Explanation: Why Ice Melts in Water
Water ordinarily occurs as a liquid, though in polar regions it exists in solid form. A thirsty person puts a few ice cubes in a glass of water. If the glass sits for more than a few minutes, the ice melts. Then the cold water warms to room temperature. Notice ice does not get warm. Instead it changes structure between molecules and melts; it changes "phase."
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Melting Point
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Every stable substance can exist in at least three forms: solid, liquid and gas. Solids convert to liquids when they reach a temperature called a melting point. Liquids convert to gases when they reach a boiling point. Solids don't convert to liquids and liquids don't convert to solids instantaneously. Rather "phase transitions" occur. A phase transition is a change in molecular architecture --- that is, in the way individual molecules align themselves as a collective body --- a chunk of ice, a puddle of water, a cloud of steam. Perhaps it is simpler to understand the phase transitions of melting and boiling by considering first their opposites: condensing and freezing.
Condensation
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Gaseous water molecules have freedom to move about, with very little interference from each other. They aren't bound together. This is because they contain enough energy to overcome electrostatic attractions between molecules. If temperature is decreased, gas molecules have less energy and speed to enable them to maintain their freedom. Instead, attractive forces draw them closer to each other. As heat energy continues to be removed, eventually attractions become overpowering, and the gas phase is replaced by the liquid phase. Each molecule now has only a little independence; it retains some fluidity.
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Freezing
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As more heat energy is removed from cold water, its temperature drops until it reaches the vicinity of the freezing point. Removing small increments of heat energy at this point doesn't lower the temperature. Rather, it forces the molecules of liquid to align themselves into a rigid, crystalline solid --- the lowest-energy configuration for water. After that is completed, removing additional energy again lowers the temperature. Ice can be cooled well below its freezing point without further structural change.
Reversing the Process
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In a two-phase system of ice in water, water molecules provide heat to the slightly cooler ice, which absorbs it. The additional energy induces a phase change by enabling the ice molecules to escape their lowest-energy configuration and form liquid water. As external heat is provided by the surroundings it is absorbed by the increasing volume of water, which repeats the process until there is no ice left. Yet further heating raises the temperature of the now single-phase system consisting solely of liquid water.
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