A substance may form several types of crystals.
A triple point is a chemistry term that refers to the pressure and temperature at which a substance changes between three forms. A phase diagram may include the gas, liquid, solid and vapor forms of a compound; it may also include different types of crystals that a substance can form. Substances at the triple point are present in three forms.
The Phase Diagram
A phase diagram describes the changes in a chemical substance. It includes two axes, with one axis representing the atmospheric pressure and the other axis representing the temperature. Areas are drawn on this phase diagram, representing the states of the chemical substance. This allows chemists to see which state the substance is in at a given temperature and pressure.
Pressure and Temperature
Changes in temperature and pressure affect the state of the substance. Chemicals are solid at lower temperatures. Some solids melt with temperature increases and form liquids; other solids directly vaporize into gases when chemists apply heat to them. Pressure affects the boiling point, so it takes less energy for a solid to melt or boil at lower pressures. A high pressure can greatly increase the temperature at which a substance changes form. A triple point may also include the vapor, solid and liquid forms of a substance, according to Arizona State University.
Geology
Geologists study triple points when studying rock formations in the earth. The rock formations are deep underground, underneath layers of soil and rock, and they're at a higher pressure and temperature than materials on the surface. According to Tulane University, a triple point occurs in aluminum silicon oxide, or Al2SiO5. This material forms three different crystal structures, including kyanite, sillimanite and andalusite; the point at which all three types of crystals are present is the triple point.
Points on the Phase Diagram
Substances may not have a triple point, and some may have additional points. A mineral that only forms two types of crystals doesn't have a triple point when crystal structure is shown on a phase diagram. A chemical that has no liquid form may not have a triple point on a solid and gas phase diagram, unless it has another form such as vapor. A mineral that's present in four different crystal structures may have a quadruple point on a phase diagram.
Solutions
Triple points can include multiple substances, forming a chemical solution; this may allow many more than three phases. For example, a solution may include crystals and liquid at one temperature, and just liquid solution at another temperature. The temperature and pressure of the solution affect whether solids completely dissolve in the liquid. A triple point occurs whenever three phases are present, so there may be more than one triple point on the phase diagram.
Tags: triple point, triple point, phase diagram, types crystals, have triple, have triple point
