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Molecules in a solid are tightly held together by strong inter-molecular forces of attraction.They all are closely packed in a regular manner.
But, when a solid is heated , at a certain temperature, the molecules in the solid get activated, begin to move apart from each other and exhibit random motion due to increase in their  kinetic energies with increase in temperature.Even the inter-molecular forces of attraction between the molecules of the solid weaken.
Thus, the solid exhibits melting and its molecules are now loosely packed(-a physical property of liquids).
Hence, solids (such as ice cubes) transform into liquid state on heating.

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The Brainliest Answer!
On heating solids, temperature rises.
i) Particles gain energy.
ii) Intermolecular attraction decreases i.e., forces acting between them decreases.
iii) Intermolecular space between molecules of solids increases.
iv) At melting point, particles become free from fixed position and convert into liquid (in liquids the molecules are loosely packed than solids, intermolecular space is more and intermolecular forces of attraction is less and particles are free to move within the liquid). 

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