chemistry- water

• hard water contains dissolved compounds such as calcium and magnesium salt 
• the calcium and/or magnesium ions in hard water reaacts with soap producing a precipitate called scum 
• temporary hard water can produces a solid scale when it is heated, reducing the effiency of heating systems and kettles
• hard water is better than soft water for developing and maintaining teeth and bones.
• hard water does not produce lather like soft water 

• soft water does not contain salts that produce scum or scale 
• hard water can be made soft by removing the dissolved calcium and magnesium ions that react with soap 
• some types of hard water are affected by heating while other are not. 
• temporary hard  water is softened by boiling because when it is heated the calcium and magnesium compounds form insoluble scale and this removes them from the water. 
• permanent hard water is not softened by boiling and does not produce scale when heated. 

One method of softening either type of hard water is by adding washing soda, which is sodium carbonate. the sodium carbonate reacts with the calcium and magnesium ions in water to form solid calcium carbonate and magnesium carbonate that cannot react with soap 

Another method is to use an ion-exchange column pack with  resin containing sodium or hydrogen ions. when hard water is passed through the resin, calcium and magnesium ions become attached to the resin and the sodium or hydrogen ions take their place in the water. 

Water that has been contaminated with certain types of microorganisms can be very unsafe. It can cause diseases such as typhoid, cholera and dysentery.

Treating water with chlorine kills microorganisms that may exist in domestic water supplies. Chlorination of water began in the early part of the twentieth century, having a dramatic effect of reducing many waterborne diseases and so increasing public health.

There may be disadvantages of consuming water that has been treated with chlorine. When water is treated with chlorine some disinfection byproducts called tirhalomethanes (THMs) can form. They form when chlorine reacts with naturally occurring organic matter such as leaves.

There is a notion that THMs can cause cancer, although no firm evidence has been found to support this theory.

Distillation of sea water

In principle, distillation can be used to make large volumes of fresh water from sea water. However, it takes a lot of energy to boil water for this process. This would make the fresh water produced much more expensive than water from traditional sources. In addition, there are problems disposing of the salt-rich waste water left over by the process. However, distillation is used in hot countries such as Saudi Arabia.

water treatment- 

• Get water from source
• Filter to remove large debris (sticks etc).  
• Several stages of filtration to remove increasingly smaller imprities such as algae/insects
• Add coagulant – makes other impurities stick together
• Filter through fine sand to remove last remaining impurities
• Disinfect by adding Cl