Steam is water in the gas phase, which is formed when water boils. Steam is usually white or translucent in nature making it invisible; however, ‘steam’ often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapor condenses. Chemically, steam and water vapor are identical and the difference between them is the result of temperature. Water vapor is the same temperature as the air in which it is spread and it can be clear or translucent, and both steam and water vapor are referred to as the gaseous state of water. Steam is above the local boiling-point of water (usually taken to be 100°C, but this may vary with local weather and altitude). Water vapor is when water molecules are present in the air, while steam is water heated to the point that it turns into gas.
In liquid water, H2O molecules are constantly being joined together and separated. As the water molecules are heated, the bonds connecting the molecules start breaking more rapidly than they can form. Eventually, when enough heat is supplied, some molecules will break free. These ‘free’ molecules form the transparent gas we know as steam, or more specifically dry steam. Dry steam is a term that is applied to steam when all its water molecules remain in the gaseous state. It’s a transparent gas. Wet steam applies to steam when a portion of its water molecules have given up their energy (latent heat) and condense to form tiny water droplets.
The general assumption is that ‘a cloud of steam’ will be largely made up of steam and water vapor, whereas ‘water vapor’ is typically only a few percent of the volume of the air. Steam is water in a high state of rarefaction, or so impregnated that it exhibits motion which constitutes heat, thus steam is able to assume the state of an elastic fluid. When steam is confined in a close vessel in contact with the water that produces it, the effort by which it endeavors to expand itself and enlarge its volume, or to separate the parts of the vessel that confines it and set itself free, is called the elastic force of the steam. In estimating the mechanical power of steam, we must take into account both the intensity of its elastic force and the distance through which that force acts. The intensity of its elastic force must be referred to by some known standard measure, such as the pressure which it exerts against a square inch of the surface that contains it, and this is usually resolved by a certain amount of pounds avoirdupois upon the square inch.
The intensity of the elastic force may be estimated by the inches in height of a vertical column of mercury, whose weight is equal to the pressure exerted by the steam on a surface equal to the base of the mercurial column. It may also be estimated by the height of a vertical column of water measured in feet, or the elastic force of any fluid may be compared with that of atmospheric air when in its usual state of temperature and density.
The elasticity or pressure of vapors is best illustrated in the case of steam, which may be considered as the type of all vapors. When a quantity of pure steam is confined in a close vessel, its elastic force will exert on every part of the interior of the vessel a certain pressure directed outward, having a tendency to burst the vessel. When steam is generated in an open vessel its elastic force must be equal to the elastic force or pressure of the atmosphere, or otherwise the pressure of the air would prevent it from forming and rising. Steam that is produced from boiling water at 212° F, is capable of exerting a pressure of 15 pounds upon every square inch of surface, or one ton on every square foot, a force equivalent to the pressure of the atmosphere.