Chemical substances exist in different physical states – solid, liquid and gas. Let us take the example of H2O. It exists as water, ice and steam. We know that when it undergoes a change in state, it is only a physical change. Its chemical properties do not change; it is still the same H2O molecule. However, the rate at which the chemical reactions occur does change with the physical state. Thus, to understand the chemistry of the reactions, we must know how the behaviour of substances changes with the change in their physical states. For this, we must know how the inter-molecular forces and the thermal interactions affect the substance.
Inter-molecular Forces vs. Thermal Interactions
- Inter-molecular force is the attractive force acting between neighbouring molecules. Whereas thermal energy is the measure of the sum of the kinetic energy of the individual molecules and particles. So the inter-molecular force of attraction keeps the particles together while the thermal interactions (kinetic energy) make them move apart.
- Let us say that under standard conditions, a substance X is solid. At this stage, the inter-molecular forces dominate over the thermal energy of the particles. Thus, the molecules cling together and take up the solid state.
- When a substance is to be converted from its gaseous state to a solid state, its thermal energy (or temperature) has to be reduced. On reducing the thermal energy, the particles lose their kinetic energy and consequently, the molecules cling together marking the dominance of inter-molecular forces.
- Similarly, if a gas is to be liquefied, reducing its thermal energy by reducing the temperature can do what is necessary.
Thus, the existence of the different states of matter is nothing but a balance between its inter-molecular forces and the thermal interactions between the particles. The predominance of inter-molecular forces is the least in gases and most in the solids; the predominance of thermal energy is the least in solids and the most in gases.
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