1. Write the molecular formula of water?

        The molecular formula of water is H₂O, relative molecular mass - 18 amu. One molecule of water contains two atoms of hydrogen and one molecule of oxygen.

2. List out the importance of water in our daily life?

         The important uses of water in our daily life are

• For everyday uses like cleaning, bathing and washing
• For drinking to satisfy our biological needs
• For growing crops
• For various manufacturing processes
• For generating electricity
• For providing nutrients to aquatic plants and animals
• It also helps to control the earth's climate

3. What are the sources of water?

        Water is widely distributed on the earth. It covers nearly three - fourths of the earth's surface. Water exists in three states i.e., as solid ice, as liquid water and as water vapour.

      Depending on its source and its degree of impurity, natural water is classified into the following types:

• Ocean and Sea water
• River and lake water
• Spring and well water
• Rain water

4. Explain the cyclic process water cycle?

      Water is a substance that is capable of existing in all the three states of matter. From one state it changes into another rather easily.

         The change of water from one state to another occurs all the time in nature, too. This process of interconversion helps in balancing the amount of water both in the atmosphere and on the earth's surface, and is known as the water cycle.

      " The change of water from one state to another in nature takes place all the time. It involves the process of evaporation, condensation and precipitation (rainfall). This repeated process is known as the water cycle".

5. Why do we say that sea water cannot be used for human consumption?

        Water is necessary for all living beings. We cannot stay alive without water.

       The earth's surface contains a very large amount of water. About 75% of the earth's surface is covered with oceans and sea. They contain 97% of the earth's total water resources.      This water has a very large amount of salts dissolved in it. Therefore, sea water cannot be used for human consumption.

6. What are the types of impurities found in natural water?

      The impurities found in our natural water are

• Dissolved impurities
• Suspended impurities
• Germs (Bacteria)

7. What is mean by potable or drinking water?

            Water that is suitable for human consumption is called potable water or drinking water.

8. What are the characteristics of potable water?

         The main characteristics of potable water are

• It should be colourless and odourless
• It should be clear and transparent
• It should be free from harmful micro organisms
• Potable water should contain the minerals necessary for our body

9. What is meant by saline water?

   The spring and well water contains a greater amount of salt (2%) than is tolerable for direct use. Such water is called saline water. It is unfit for both drinking and irrigation purposes.

10. Define the term sedimentation?

      The setting of suspended solid matter at the bottom of a liquid is called sedimentation; and the settled solid matter is called sediment.

11. Explain the process filteration?

       Filteration is the process by which a liquid with insoluble , suspended fine particles is allowed to pass through a filter. The insoluble resuide is left behind on the filter, while clear liquid is collected in another container as filterate.

12. What is sterilization?

     The process of the removal of germs from water is called sterilization.

13. What is tap water?

     Water, after the removal of suspended impurities and germs is collected in overhead tanks and then supplied for domestic use through pipes. Therefore the portable water so obtained is also called " tap water".

14. What are the household methods to get drinking water?

    The household methods to get drinking water are

• By boiling water
• By water purifiers made available by different companies
• By adding calculated amount of chlorine to the water stored in tanks on the roofs of house to kill the germs

15. What is distillation?

      The process of converting a liquid into vapour by heating and the subsequent condensation of the vapour back into a liquid is called " distillation".