Explain the following reasons

cbse
statesofmatter

#1

Explain the following.
(i) The boiling point of a liquid rises on increasing pressure.
(ii) Drops of liquids assume spherical shape.
(iii) The boiling point of water (373 K) is abnormally high when compared to that of ${ H }_{ 2 }$S (211.2 K).
(iv) The level of mercury in a capillary tube is lower than the level outside when a capillary tube is inserted in the mercury.
(v) Tea or coffee is sipped from a saucer when it is quite hot.


#2

(i) A liquid boils when its vapour pressure becomes equal to the atmospheric pressure. An increase in pressure on liquid, therefore, causes a rise in the boiling temperature of the liquids.
(it) Liquids have a property, called surface tension, due to which liquids tend to contract (to decrease the surface area). For a given volume of a liquid, since a sphere has the least surface area, hence the liquids tend to form spherical droplets.
(iii) The extensive hydrogen bonding in water gives a polymeric structure. This makes the escape of molecules from the liquid more difficult.
Therefore, water requires higher temperature to bring its vapour pressure equal to the atmospheric pressure. On the other hand, sulphur being less electronegative, does not form hydrogen bonds with H of ${ H }_{ 2 }$S.

As a result, ${ H }_{ 2 }$S has low boiling point.
(iv) The cohesive forces in mercury are much stronger than the force of adhesion between glass and mercury. Therefore, mercury-glass contact angle is greater than 90°. As a result, the vertical component of the surface tension forces acts vertically downward, thereby lowering the level of mercury column in the capillary tube.
(v) Evaporation causes cooling and the rate of evaporation increases with an increase in the surface area. Since, saucer has a large surface area, hence tea/coffee taken in a saucer cools quickly.