Phase state changes

A change of state occurs when matter is converted from one physical state to another. For example, when water is heated, it changes from a liquid to a gas—when cooled water will eventually freeze into a solid: ice . A change of state is usually accompanied by a change in temperature and/or pressure.

Matter commonly exists in one of three forms, or states: solid, liquid, or gas. One fundamental way in which these three states differ from each other is the energy of the particles of which they are made. The particles in a solid contain relatively little energy and move slowly. The particles in a liquid are at a higher energy level and move more rapidly. The particles in a gas are at an even higher energy level and move most rapidly.

The state in which matter occurs can be changed by changing the energy state of the matter or the system surrounding matter that has the capacity to come into equilibrium with that system. When water is heated, molecules begin to move more rapidly. Eventually, they are moving fast enough to change to the gaseous, or vapor, state. The term vapor is used to describe the gaseous state of a substance that is normally a liquid at room temperature.

Imagine a block of ice at 14°F (−10°C). The molecules of water in the ice are vibrating in a crystalline array. As heat is added to the ice, the molecules begin vibrate more rapidly. At some point, they vibrate rapidly enough to break the lattice array and move freely in a liquid state. The point at which this occurs is the melting point. The melting point is the temperature at which a solid changes to a liquid. The melting point of ice is 32°F (0°C).

If additional heat is added to the liquid water, water molecules move even faster. The increase in speed with which they move is measured as an increase in temperature. The temperature of the liquid water increases from 32°F (0°C) to 212°F (100°C). At a temperature of 212°F (100°C), the water molecules are moving fast enough to change to a vapor, called steam. The temperature at which a liquid changes to a gas is called its boiling point. The temperature is a function of atmospheric pressure . The lower the pressure (e.g., lower pressures found at altitude) the lower the boiling point.

If the steam formed in this process is heated further, its temperature continues to increase.

Changes of state occur also when a material is cooled. Suppose the steam in this example is cooled below 212°F (100°C). When that happens, the water reverts to a liquid. The steam is said to condense to a liquid. The condensation point is the temperature at which a gas or vapor changes to a liquid. It is the same as the boiling point of the liquid.

Under the proper conditions, at some point, the liquid cools sufficiently to change to a solid. At this point, the liquid becomes frozen. The freezing point of a liquid is the temperature at which the liquid changes to a solid. The freezing point is the same as the melting point of the solid.

Some materials behave differently from water when they are heated. They may pass directly from the solid state to the gaseous state. Iodine is an example. When solid iodine is heated, it does not melt. Instead, it changes directly into a vapor. Substances that behave in this way are said to sublime. The sublimation point of a substance is the temperature at which it changes directly from a solid to a vapor.

Dry ice (solid CO₂), which rapidly undergoes sublimation from solid to vapor at room temperatures, is often used to create fog on stage and movie sets. A white, opaque solid, it is

also widely used as a cryogenic agent in industry to reduce bacteria growth and maintain low temperatures. At atmospheric pressures found on Earth, dry ice undergoes sublimation at −109.3°F (−78.5°C). Special care must be taken when working with dry ice to avoid frostbite. In addition, dry ice must be used in a well-ventilated environment because, as it undergoes sublimation, dry ice will reduce the percentage of available oxygen .

See also Atmospheric chemistry; Atoms; Hydrothermal processes; Ice heaving and ice wedging; Rate factors in geologic processes