Chemistry-Water(C3.2)

• Hard water contains dissolved compounds, usually of calcium or magnesium.
• There are two types of hard water.
• Permanent hard water remains hard when it is boiled.
• Temporary hard water is softened by boiling.
• Temporary hard water contains hydrogencarbonate ions (HCO₃^-) that decompose on heating to produce carbonate ions which react with calcium and magnesium ions to form precipitates.
•  Using hard water can increase costs because more soap is needed.
• When temporary hard water is heated it can produce scale that reduces the efficiency of heating systems and kettles.
• Hard water has some benefits because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart disease.
• Hard water can be made soft by removing the dissolved calcium and magnesium ions
• Done by adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate

• Rainwater is naturally weakly acidic 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 from the rocks dissolve into the water.

•  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)

• Soft water 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, so more soap is needed. Soapless detergents do not form scum with hard water.