Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!

Home > Science > AQA Triple Science Topics > Water > Hard and soft water


Hard and soft water


Water in different parts of the UK varies in the amount of dissolved mineral ions it contains. This determines whether it is hard or soft water. There are advantages and disadvantages to each, but the damage that can be caused to water pipes and appliances by hard water means that water may need to be softened.

Hard and soft water

Rainwater is naturally weakly acidic [acidic: Having the properties of an acid, such as a pH below 7. because it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks or soaks into the ground and then passes through layers of rock. Compounds [compound: A substance formed by the chemical union (involving bond formation) of two or more elements. from the rocks dissolve into the water.

Hard water [hard water: Water, containing magnesium ions and calcium ions, that does not easily form lather with soap. contains dissolved compounds, usually calcium or magnesium compounds. For example, limestone contains calcium carbonate, CaCO3. Carbonic acid in rainwater reacts with this to produce soluble calcium hydrogencarbonate:

carbonic acid + calcium carbonate → calcium hydrogencarbonate

H2CO3(aq) + CaCO3(s) → Ca(HCO3)2(aq)

The presence of calcium ions [ion: The charged particle formed when an atom, or a group of atoms, lose or gain electrons. Ion charge helps determine a substance's acidity or alkalinity. and magnesium ions in the water makes it hard. Soft water [soft water: Water that readily forms lather with soap. readily forms lather with soap, but it is more difficult to form lather with hard water. The dissolved calcium ions and magnesium ions in hard water react with the soap to form scum [scum: An insoluble precipitate that forms with soap and hard water., so more soap is needed. Soapless detergents do not form scum with hard water.

The types of rocks found in different regions determines how hard or soft the water will be.

The water in some parts of the country is soft because it has low levels of dissolved calcium and magnesium compounds, while the water in other parts of the country is hard because it has higher levels of dissolved calcium and magnesium compounds.

There are varying degrees of hardness in water - from slightly hard to very hard.

Map of water softness in England and Wales. The softest water is found in Cumbria, west Wales and Cornwall. The south and east of England have the hardest

Water softness in England and Wales



boy smiling wearing headphones

Science Audio Bites

Put down that pen and listen to some Science audio.

More audio


Mia Cadaver illustration

Mia Cadaver's Tombstone Timeout

Revision topics from beyond the grave!

More games

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.