If the pressure is reduced further at this temperature, the ice, instead of melting, sublimates directly into steam. This pressure is very close to a perfect vacuum. The triple point of H 2O, where the three phases of ice, water and steam are in equilibrium, occurs at a temperature of 273.16 K and an absolute pressure of 0.006 112 bar. Triple pointĪll the three phases of a particular substance can only coexist in equilibrium at a certain temperature and pressure, and this is known as its triple point. The degree of excitation of the molecules determines the physical state (or phase) of the substance.
The molecular arrangement of ice, water, and steam is still not fully understood, but it is convenient to consider the molecules as bonded together by electrical charges (referred to as the hydrogen bond). In the case of H 2O, the terms ice, water and steam are used to denote the three phases respectively. Most mineral substances can exist in the three physical states (solid, liquid and vapour) which are referred to as phases. Carbon is another element of significant abundance, and is a key component in all organic matter. The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst the most abundant elements in the universe.
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