Uses of chlorine and chlorate(I): The use of chlorine in water treatment.

The idea

Chlorine is added to drinking water and to swimming pools to kill bacteria and other micro-organisms, which is why water-borne diseases such as cholera and typhoid are rare where supplies are treated. The chlorine is not itself the direct agent: it reacts with water, Cl₂ + H₂O ⇌ HClO + HCl, and the chloric(I) acid or chlorate(I) ion oxidises and destroys the organisms. A small residual concentration is deliberately left in the water so that it stays protected all the way through the pipework to the tap. The amount added must be carefully controlled, because chlorine is itself toxic and in excess would poison the people drinking it. The concentration used is only a few parts per million.

What earns the marks

• Stating that chlorine kills bacteria or micro-organisms in the water.
• Naming a disease such as cholera or typhoid that is prevented.
• Stating that the amount added must be controlled because chlorine is toxic or poisonous.
• Identifying chloric(I) acid or the chlorate(I) ion as the species that kills the organisms.
• Stating that a residual amount keeps the water safe along the pipework.

Common mistakes and how to put them right

• Saying chlorine removes dirt or makes the water clear rather than killing bacteria.

• Stating that all the chlorine is removed before the water reaches the tap.

• Giving the smell or taste of chlorine as the reason for controlling the dose, rather than its toxicity.

• Confusing the role of chlorine with that of fluoride.

• Naming chloride ions as the species that kills the bacteria.

Exam tips

• 'Toxic' or 'poisonous' is the key word for the dose-control mark; vaguer answers about taste do not score.
• Name a specific disease; 'stops you getting ill' is not enough for the mark.