Boiling
Boiling is a physical process in which heating converts a liquid into a gas or vapor. The boiling point or boiling temperature is that point where the vapor pressure of a substance is equal to the atmospheric pressure. For a given atmospheric pressure the boiling point for pure substances is well defined and impurities in a substance broaden the boiling point range.
All matter generally exists in one of three physical phase states commonly described as solid, liquid, or gas. When matter undergoes a phase change, for example from solid to liquid or liquid to gas, there is a significant change in the amount of atomic or molecular motion (i.e., heat). For example, as the motion of molecules in liquid water increases (i.e. the water heats) the collisions of water molecules at the liquid/gas boundary increase in intensity due to the increased kinetic energy of the molecules. As a result, as water heats, more molecules gain sufficient energy to escape into the vapor phase (i.e. the vapor pressure of the substance increases). Because temperature is a measure of heat, the boiling temperature is that temperature where vapor pressure of the water equals the atmospheric pressure.
The exact temperature at which a liquid begins to boil at normal atmospheric pressure is called the normal boiling point and is a unique physical characteristic of individual elements and compounds. For example, the normal boiling point of water is 212°F (100°C) while the normal boiling point of ethanol, is 172.9°F (78.3°C).
The boiling point of liquids changes in response to changes in pressure. As pressure lessens, the boiling point of liquids decreases. For example, water boils at lower temperatures as pressure decreases with increasing elevation. This is the reason that food cooked at mountain elevations often seems undercooked. Similarly, a glass of water can be made to boil at room temperature if it is placed inside of a vacuum chamber. At the other extreme, because the boiling point temperature of water will increase as pressure increases, pressure cookers cook food much faster than normal cooking methods.
Boiling of liquids is also influenced by the presence of dissolved substances, or solutes. In general, when liquids contain dissolved substances, their boiling point temperatures are elevated. For example, when salt is added to water, it boils at a higher temperature. Likewise, if sugar is added to water, boils at a higher temperature. This characteristic is used in automobile engine coolants that must be altered to boil at temperatures much higher than the normal boiling point of water.
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