Potable water: Water of appropriate quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes. Water that is safe to drink is called potable water. Potable water is not pure water in the chemical sense because it contains dissolved substances.

The idea

All living organisms need water, but the quality required differs. For humans, drinking water must contain sufficiently low levels of dissolved salts and microbes so that it is safe to drink; this is called potable water. Potable water is not pure water in the chemical sense, because it still contains dissolved substances such as mineral ions. In chemistry, a pure substance contains only one substance, so potable water is a mixture. Water sources include fresh water from rivers, lakes and aquifers, and seawater. Fresh water is usually treated by filtration to remove insoluble particles, then sterilisation using chlorine, ozone or ultraviolet light to kill microbes. Seawater or salty water requires desalination, for example by distillation or reverse osmosis, which removes dissolved salts but uses large amounts of energy.

What earns the marks

• State that water of appropriate quality is essential for life and that humans need drinking water with sufficiently low levels of dissolved salts and microbes.
• Define potable water as water that is safe to drink, distinguishing it from chemically pure water.
• Explain that potable water is not pure in the chemical sense because it contains dissolved substances, such as mineral ions.
• Describe a treatment method for fresh water, such as filtration to remove insoluble particles followed by sterilisation with chlorine, ozone or ultraviolet light.
• Describe desalination of salty water or seawater by distillation or reverse osmosis, and note that it requires significant energy.

Common mistakes and how to put them right

• Confusing potable water with pure water.

  Put it right: Potable water is safe to drink but still contains dissolved substances, so it is a mixture, not a pure substance.

• Thinking that filtration alone makes water potable.

  Put it right: Filtration removes insoluble particles, but sterilisation is also needed to kill microbes.

• Believing that all water sources are treated in the same way.

  Put it right: Fresh water is usually filtered and sterilised, while salty water or seawater needs desalination.

Exam tips

• Use the term 'potable water' precisely and define it as safe to drink, not as chemically pure.
• When describing treatment, name both the process and its purpose, for example filtration to remove insoluble particles.
• Link desalination to its high energy cost when discussing why it is used only when other sources are limited.