Students should be able to: distinguish between potable water and pure…

The idea

Potable water is water that is safe to drink; it contains dissolved substances but has sufficiently low levels of microbes and contaminants. Pure water, in a chemistry context, contains only H₂O molecules and no dissolved substances. Ground water is usually treated by filtration to remove insoluble particles and sterilisation, often by chlorination, to kill microbes. Salty water needs desalination, for example by distillation or reverse osmosis, because dissolved salts must be removed. Each step has a purpose: filtration removes solids, chlorination kills microorganisms, distillation boils and condenses water leaving salts behind, and reverse osmosis forces water through a membrane that retains ions. Understanding these differences allows students to select and justify treatment methods for a given water source.

What earns the marks

• Defines potable water as water that is safe to drink and pure water as water containing only H₂O molecules.
• States that potable water may contain dissolved substances but must have low levels of microbes and harmful contaminants.
• Describes ground water treatment as filtration followed by sterilisation, such as chlorination.
• Describes salty water treatment as desalination by distillation or reverse osmosis.
• Explains that filtration removes insoluble particles and chlorination kills microorganisms.
• Explains that distillation removes dissolved salts by boiling and condensing water, while reverse osmosis uses pressure and a membrane.
• Gives reasons for each step in terms of the type of impurity being removed.

Common mistakes and how to put them right

• Confusing potable water with pure water.

  Put it right: Potable water is safe to drink but may contain dissolved substances, whereas pure water contains only H₂O molecules.

• Thinking that filtration removes dissolved salts.

  Put it right: Filtration removes insoluble particles, while dissolved salts require desalination.

• Believing that chlorination removes dirt.

  Put it right: Chlorination kills microorganisms, and filtration removes insoluble particles.

Exam tips

• When comparing ground water and salty water, name the specific treatment steps and the impurity each step removes.
• Use the term 'sterilisation' or 'chlorination' for killing microbes, and 'desalination' for removing salts.
• If asked to give reasons, link each step to the problem it solves, such as 'filtration removes solid particles that would make water cloudy or unsafe'.