Liquid State
Liquid is an intermediate state of matter between solid and gaseous state. Liquid exists before the melting point of the substance. Liquid is composed of molecules which are held with each other by the intermolecular force of attraction. The molecules of liquid can flow and diffuse in other liquid.

Evaporation: The molecules of liquid are held with each other by intermolecular force of attraction. The kinetic energy of molecules is balanced with intermolecular force of attraction. Intermolecular force of attraction tends to keep the molecules together while kinetic energy tends to eject the molecules from surface. When kinetic energy exceeds intermolecular force of attraction, the molecules escape from the liquid surface in the form of vapour. This phenomenon is known as evaporation.

Evaporation is spontaneous and natural process of escaping molecules from liquid surface in the form of vapour at any temperature due to its higher kinetic energy than that of intermolecular force of attraction. Evaporation is spontaneous and natural process. This process is accompanied by the lowering of free energy.

Evaporation is endothermic process: When temperature is increased, the kinetic energy of molecules also increases. So, more molecules escape from the liquid surface. That means rate of evaporation increases. For eg: lakes dry in summer faster than in winter.

Evaporation causes cooling: Evaporation causes cooling of liquid surface because liquid provides energy to the molecules of surface. So, kinetic energy of molecules of surface increases and evaporation takes place. This is one of the reasons why we feel cold when a drop of ether, alcohol or water on our skin. During summer, animals like dog enjoy cold by exposing their tongue outside their mouth. Similarly, the water stored in earthen pots remain cold during summer season.

Evaporation is surface phenomenon: Only the molecules which possess higher kinetic energy than that of intermolecular force of attraction escape from the surface of the liquid. If molecules in bulk, possess higher kinetic energy, it comes on the surface by jostling the mass of liquid and escape from the surface.

FACTORS AFFECTING EVAPORATION

Nature of liquid: Evaporation depends on the strength of the intermolecular force of attraction. Greater the strength of the intermolecular force of attraction, the probability of escaping molecules from the surface is less. Non-polar liquids evaporate faster than polar liquids. The presence of hydrogen bond decreases the rate of evaporation.

Temperature: Evaporation is endothermic process. So the rate of evaporation increases with presence in temperature.

Surface area: Rate of evaporation increases with increase in surface area.

Presence of impurities: Rate of evaporation increases if volatile impurities are present but rate of evaporation decreases if non-volatile impurities are present. Since non-volatile impurities occupies an area which decreases surface for evaporation.

Vapour Pressure: When liquid is heated in a closed vessel, liquid evaporates and with increase in temperature, rate of evaporation increases. So, number of molecules in the vapour phase starts to collide with each other and condensed back into a liquid. After a certain interval of time, the rate of evaporation and the rate of condensation becomes equal which is known as the dynamic equilibrium of physical state. The pressure exerted by a liquid molecule when the rate of evaporation and the rate of condensation becomes dynamically equilibrium at constant temperature is called vapour pressure.

Factors affecting vapour pressure

Nature of liquid: The vapour pressure of liquid depends on the strength of the intermolecular force of attraction. Greater the strength of the molecular force of attraction, less the rate of evaporation. So, vapour pressure decreases with increase in strength of intermolecular force of attraction.

Temperature: With increase in temperature, the rate of evaporation increases.

Surface area: With the increase in surface area, the rate of vapour pressure increases in the rate of evaporation.

Presence of non-volatile liquid: With the presence of non-volatile liquid, surface area decreases due to which the rate of vapour pressure decreases.

Boiling point: When temperature gradually increases, the vapour pressure of liquid increases. The temperature at which the vapour pressure of the liquid becomes equal to atmospheric pressure is called the boiling point. Similarly, the temperature at which the vapour pressure of the liquid becomes equal to one atmospheric pressure is called normal boiling point. When water is heated at 100degree Celsius, the vapour pressure of the liquid becomes one atmospheric pressure. So, the normal boiling point of water is 100degree Celsius. Rapid vaporization of liquid which occurs pressure of a liquid equals to atmospheric pressure is called boiling.

Differences between evaporation and boiling;