Melting – Conversion of solid to liquid
Melting refers to the phase change of a substance from a solid state to a liquid state. This is a phase change phenomenon which means in this process, a substance is transformed from one state of matter to another state.
How does melting occur?
Most solids are packed in a tight crystal lattice with strong intermolecular forces of attraction. On passing heat, the internal binding energy of the crystal lattice gets overcome by the heat energy and the intermolecular forces of attraction weaken. This weakening of force causes instability in the crystal lattice and molecules to separate out from each other and start moving in random directions. The instability in the crystal lattice initiates the melting of a substance. According to the accepted theory of melting, when the temperature of a substance is increased by the virtue of supplying heat or increasing pressure, then the molecules of the substance start vibrating at their places. When the amplitude (or distance covered) of the vibration exceeds the inter-atomic distance of the substance, it causes vibrational instability and leads to the melting of the substance.
Melting point is the temperature of a solid at which it changes its state from solid to liquid at atmospheric pressure. At melting point, these two phases viz. solid and liquid stay in equilibrium i.e. at this point both solid state and liquid state exist simultaneously. The melting point of a substance depends upon atmospheric pressure.
Latent heat of fusion
After a substance is raised to the temperature of melting point, the heat added after this point doesn’t raise the temperature of the substance, rather it is used completely to change the phase of the substance from solid to liquid. This heat supplied to the solid at its melting point is the latent heat of fusion. It is defined as the heat energy required for completely changing 1 kg of a solid into a liquid state at atmospheric pressure. The term latent meaning hidden is used because this heat is figuratively hidden as it doesn’t appear on the temperature scale.
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