Which of the following is not true with reference to evaporation of liquid?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The evaporation rate follows the order of Water > Alcohol > Ether.
In evaporation, molecules at the surface of a liquid escape into the vapour when they have enough energy to overcome the attractive forces holding them in the liquid. The rate rises when these forces are weaker, when more molecules have high energy (higher temperature) and when more molecules are at the surface (larger area).
Option (1) The liquids having low inter-molecular forces evaporate faster. — True about evaporation. Weaker forces mean less energy is needed for a molecule to escape from the surface.
Option (2) The higher the temperature, the higher is the rate of evaporation. — True about evaporation. At a higher temperature a larger fraction of molecules has enough kinetic energy to escape.
Option (3) Larger the surface area, larger is the rate of evaporation. — True about evaporation. Evaporation happens only at the surface, so more surface exposes more molecules.
Option (4) The evaporation rate follows the order of Water > Alcohol > Ether. — Not true about evaporation. Diethyl ether has only weak dipole–dipole and dispersion forces; ethanol molecules are held by hydrogen bonds through one –OH group each; water molecules form a more extensive network of hydrogen bonds. The forces increase from ether to water (boiling points: ether 308 K, ethanol 351 K, water 373 K), so the rate of evaporation is Ether > Alcohol > Water, the reverse of the printed order.