1.2    Availability of water
    Water is available in plenty on earth. More than three-fourth of the earth’s surface is covered with water in the form of seas, rivers and lakes. It is also found inside the earth’s crust Most of the water that we get from the wells comes from this source.
    About the 71 % of the earth’s surface is covered with water. 

1.3    Physical properties of water 
        Water possesses certain significant physical properties which are responsible for making it a vital liquid.
(i)    Nature :  It is a colourless, odourless and tasteless liquid.
(ii)    Freezing point :  Water freezes to ice at 0°C under normal atmospheric pressure that is 76 cm of Hg.
(iii)    Boiling point : Water gets converted to steam at 100°C under normal atmospheric pressure that is 76 cm of Hg. The boiling point of water is proportional to external pressure.
(iv)    Density : In general, solids possess greater densities than liquids. Therefore, for such liquids, the density decreases with an increase in temperature as the liquids expand on heating. However, in the case of water, it is different. Ice has lower density than water at the same temperature. As ice gets converted to water at 0°C its density increases and this trend continues till the temperature rises to 4°C. Beyond 4°C, water shows normal trend of decrease in density with increase in temperature. Therefore, density of water is maximum at 4°C (1 g /cc). From 4°C to 0°C, water undergoes expansion while all other liquids undergo contraction. Since this trend is opposite to the normal trend, this is called anomalous expansion of water.

(v)    Specific heat : The amount of heat energy required to raise the temperature of unit mass of a substance through 1°C is called the specific heat capacity of that substance. Specific heat capacity of pure water is 1 calorie / g / °C. Among all substances, water has the highest specific heat capacity.
(vi)    Latent heat : When a substance changes from one state to the other, certain amount of heat is required to overcome the intermolecular force of attraction. For instance, when ice of certain mass at 0°C is converted to water of same mass at 0°C, the heat supplied brings about conversion of state without change in temperature. This is stored as potential energy in the water molecules and is called latent heat of fusion and is equal to 80 cal/g.
Similarly, the conversion of 1 g of water to steam at 100°C requires 540 cal heat and thus called as latent heat of vapourization.
At any temperature between 0°C and 100°C, conversion of water to water vapour takes place rather slowly. This process is called evaporation. The rate of evaporation increases with increase in temperature.
(vii)    Conductivity :  Pure water is a poor conductor of electricity. However, the presence of dissolved salts renders normal water a good conductor.
(viii)    Solvent property : Water can dissolve most of the substances in it. Hence it is known as universal solvent.            

1.4    Composition of water
        Water is essential for life. Water is made up of hydrogen (H2) and oxygen (O2) elements.
        The molecular formula for water is H2O.The ratio between hydrogen and oxygen in water is 1 : 8 by mass.  

 

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