The physical state of a solid substance can be changed by simple heating. Heat increases the kinetic energy of the particles and they start vibrating at a higher frequency. At a particular temperature their vibrational motion becomes so fast that it overcomes the cohesive forces. As a result the solid starts melting and this temperature is called the melting point of the solid. At the melting point, the particles of a solid not only lose their mean positions but the arrangement as well. The solid collapses and turns to a liquid.
Heating a solid below its melting point increases its kinetic energy and temperature that weakens the force of attraction between its particles. Further heating the solid after it has started melting does not increase its temperature. Instead, all the heat energy provided at this moment is utilized to convert the solid into its liquid.
Figure 14.1: Melting of ice — solid to liquid transition.
The molecules of a liquid keep on coming out from the surface of a liquid at all temperatures and such an escape of molecules from surface is called Evaporation. Heating a liquid increases the kinetic energy of its molecules and so does the process of evaporation. Heating the liquid further, increases the kinetic energy so much that the inter-particle forces are weakened to a large extent. At this point the bubbles start coming out of the liquid at a rapid pace. At this stage if the vapour pressure of the liquid becomes equal to the external pressure, the liquid starts boiling. The temperature of a liquid at which its vapour pressure becomes equal to the atmospheric or external pressure is called its boiling point.
At the boiling point, the heat provided to the liquid is used to convert it into gaseous form and during this its temperature remains constant. In other words, the heat provided is used only to break the forces of attraction between its particles.
When a gas is cooled, the kinetic energy of its molecules decreases, as a result the molecules come closer with a significant force of attraction between them. At a suitable lower temperature the increased attractions bring the molecules so close that they are changed into a liquid form. This is called condensation. During this transition the temperature of the gas remains constant until all the gas is changed into its liquid.
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