In which of the following process entropy increases?
a) A liquid evaporates to vapour.
b) Temperature of a crystalline solid lowered from 115 K to 0 K.
c) `CaCO_(3(s))toCaO_((s))+CO_(2(g))`
d) `Cl_(2(g))to 2Cl_((g))`

Explanation

a) A liquid evaporates vapour: During the vapourisation, the liquid absorbs heat at constant temperature. In this process the liquid state converts into gaseous state. In the liquid state the particles are close to one another and somewhat the order of the particles is more. In gaseous state the order of the particles is less and disorder increases due to the random motion of gaseous particles. So entropy increases during the vapourisation of a liquid.
b) Temperature of a crystalline solid lowered from 115 K to 0 K : In a crystalline solid the particles are arranged in an orderly manner. Due to decrease in temperature there will be no change in the order but due to decrease in vibrational energies the entropy decreases.
c) `CaCO_(3(s))toCaO_((s))+CO_(2(g))` : Here on of the product is gas in which randomness increases than in the solid reactant. So entropy increases.
d) `Cl_(2(g))to2Cl_((g))` : Here both are gases but one `Cl_(2)` molecular converts into `2Cl` atoms due to which randomness increases. So entropy increases.