About 71% of the earth’s surface is covered with water. All living things need water to survive. Water exists in the air as water vapour and under the ground as ground water.
Properties of Water
(i) Pure water has no colour, taste or odour.
(ii) Water is a very good solvent as it dissolves many things. It is called the universal solvent.
(iii) Water can absorb a lot of heat. That is why water is valuable to industries. It is also used as a coolant in vehicles.
(iv) Water tends to clump in drops. That is why it can move through the roots and stem of plants and through the tiny blood vessels of our body.
(v) Water is the only natural substance that is found in all three states of matter-liquid (water), solid (ice) and gas (steam).
Water dissolves many things
Water dissolves many things in it. Half fill a test-tube with water and add two spoons of sugar to it. Repeat this activity with salt, sand, copper sulphate, gravel, tea leaves and talcum powder.
You will observe that some solids dissolve, in water. When the solute dissolves in water (solvent), a solution is formed. Which of the above solids do not dissolve in water?
Removal of Insoluble Impurities
Insoluble impurities present in water may be removed by following processes:
(1) Sedimentation and Decantation : Take a mixture of soil and water in a beaker. Stir it with a glass rod. Have the mixture to stand for a few minutes. The heavier soil particles settle down at the bottom of the beaker. Carefully pour this water into another beaker.
Sedimentation The process by which insoluble substances which are heavier than water are separated, is called sedimentation and decantation.
(2) Filtration : Make a cone of a circular filter paper as shown in the figure. Keep the cone inside a funnel. Keep a beaker below the funnel. Pour the mixture of soil and water on the filter paper by using a glass rod. Drops of pure water start trickling into the beaker kept under the funnel and collect as the filtrate. Soil filtration particles are left on the filter paper.
Removal of soluble impurities
Methods of removing soluble impurities from water are:
(1) Evaporation : Mix a little salt in a glass of water. Pour this into a shallow dish and heat the water. The salt will be left behind and the water will be evaporated.
(2) Distillation : In the process of distillation, water is first heated in a Mirtillation flask. The water evaporates and steam enters the condenser. The condenser is kept cool by the cold water circulating around it. When hot stream enters the cool condenser, the steam changes into water again. It is collected in another flask. This water is called distilled Distillation water and is very pure water.
Purification of Water
A supply of clean water for drinking, cooking and other human activities is necessary for all living beings. Impure water is harmful for health as it carries disease causing germs of cholera, jaundice and dysentry. In large towns, water is purified and filtered in the public water supply before being piped for domestic use.
Underground water from deep springs and wells is usually free from insoluble impurities and is safe to drink.
Following are the methods which purify water and make it fit for drinking:
(1) Boiling water for at least ten minutes is the easiest and best way to make a small quantity of water safe for drinking. Boiled water should be strained through a clean cloth and stored in a clean container.
(2) Use a water filter to store drinking water. Clean the filter regularly. Dirty filter is a breeding ground for disease causing germs.
(3) Potassium permaganate crystals should be added in wells for killing germs.
(4) For treating water in the town water supply, following methods are mainly used:
(i) Sedimentation : In large open tanks, collected water is left undisturbed for a few days. This way the heavier suspended particles sink to the bottom in the form of impurities. Air and sunlight kill many harmful germs. Alum is also used to purify the water.
(ii) Filtration : After sedimentation, water is filtered through a clean sandbed to remove finer suspended particles and bacteria.
(iii) Chlorination : Filtered water is then disinfected with a very small quantity of chlorine gas to kill the bacteria. This water is now safe for drinking.