Boiling point is the temperature at which a liquid bubbles and changes into vapor. At this temperature, the vapor pressure (the pressure exerted by the vapor) equals the atmospheric pressure. The motion of vapor molecules produces the vapor pressure, which rises as the temperature of the liquid rises. The lower the atmospheric pressure is, the lower is the temperature needed to produce a vapor pressure equal to the atmospheric pressure. So the boiling point of any substance depends on the atmospheric pressure.
Unless otherwise specified, boiling points are based on a pressure of 1 atmosphere (14.696 pounds per square inch [101.325 kilopascals]), the average pressure of the atmosphere at sea level. As altitude increases, atmospheric pressure decreases. Thus, the boiling point of a substance also decreases as the altitude increases. The boiling point of water at sea level is 100 °C (212 °F). But at 10,000 feet (3,050 meters) above sea level, water boils at about 90 °C (194 °F).
How pressure affects boiling.
If a liquid is put into a closed vessel, leaving space above the liquid, some of the liquid turns into vapor. The pressure produced by the vapor is the liquid’s vapor pressure. The vapor pressure stabilizes when it equals the pressure of the liquid.
If the vessel is opened and the atmospheric pressure exceeds the vapor pressure, nothing noticeable happens to the liquid. The pressure of the air holds the liquid’s vapor above the liquid, maintaining a fairly stable vapor pressure. But if the atmospheric pressure equals or is less than the vapor pressure, the liquid boils. In boiling, bubbles of vapor form in the liquid and rise to the surface. The vapor then pushes out against the air and escapes from the space above the liquid. Because the vapor escapes, its pressure never stabilizes, and the liquid evaporates completely.
A liquid does not have to reach its boiling point to evaporate completely. This fact can be demonstrated by placing a pan of cool water in the sun on a hot day. The atmospheric pressure exceeds the vapor pressure, and so the water vapor becomes trapped above the water. If air currents sweep vapor molecules away, more water turns into vapor and evaporation continues. As a result, the water in the pan soon dries up.
Why boiling points differ.
Substances have different boiling points because they vary in the strength of the bonds between their molecules. The stronger the forces of attraction between the molecules of a substance, the lower the vapor pressure of the substance at a given temperature. In turn, the lower the vapor pressure of a substance, the higher its boiling point. For example, water molecules are strongly attracted to one another. Water thus has a fairly low vapor pressure, and it boils at 100 °C. Nitrogen, whose molecules are not as strongly held together as water molecules, has a higher vapor pressure and a much lower boiling point, -195.8 °C.
Some substances have especially strong bonds between molecules and almost no vapor pressure at ordinary temperatures. These substances boil only at extremely high temperatures. For example, gold turns into liquid at 1064.43 °C and has a boiling point of 2807 °C. The boiling point of iron, which becomes liquid at 1535 °C, is 2750 °C.
