A discussion of temperature and heat is essential to an explanation of how water becomes steam.
Temperature is a measurement of the molecular motion within a substance (water in a tea pot, for example). Faster-moving molecules result in higher temperature. We measure temperature on a thermometer, which sets the freezing point of water at 32 degrees Fahrenheit (0 degrees Celsius) and boiling point of water at 212 degrees Fahrenheit (100 degrees Celsius) at sea level.
Heat is a measurement of the movement or transfer of energy from a body with a higher temperature to a body with a lower temperature. We measure heat in British Thermal Units (Btu) or kilojoule. One Btu is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. In SI units, the kilojoule is equal to a kilowatt-second.
Water can absorb only so much heat. This is called sensible heat because it produces a temperature change that can be measured. The amount of sensible heat that is required to raise the temperature of water from freezing to boiling is called the heat of the saturated liquid. At sea level, atmospheric pressure, it is about 180 Btu per pound (418.8 kJ/kg) of water.
Adding more heat to the water has no effect on temperature. Instead, it causes the water to change state (or vaporize) from a liquid to a gas. About 970 Btu of additional heat is required to vaporize one pound of water into one pound of steam (2256.2 kJ/kg). This is called latent heat because it cannot be measured by a thermometer.
Water is made up of hydrogen and oxygen atoms. At lower temperatures, these molecules are loosely bonded to each other. When heat is transferred to the water, the molecules begin to vibrate and the temperature starts to rise.
As the heat increases, the molecules vibrate more and more until the bond holding them together is severely weakened. When sufficient heat has been transferred to the water to break the bonds, the molecules explode apart, creating steam.
After the steam has been transported to the heat exchanger, it gives up its latent heat to raise the temperature of some product or to power some operation. As the steam gives up its latent heat to the work effort, it condenses back into water.