Properties of water 18

1. Give reasons:

(a) Even when the water at the surface of a lake is frozen, aquatic animals continue to live below the ice.

ANS: (1) The density of water has a special characteristic, Normally, as the temperature of a substance rises, its density decreases, which means that its volume increases.

(2) As the temperature of water rises above 0°C up to 4°C its density increases. It is maximum at 4°C. After that, like other liquids, its density decreases with rising temperature. This property of water is called the anomalous expansion of water.

(3) The surface layer in the reservoir freezes at 0°C. But the temperature of the water below being at about 4°C has a greater density also does not freeze.

(4) Hence, that water stays down and the aquatic animals in the reservoir can survive in the water.

(b) It is easier to swim in sea water than in ordinary water.

Ans: (1) The quantity of salts in the sea goes on increasing as water flows, salts from the soil dissolve in it. At the same time water in the sea is continually evaporating.

(2) This results in the increase in the concentration of salt in the water.

(3) It is about 35 grams per litre.

(4) Hence, the density of sea water is more than that of ordinary water. That makes it easier to swim in sea water.

(c) Water is said to be a universal solvent.

Ans. (1) Chemically, water is a neutral substance. (2) This property makes water an ideal medium for most chemical reactions. (3) Many substances dissolve in water. (4) Hence water is said to be a universal solvent.

(d) Water is an ideal medium for chemical reactions.

Ans. (1) Water is colourless tasteless and odourless. (2) It is transparent and liquid at normal temperature. (3) Many substances dissolve in water. (4) Chemically water is neutral. therefore, water is an ideal medium for chemical reactions.

2. Fill in the blanks:

(1) The density of water is 1 g/cc.

(2) A substance that dissolves in a liquid is called a solute.

(3) When a solute and a solvent mix in each other completely we get a solution.

2. Match the columns:

(a) Anomalous expansion ------- 2. Water

(b) Density -------------------------- 1. Mass/ volume

(c) Dead Sea ------------------------ 4. 25%o salts

(d) Water ---------------------------- 3. Compound