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Effect of Change of Pressure on State Change of Matter

Liquid which ...

Question

Liquid which decompose below its normal boiling point can be distilled at lower temperature by:

A

increasing the pressure

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B

decreasing the pressure

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C

heating in water bath

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D

heating on sand bath

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Solution

The correct option is D decreasing the pressure
Suon liquids are made to boil at a temp below than their normal boiling joint by reducing pressure on their surface. A water pump or vacuum pump is used to reduce pressure. This method is used for the purification of glycerol from spent lye in soap industry.

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Similar questions

H_{2} O_{2}

decomposes below its boiling point if it is distilled at ordinary pressure.

Q. Assertion :Glycerol is purified by distillation under reduced pressure. Reason: Method of distillation under reduced pressure is used to purify liquids having very high boiling points and those, which decompose at or below their boiling point.

Q. Match the following.

(A) Simple distillation	(i) To separate the liquids which are steam volatile, insoluble in water and contains non-volatile impurities.
(B) Fractional distillation	(ii) To separate liquids which decompose at a temperature below their normal boiling points
(C) Vacuum distillation	(iii) To separate two or more liquids which have boiling points close to each other.
(D) Steam distillation	(iv) To separate liquid from non-volatile impurities.

Comprehension # 2

Following passage explains the effect of temperature on the vapour pressure of the liquid.

Effect of temperature on vapour pressure

The quantity of heat required to evaporate a given liquid at constant temperature is defined as the heat of vaporisation. Variation of vapour pressure with temperature is given by,

Clausius-Clapeyron equation.

l o g_{e} P = - \frac{Δ H_{v a p}}{R T} + l o g_{e} A

A liquid is said to be at its boiling temperature if its vapour pressure is equal to external pressure. Therefore, the boiling point of water in particular and of liquids in general decreases as an altitude of a place increases where the external pressure is less than

1 a t m o s p h e r e

(normal b.p. of water is

373.15 K

1 a t m o s p h e r e

On top of mount everest, for example, where the atmospheric pressure is only about

260 m m H g

, water boils at approximately

71^{\circ} C

. Conversely, if the external pressure on a liquid is greater than

1 a t m

, the vapour pressure necessary for boiling than normal boiling is reached later, and the liquid boils at a temperature greater than the normal boiling point.

Clausius-Clapeyron equation can be written in the following form:

Q. Which liquid among water or alcohol, has a lower boiling point (and boils first) and can be used in distillation?

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