A cooling curve is a type of graph used in chemistry, physics, engineering, and other disciplines to chart the progress of a cooling substance. One axis of graph, usually the x axis, charts time, while temperature is represented on the other axis. As such, a cooling curve generally slopes downward from left to right as the temperature decreases over time. It is important to note that such a curve does not always progress downward at a uniform rate over the course of the graph because cooling curves are often used to depict physical phase changes, such as the change from water to ice. The temperature decreases at a uniform rate as the water cools to the freezing point, but the curve flattens out at the freezing point when the liquid water freezes to solid ice.
Many different factors can influence the progression of a cooling curve. Two of the most important factors are the initial temperature of the cooling substance, often called the "pouring temperature," and the temperature of the environment into which the substance is poured. The specific traits of the cooling substance are major determinants of the progression of the cooling curve. Other factors, such as pressure and the volume of the cooling substance, can also drastically affect the curve.
It is not uncommon for a cooling curve to depict a phase change, such as that from gas to liquid or from liquid to solid. The transformation from water to ice is one of the best-known and most widely-recognized examples of a phase change. When water at a relatively high temperature is added to an environment at a temperature below the freezing point, it will cool at a generally uniform rate until it reaches the freezing temperature. At this point, the temperature will cease to decrease until all of the water has solidified into ice. Cooling water loses energy in the form of heat and therefore causes a temperature decrease, but the same energy loss is also necessary for the transition from liquid to solid, only without the corresponding change in temperature.
Researchers and students can use a variety of methods to make cooling curves. The simplest method involves adding a substance to a temperature-controlled environment and, using a thermometer, recording the temperature of the substance at regular intervals. This method, however, is susceptible to human error. Other methods rely instead on electronic temperature sensors and computerized recording software. Such devices and software can be used to produce an accurate cooling curve in real time, while reducing the chance that experimental error will render the cooling curve useless.