Question

Why does water changes into ice on cooling and back into the water on heating??

Answer 1

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There's nothing left to increase the kinetic energy of the molecules in the liquid state until all the bonds holding the molecules in a crystal structure have been broken. Consequently, the temperature remains constant until all the ice has melted.

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Answer 2

On the other hand, when the temperature drops and water freezes, water molecules form a crystal structure maintained by hydrogen bonding (as there is too little heat energy left to break the hydrogen bonds). This structure makes ice less dense than liquid water.

When a solid is heated, the particles are given more energy and start to vibrate faster. At a certain temperature, the particles vibrate so much that their ordered structure breaks down. At this point the solid melts into liquid.

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