The Chemistry of Swimming Pools – Why You Smell Chlorine

 

The Chemistry of Swimming Pools – Why You Smell Chlorine

You know that “chlorine smell” you notice the moment you walk into a swimming pool?
Here’s the twist — it’s not actually chlorine you’re smelling.

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💧 What’s in Pool Water?

Most pools are disinfected with chlorine-based compounds, often sodium hypochlorite or calcium hypochlorite.
When added to water, these release hypochlorous acid (HOCl), a powerful disinfectant that kills bacteria, viruses, and algae.

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🧪 So Where Does the Smell Come From?

The familiar “pool smell” comes from chloramines, which are formed when chlorine reacts with:

• Sweat

• Urine

• Dead skin cells

• Other organic material brought in by swimmers

One common culprit is trichloramine (NCl₃), which has a sharp, pungent odour.

Trichloramine (NCl₃), also known as nitrogen trichloride, is a chemical compound that's a volatile, irritating by-product of chlorination in swimming pools. It's formed when chlorine reacts with nitrogen-containing compounds like ammonia and urea present in the water. Trichloramine is known for its strong, irritating odour and can cause respiratory and eye irritation, especially in indoor swimming pools. 

Here's a more detailed explanation:

Formation:

• Chlorination Byproduct: Trichloramine is a disinfection byproduct, specifically formed when chlorine reacts with nitrogenous compounds in water.
• Precursors: Common precursors include ammonia, ammonium ions, urea, and α-amino acids, which are often found in swimming pool water.
pH Dependence: The formation of trichloramine is favoured at acidic and neutral pH levels. 

Properties:

Volatility: Trichloramine is a volatile compound, meaning it readily evaporates into the air. 
Irritant: It's a known irritant, causing irritation to the eyes and upper respiratory tract. 
Odour: Trichloramine has a distinct, pungent odour. 
Explosive: In higher concentrations, it can be explosive. 

Health Effects:

Eye and Respiratory Irritation:
Swimmers and pool workers are often exposed to trichloramine, leading to eye and respiratory irritation. 
Asthma:
Some studies suggest a link between trichloramine exposure and an increased risk of asthma development, particularly among children who frequent swimming pools. 
Occupational Asthma:
There have been reports of occupational asthma among swimming pool workers exposed to trichloramine. 
Lung Function:
Studies have investigated the impact of trichloramine exposure on lung function, with some showing changes in lung permeability and respiratory symptoms. 

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🧠 The Irony

The stronger the smell, the dirtier the water might be — because more contaminants mean more chloramine formation.
A well-maintained pool with balanced chlorine levels often smells far less.

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⚖️ Pool Chemistry in Balance

Keeping a pool safe (and pleasant) involves:

• Free chlorine: The amount available to disinfect.

• Combined chlorine: The amount tied up in chloramines.

• pH levels: Should be kept between 7.2 and 7.8 for optimal chlorine efficiency.

• Shock treatments: Adding a higher dose of chlorine to break down chloramines.

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🔬 Classroom Demonstrations

Mini Pool Chemistry Experiment

1. Use small beakers of water with added sodium hypochlorite.

2. Introduce small amounts of ammonia solution to simulate contamination.

3. Measure free and combined chlorine using test strips or a Chlorine Meter.

4. Discuss how pH and temperature affect results.

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🎓 Curriculum Links

• GCSE & A-Level Chemistry: Reactions between acids, bases, and ammonia derivatives.

• Real-life chemistry in public health and hygiene.

• Equilibrium concepts and how they apply to water treatment.

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💡 Fun Fact for Students

Olympic-sized pools contain millions of litres of water — but it only takes a small chemical imbalance to make them smell and irritate your eyes.

At Philip M Russell Ltd, we use real experiments, high-quality video demonstrations, and engaging stories to bring chemistry to life. Understanding why pools smell the way they do helps the students understand more about life and the chemistry that is going on around them..

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📅 Now enrolling for 1:1 GCSE and A-Level Chemistry Tuition
In our lab, classroom, or online via Zoom.
🔗 www.philipmrussell.co.uk