C3.2.1 Hard and Soft Water
Hard water contains magnesium/calcium ions. There are two types of hard water: temporary and permanent.
Hard water is formed when rain water flows over rocks before entering resevoirs. This is because rainwater is naturally acidic as it contains carbonic acid, formed by the reaction between water and carbon dioxide in the atmosphere. When the rain falls, it flows over rocks, the magnesium/calcium ions from the rocks dissolve into the water. The presence of the calcium/magnesium ions makes the water hard.
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. When heated, temporary hard water produces scale (an unsoluble solid). Scale can become deposited in kettles, boilers and pipes, reducing the efficiency of these heating systems.
C3.2.2 Removing Hardness
Hard water can be softened by removing the dissolved calcium/magnesium ions that react with the soap.
Temporary hard water can be softened by boiling as it contains hydrogencarbonate ions, HCO3–, which decompose when heated, producing carbonate ions, water and carbon dioxide.
2HCO3– --> CO32–+ H2O + CO2
The carbonate ions react with the calcium/magnesium ions in the water to produce precipitates of calcium/magnesium carbonates that are deposited as scale removing all hardness from the water.
Permanent hard water cannot be softened by boiling because it contains dissolved sulfate ions, SO42–. These do not decompose when heated.
Either types of hard water can be softened in an ion exchange column- which is packed with resin containing sodium/hydrogen ions. When the water is passed through the column, the calcium/magnesium ions are replaced by the sodium/hydrogen ions, removing all hardness.
C3.2.3 Water Treatment
Drinking water should only contain suffieciently low levels of dissolved salts and microbes. Water from appropraite sources can be treated to make it suitable to drink. Water is treated by:
- Filtration: Solids in the water, such as leaves and soil, must be removed. The water is sprayed onto specially-prepared layers of sand and gravel called filter beds.
- Sedementation: The water is passed into a sedimentation tank. Aluminium sulfate is added to clump tiny particles together to make larger particles. The water is then passed through a fine filter, such as carbon granules, to remove very small particles.
- Disinfection: Chlorine is added to the water to kill all microbes and sterilise it.
Water filters are used to improve the taste of water. These often contain carbon and ion-exchange resin which removes some soluble substances to prevent bacteria growth.
Distillation produces pure water, but this requires large amounts of energy to boil the water, and so would be very expensive to do in a large scale.
C3.2.4 Water Issues
- Hard water causes difficulties with the efficiency of water heating appliances and washing
- The calcium compounds in hard water are good for developing strongs bones and teeth, and can also reduce the risk of heart disease
- Chlorine is particularly effective for killing microbes in water
- Chlorine is poisonous and can produce other toxic compounds
- Fluoride compounds are added to toothpaste and water to supplies to help prevent tooth decay
- Large amounts of fluoride can result in fluorosis
- People feel they should be able to choose whether they take extra fluoride or not