Temperature and Pressure -

When a liquid boils, a phase change occurs a substance changes from a liquid state of matter to a vapor state. The boiling point of a liquid is affected by temperature and pressure. The boiling temperature of water, for example, is different at different pressures. At atmospheric pressure, water boils at 212 degrees Fahrenheit. At pressures lower than atmospheric, the boiling temperature is lower. At pressures above atmospheric, the boiling temperature is higher. A graph can be constructed to show how temperature, pressure, and heat are related in the boiling of water. The vertical axis represents temperature in degrees Fahrenheit. The horizontal axis represents heat. The heat is measured in BTUs per pound of water. A BTU is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. At atmospheric pressure, which is 14.7 psia, water boils at 212 degrees Fahrenheit. A point can be placed on the graph to show the temperature at which water first starts boiling at atmospheric pressure. Another point shows where all of the water is boiled into steam. The length of the horizontal line connecting the two points represents the amount of heat required to boil all the water. In other words, this line represents the latent heat of vaporization. For one pound of water at atmospheric pressure, it takes about 970 BTUs of heat to boil all the water into steam. At 4.9 psia, which is less than atmospheric pressure, a pound of water boils at 162 degrees, and the latent heat of vaporization is around 1001 BTUs per pound. When the pressure is reduced, water boils at a lower temperature. However, it takes more heat to boil all of the water into steam than it does at atmospheric pressure. At lower temperatures, the water molecules move more slowly. As a result, it is harder for them to escape the body of water as steam. Therefore, it takes more heat to change the water completely to steam. At 114.7 psia, which is higher than atmospheric pressure, water will not boil until its temperature reaches 338 degrees. The latent heat of vaporization is about 881 BTUs per pound, which is less than the latent heat of vaporization at atmospheric pressure. At higher temperatures, the water molecules move more quickly than they do at lower temperatures. This means that it is easier for the molecules to escape the water as steam. In other words, it takes less heat to change the water completely to steam. As the pressure and boiling temperature of water increase, the latent heat of vaporization decreases. There is a point at which the latent heat of vaporization is zero BTUs per pound. This point occurs at a pressure of 3206 psia and a boiling temperature of 705 degrees Fahrenheit. This point is called the critical point. Above the critical point, water "flashes" to steam, but there is no latent heat transfer. Above this point, water exists as a superheated vapor. A superheated vapor is a vapor whose temperature is higher than its boiling temperature. On the heat/temperature graph, a curved line called the saturation curve connects the boiling points of water at specific pressures. The points on the curve to the left of the critical point represent liquid at the boiling temperature for a given pressure. The points on the curve to the right of the critical point represent vapor at the boiling temperature.