Most substances are solid at low temperatures; as the temperature rises, they become liquid; and at still greater temperatures, they become gaseous.
Melting is the conversion of a solid into a liquid (an older term that you may see sometimes is fusion). When a substance transitions from one phase to another, its temperature stays constant. Phase changes are therefore isothermal (isothermal is defined as "constant temperature"). Take H2O as an example once more. At 0°C, solid water (ice) can exist. When matter transitions from one state (solid, liquid, gas, or plasma), it goes through a phase shift. When the system receives enough energy (or loses enough), these adjustments take place.
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