1. Water CHEMISTRY 2 Additional Science Chapter 8

2. The Importance of Water Water is essential to life on Earth Water is used widely as a solvent in industrial processes Water is used widely as a coolant in industrial processes 75% of the worlds surface is covered by water Additional Science Chapter 8

3. The public water supply Water from river, lake or underground well stored in a reservoir Solids allowed to settle Filtration to remove smaller particles Cl 17 35 Chlorination to kill bacteria Storage in tank or water tower before use by schools. Factories etc. This is the process that takes place to ensure that we have a supply of drinking water. Some water authorities add other chemicals to drinking water e.g. many add fluoride ions which prevent tooth decay at controlled levels. Additional Science Chapter 8

4. Hard water As rain water flows through rocks, it picks up calcium and magnesium ions. The result is - hard water. Hard water forms scale (calcium carbonate) in pipes, and restricts water flow. Hard water doesnt produce soap suds easily. This fact can be used to prove for water hardness, and to compare the hardness of different samples. Hard water mixes with soap to form calcium or magnesium stearate which is insoluble – this is why soap scum forms. Additional Science Chapter 8

5. Types of hardness Reason:- Presence of calcium or magnesium carbonate in the water Temporary hardness Solution:- Boiling the water. The hydrocarbonates decompose, leaving insoluble calcium carbonate or magnesium carbonate – this is the scale found in kettles and boilers. Equation:- Ca(HCO 3 ) 2 Ca CO 3 + CO 2 + H 2 O Permanent hardness Reason:- Presence of calcium or magnesium sulphate in the water. Solution:- Soften the water by adding sodium carbonate or by ion exchange. Equation:- Na 2 CO 3 + CaSO 4 Ca CO 3 + Na 2 SO 4 Additional Science Chapter 8

6. Hard water Advantages Disadvantages It produces boiler scale Uses more energy to boil the water More soap has to be used, resulting in soap scum in the wash. Modern detergents are used instead of soap in washing machines. Removing boiler scale is expensive Reduces the efficiency of kettles, boilers, coffee machines etc. Hard water tastes better than soft water. Beer production often involves artificially hardening the water. Additional Science Chapter 8

7. Solubility curves water sugar saturated solution. The sugar doesnt dissolve in the water any longer – you have a saturated solution. Terms Solute – the substance that dissolves (sugar). Solvent– the liquid doing the dissolving (water). Solubility – mass of solute that dissolves in a certain mass of solvent. Usually the solubility of a solute increases with an increase in temperature potassium chlorate sodium chloride Temperature °CSolubility/ g per 100g of water Solubility curve Can you think which solutes solubility is most affected? Additional Science Chapter 8