CHEMISTRY [3 Mark Question Answer]

Air dissolved in water is biologically very important.

• Oxygen dissolved in water is used by marine life like fish for respiration, and thus, marine life is sustained.
• Aquatic plants make use of dissolved carbon dioxide in photosynthesis to prepare food.
• Carbon dioxide dissolved in water reacts with calcium carbonate to form calcium bicarbonate.

Marine organisms such as oysters and snails extract calcium carbonate from calcium bicarbonate to build their shells.

If hard water is used, calcium and magnesium ions of the water combine with the negative ions of the soap to form a slimy precipitate of insoluble calcium and magnesium usually called soap curd (scum).
Formation of soap curd will go on as long as calcium and magnesium ions are present. Till then, no soap lather will be formed and cleaning of clothes or body will not be possible. Moreover, these precipitates are difficult to wash from fabrics and sometimes form rusty spots if iron salts are present in water.

Steam is usually made in boilers which are made of a number of narrow copper tubes surrounded by fire. As the cold water enters these tubes, it is immediately changed into steam, while the dissolved solids incapable of changing into vapour deposit on the inner walls of the tubes. This goes on and makes the bore of the tubes narrower. The result is that less water flows through the tubes at one time and less steam is produced. When the bore of the tube becomes very narrow, the pressure of the steam increases so much that at times the boiler bursts.

Advantages of hard water:
Water free from dissolved salts has a very flat taste. The presence of salts in hard water makes it tasty. So, hard water is used in making beverages and wines.
Calcium and magnesium salts present in small amounts in hard water are essential for bone and teeth development.
Hard water checks the poisoning of water by lead pipes. When these pipes are used for carrying water, some lead salts dissolve in water to make it poisonous. Calcium sulphate present in hard water forms insoluble lead sulphate in the form of a layer inside the lead pipe and this checks lead poisoning.

Advantages of soft water:
When the water is soft, you use much less soap and fewer cleaning products. Your budget will reflect your savings.
Plumbing will last longer. Soft water is low in mineral content and therefore does not leave deposits in the pipes.
Clothes last longer and remain bright longer if they are washed in soft water.