Properties of Liquids

Properties of liquids can be explained in terms of molecular arrangement and inter-molecular forces.

Volume and Shape

The liquids (for example water) take the shape of the container in which they are kept. However, they have a definite volume. In liquids, the attractive forces are strong enough to keep the molecules moving within a definite boundary. Thus, they maintain a definite volume. These inter-molecular forces are not strong enough to keep them in definite positions. Therefore, the molecules can move around and take the shape of the container in which they are kept.

Compressibility

Compressibility of a substance is its ability to be squeezed when a force is applied on it.

Liquids are largely incompressible. It is because there is very little empty space between the molecules. In contrast, the gases are highly compressible because of large empty spaces between their molecules.

The large difference in the free space in gaseous and liquid states becomes evident from the fact that the volume occupied by a given amount of a substance in liquid state is 100-1000 times less than that in the gaseous state.

Diffusion

Diffusion is the process of spreading of a substance from a region of higher concentration to a region of lower concentration.

Diffusion occurs in liquids because the molecules of the liquids are moving and help in the diffusion process.

Evaporation

Evaporation is the process by which a liquid changes into vapor. It occurs at all temperatures from freezing point to boiling point of the liquid.

In a liquid, at any temperature, a small fraction of the molecules is moving with relatively high velocity. Such molecules have high kinetic energy. These can overcome the inter-molecular attractive forces and escape through the surface of the liquid.

Rate of evaporation of a liquid depends on a number of factors. For example, more is the surface area, faster will be the evaporation. For faster drying, you increase the surface area by spreading the wet clothes. If you supply heat to the liquid, evaporation is faster. The wet clothes dry faster in the sun. The increase in temperature increases the kinetic energy of the molecules of the liquid and the liquid evaporates at a faster rate.

The number of molecules escaping from a liquid depends upon the attractive forces. When these forces are stronger, fewer molecule escape. In alcohol, these attractive forces are weaker than those in the water. Hence, alcohol evaporates faster than water.