States of Matter

As the temperature of a liquid is increased, the particles gain more energy and move faster and faster. Jostling about and colliding increases until eventually the particles at the surface gain enough energy to overcome the attractive forces from their neighbors and break away into the surrounding space. At this point, the liquid is becoming a gas (also called a vapor). The temperature at which this happens depends on what the substance is. This temperature, known as the boiling point, remains constant during the entire process of boiling because the added heat is being used up to break the attraction between the particles. The reverse process, condensation, occurs at the same temperature as boiling. Like the melting point, the boiling point is unique for each pure substance, and can be used as an analytical tool for determining the identities of unknown substances.

The amount of energy required for a given amount of a liquid to vaporize or become a gas is called the heat of vaporization (or condensation). It varies from substance to substance because the particle of different substances may be heavier or lighter and may exert different attractive forces. The amount of energy absorbed when 1 gram of water completely changes to a vapor is 540 calories. Conversely, 540 calories are released when 1 gram of water vapor changes back to liquid.

When a liquid reaches the boiling point, particles on the surface actually gain enough energy to break away from the surface. But as heating continues, particles Gallium melts at 86°F (30° C). © Yoav Levy/Phototake NYC. Reproduced with permission.
throughout the liquid are also increasing in energy and moving faster. In a body of the liquid, however, the particles cannot escape into the air, as those on the surface can. That is not only because they happen to be buried deep down below the surface. It is also because the atmosphere is pushing down on the entire liquid and all the particles within it, and, in order to break away, these particles deep within the liquid must acquire enough energy to overcome this additional pressure. (The surface particles can just fly off into the spaces between the air molecules.) When a group of interior particles finally do get enough energy-get hot enough-to overcome the atmospheric pressure, they can push each other away, leaving a hollow space within the liquid. This is a bubble. It is not entirely empty, however, because it contains many trapped particles, flying around inside. The light-weight bubble then rises through the liquid and breaks at the surface, releasing its trapped particles as vapor. We then say the liquid is boiling.

Since the pressure inside the bubbles must overcome atmospheric pressure in order for the bubbles to form, the boiling point of a substance depends on atmospheric pressure. Liquids will boil at lower temperatures if the atmospheric pressure is lower, as it is on a mountain. At the top of Mount Everest, 29,000 ft (8,839 m) above sea level, where the pressure is only about one-third that at sea level, water boils at 158°F (70°C). At 10,000 ft (3,048 m) above sea level, water boils at 192°F (89°C). It would take longer to cook an egg where the boiling point is 192°F (89°C) than at sea level where the boiling point is 212°F (100°C). The normal boiling point of a liquid is defined as its boiling point when the atmospheric pressure is exactly 760 mm Hg, or 1 atmosphere.

With the diminishing supplies of fresh water today, it is increasingly important to find ways of desalinating-removing the salt from sea water in order to make it useful for human consumption, agriculture, and industry. Changes in state, both boiling and freezing, are useful for this purpose. When salt water is heated to boiling and the vapors cooled, they condense to form water again, but the salt stays behind in a very salty residue called brine. By this process, called distillation, freshwater has been recovered from salt water. Similarly, when salt water freezes, much of the salt stays behind as a very salty slush. The ice is removed from the brine and melted to produce relatively fresh water.