Chlorine in Water Treatment Revision Notes | A Level AQA Chemistry

1. Why Chlorine in Water Treatment Matters

Before chlorination became widespread in the early twentieth century, waterborne diseases such as cholera and typhoid killed thousands of people every year in the UK. Adding chlorine to drinking water is one of the most consequential public health decisions ever made — and it is also one of the most chemically interesting.

This topic sits at the intersection of Group 7 chemistry and real-world decision-making. You need to know the chemistry of what chlorine does in water, and you also need to be able to evaluate the societal argument: are the benefits worth the risks? AQA expects both the chemical detail and the evaluative reasoning.

2. The Chemistry of Chlorine in Water

When chlorine gas dissolves in water, it does not remain as $Cl_2$. Instead, it reacts with water molecules to produce two acids. This reaction is the chemical foundation of the entire water treatment process.

Notice what has happened to chlorine's oxidation state. In $Cl_2$, chlorine is in the 0 oxidation state. In $HCl$, it is $-1$; in $HOCl$, it is $+1$. The same element has been both oxidised (to $+1$) and reduced (to $-1$) in the same reaction. This is a disproportionation reaction.

The species that does the bactericidal work is $HOCl$. It is a powerful oxidising agent that penetrates bacterial cell walls and destroys the enzymes bacteria need to survive. $HCl$ is a strong acid that fully dissociates, contributing $H^+$ and $Cl^-$ ions to solution, but it is the $HOCl$ that kills pathogens.

3. Chlorine Reacting with Aqueous Alkali

The reaction of chlorine with water is reversible and only partial. To drive the reaction further and produce a more stable solution, chlorine is reacted with aqueous sodium hydroxide — a cold, dilute alkali. This is the chemistry behind the manufacture of bleach, and it is also relevant to understanding how water treatment plants control pH.

The ionic equation strips away the spectator sodium ions and shows the chemistry more clearly. The $OH^-$ ions from the alkali remove the $H^+$ produced in the water reaction, driving the equilibrium to the right and ensuring a more complete conversion to $ClO^-$.

Worked example: Verifying Disproportionation in the Chlorine–Alkali Reaction

Show that the reaction $Cl_2(aq) + 2NaOH(aq) \rightarrow NaCl(aq) + NaOCl(aq) + H_2O(l)$ is a disproportionation reaction by assigning oxidation states.

1. $Cl_2$: each Cl atom is 0 (element in standard state) — Any element in its standard state has an oxidation state of 0.
2. Na is $+1$, so Cl must be $-1$ to give overall charge of 0. — Chlorine has been reduced from 0 to $-1$: it has gained an electron.
3. Na is $+1$, O is $-2$, so Cl must be $+1$ to give overall charge of 0. — Chlorine has been oxidised from 0 to $+1$: it has lost an electron.
4. Cl goes from 0 to $-1$ (reduction) AND from 0 to $+1$ (oxidation) in the same reaction. — The same element is simultaneously oxidised and reduced — this is the definition of disproportionation.

Answer: Disproportionation confirmed: Cl is oxidised to $+1$ in $NaOCl$ and reduced to $-1$ in $NaCl$, both from an initial oxidation state of 0 in $Cl_2$.

4. Balancing the Equations — A Closer Look

AQA mark schemes reward precise equation writing. Both reactions above need to be written with correct formulae, balanced coefficients, and state symbols. It is worth pausing to check each equation carefully before moving on to the societal argument.

One detail that trips students up: the reaction with water uses a reversible arrow ($\rightleftharpoons$) because the equilibrium lies to the left under normal conditions. The reaction with sodium hydroxide uses a one-way arrow because the alkali removes $H^+$ ions, preventing the back reaction.

5. Societal Decision-Making: Benefits vs Risks

The chemistry tells us what chlorine does. The harder question — and the one AQA explicitly tests — is whether society should add it to drinking water at all. This requires weighing evidence, not just recalling facts.

The case for chlorination is compelling. Waterborne diseases such as cholera, typhoid, and dysentery are caused by bacteria and other pathogens that thrive in untreated water. Chlorination kills these pathogens reliably and cheaply. In countries where water chlorination is standard practice, deaths from waterborne disease are rare; in countries without it, they remain a leading cause of mortality, particularly in children.

The case against is not without substance. Chlorine is a toxic gas, and at high concentrations it is lethal — it was used as a chemical weapon in the First World War. When chlorine reacts with organic matter naturally present in water, it can form a group of compounds called trihalomethanes (THMs), such as chloroform ($CHCl_3$). Some THMs are suspected carcinogens at high exposure levels. There are also concerns about the taste and odour that residual chlorine gives to tap water.

However, the concentrations used in water treatment are carefully regulated and far below levels that cause harm. The World Health Organisation and UK Drinking Water Inspectorate set strict limits on both residual chlorine and THM concentrations. The scientific consensus, supported by decades of epidemiological data, is that the public health benefit of eliminating waterborne disease vastly outweighs the very small risk from trace levels of chlorine or THMs.

6. Exam Technique and Mark-Winning Responses

The three specification points for this topic map onto three distinct types of exam question. Knowing which type you are facing determines how you structure your answer.

For 'evaluate' questions, the structure that consistently wins marks is: benefit (with chemical or epidemiological evidence) → risk (with specific detail, e.g. THM formation) → conclusion that explicitly weighs the two sides. A conclusion that says 'the benefits outweigh the risks because the concentrations used are regulated and the reduction in waterborne disease is well-documented' is far stronger than a vague 'chlorine is good for water'.

For equation questions, the two most common errors are using the wrong arrow type and omitting state symbols. Practise writing both equations from memory, checking the arrow and state symbols every time. The coefficient of 2 in front of $NaOH$ in the alkali reaction is another frequent slip — two moles of $NaOH$ are required per mole of $Cl_2$ because the reaction produces one mole each of $NaCl$ and $NaOCl$, each requiring one $OH^-$ ion to form, giving two $OH^-$ ions overall.

Keep the chemistry and application linked: chlorine in water treatment matters because the equilibrium between Cl2, HOCl and OCl- determines how much effective disinfectant is present under the chosen conditions.