has watched the brightness of a chemiluminescent reaction change with temperature has a much more concrete reason to care about reaction rates.

A student who has seen blue light appear without a lamp has a reason to ask what a photon actually represents.

Those questions matter.

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The Moment When the Lights Go Out

There is also something valuable about the sheer theatre of chemiluminescence.

Science teaching does not have to be dull in order to be rigorous.

Turn down the laboratory lights.

Start the reaction.

Watch blue light appear where there was darkness.

For a moment, students simply watch.

Then comes the question:

"How is it doing that?"

That is exactly the question we want.

The spectacle gets their attention.

The science keeps it.

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Conclusion — Sometimes Chemistry Really Does Glow

Chemiluminescence is a wonderful example of why science becomes so much more interesting when we move beyond simply learning definitions.

A chemical reaction releases energy.

Some of that energy creates electronically excited molecules.

Those molecules return to lower-energy states.

Photons are released.

We see light.

But from that simple chain of events we can investigate reaction rates, temperature, catalysts, spectroscopy, forensic chemistry, biochemistry and experimental design.

We can measure the changing brightness.

We can produce graphs.

We can test hypotheses.

And we can ask whether making a reaction brighter necessarily makes it better.

So the next time somebody cracks a glow stick at a party, perhaps look at it slightly differently.

Inside that little plastic tube is a chemical reaction producing excited molecular states and releasing photons into the room.

And if we take it into the laboratory, measure what is happening and start asking questions, that glowing stick becomes a surprisingly sophisticated scientific experiment.

Sometimes the best way to illuminate a scientific idea is quite literally to make the chemistry produce the light.