| | |  | View upwards from a glacier crevasse | |
Ice – a special substance
Ice is the solid phase of water (H2O). Under normal conditions, the melting point at the Earth’s surface is 0°C. To change the phase of H2O, a certain amount of energy must be absorbed or released. In order to melt ice of 0°C to water of 0°C, a high amount of energy is needed, equivalent to 334 Joules for each gram. The same amount of energy is released in the atmosphere or ground when water freezes to ice.
The energy which is absorbed or released during a phase change (for example from water to ice, or from water to water vapour) is called latent heat. All melting, evaporation (phase change from water to gas) and sublimation (phase change from ice to gas) processes lead to a cooling of the directly surrounding space where the required energy is absorbed from. By contrast all condensation (change in phase from gas to liquid), freezing and deposition (change in phase from gas to ice) processes lead to a warming of that space.
Phase transitions between ice, water and vapour, and the energy released or absorbed during these phase transitions
The consequences of the physical properties of ice, and glaciers in particular, on Earth are crucial.
Glaciers do not instantly melt completely once they are exposed to air temperatures higher than 0°C. They melt slowly because the ice has to absorb the energy from its surroundings. Thus, glaciers or glacier parts can exist for a certain amount of time under environmental conditions above 0°C.
To melt ice of 0°C to water of 0°C requires as much energy (latent heat) as, for example, heating water of 0°C to about 80 °C. Ice can therefore be quite stable if only limited energy is available to melt it.
Ice is very sensitive to temperature changes on Earth. It survives over a longer period only if the temperature stays below 0°C. As soon as the temperature increases and is above 0°C, the ice begins to melt. If there is enough energy (heat) available to the ice, it absorbs that and changes its phase from ice to water by the process of melting.
Questions
1. What are the 3 different phases of H2O?
2. How are the 6 different phase changes called?
3. How much energy does it take to melt 0°C ice to 0°C water?
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