Melting and freezing
During freezing hydrogen bonds are formed between water molecules resulting in a crystal structure.
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Structure of ice
Depending on temperature and pressure, frozen water can have various crystal structures.
At standard atmospheric pressure, ice has a hexagonal crystal structure. This is the type we encounter in our daily lives.
Water molecules are connected by hydrogen bonds. These bonds form between the hydrogen atom of one molecule and a non-binding pair (lone pair) of electrons of an oxygen atom of the adjacent water molecule. The hydrogen bond is the strongest type of secondary bonds, it can occur between water molecules that are relatively far from each other. Therefore, water ice has a relatively low density, lower than that of liquid water.
- water molecule
- hydrogen bond
Melting and freezing
- heat transfer
- heat reduction
When exposed to heat, the temperature of water ice will rise to 0 °C, to its melting point at 1 atmosphere of pressure. At this point it starts to melt, and the heat works to break the ice crystals. Therefore, until the process of melting is complete, further exposure to heat will not raise its temperature.
When exposed to heat reduction, the temperature of water will decrease until it reaches its freezing point, which is 0 °C at 1 atmosphere of pressure. Further heat reduction puts the creation of ice crystals into motion. Until this is complete, the temperature does not decrease further.
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