Instant knowledge: A case for steam

A better understanding of the properties of steam may be achieved by understanding the general molecular and atomic structure of matter, and applying this knowledge to ice, water and steam.


What is steam?

A better understanding of the properties of steam may be achieved by understanding the general molecular and atomic structure of matter, and applying this knowledge to ice, water and steam.

The specific combination of these atomic elements provides compound substances. One compound is represented by the chemical formula H2O, having molecules made up of two atoms of hydrogen and one atom of oxygen.

The reason water is so plentiful on the earth is because hydrogen and oxygen are among the most abundant elements in the universe. Carbon is another element of significant abundance, and is a key component in all organic matter.

Most mineral substances can exist in the three physical states (solid, liquid and vapor) which are referred to as phases. In the case of H2O, the term ice, water and steam are used to denote the three phases respectively.

The molecular structure 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). The degree of excitation of the molecules determines the physical state (or phase) of the substance.

Triple point

All 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 triple point of H2O, where the three phases of ice, water and steam are in equilibrium, occurs at a temperature of 32 F and an absolute pressure of 0.088 psi. This pressure is very close to a perfect vacuum. If the pressure is reduced further at this temperature, the ice, instead of melting, sublimates directly into steam.


In ice, the molecules are locked together in an orderly lattice type structure and can only vibrate. In the solid phase, the movement of molecules in the lattice is a vibration about a mean bonded position where the molecules are less than one molecular diameter apart.

The continued addition of heat causes the vibration to increase to such an extent that some molecules will eventually break away from their neighbors, and the solid starts to melt to liquid state. At atmospheric pressure, melting occurs at 32 F. Changes in pressure have very little effects on the melting temperature, and for most practical purposes, 32 F can be taken as the melting point. However, it has been shown that the melting point of ice falls by 0.01 F for each additional atmosphere of pressure. For example, a pressure of 200 psig would be needed to reduce the melting temperature by 0.18 F.

Content provided by Spirax Sarco, originally published in Steam News Magazine.

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