When heat energy is applied to a substance, its temperature typically rises, reflecting an increase in the kinetic energy of its molecules. This relationship holds true in most scenarios, such as heating water on a stove. However, a temporary halt in this temperature increase is observed when a substance absorbs heat without getting warmer. This phenomenon is a predictable characteristic of matter as it transitions from one physical state to another.
Defining the Temperature Plateau
A temperature plateau is a period during a physical transformation when a substance’s temperature remains fixed, even as energy is continuously transferred into or out of it. This steady temperature point is observed on a heating or cooling curve as a distinctive “flat line” where the graph of temperature versus time or energy becomes horizontal. The plateau marks the precise temperature at which a substance begins to change its physical state, such as when a solid becomes a liquid or a liquid becomes a gas.
The existence of this plateau means the energy being supplied is not increasing the average kinetic energy of the particles, which is what temperature measures. Instead, the continuous energy input is entirely dedicated to facilitating the phase transition. This constant temperature persists until the substance has completed its transformation into the new state.
The Physics Behind Constant Temperature
The reason temperature remains constant during this period lies in the concept of “latent heat,” which is energy absorbed or released without causing a change in temperature. During a phase change, the heat being added is not increasing the speed of the molecules. Instead, it is used to overcome the attractive forces that hold the substance in its current arrangement.
For a solid to melt into a liquid, the energy supplied must break the rigid bonds between the molecules in the crystalline structure. This energy, known as the latent heat of fusion, increases the potential energy stored within the substance by pulling the molecules apart, but it does not increase their kinetic energy. Once the substance has become a liquid, further heat added will increase the molecules’ kinetic energy, causing the temperature to rise again. A similar process occurs when a liquid boils into a gas, requiring the latent heat of vaporization to separate the molecules.
Common Examples in Daily Life
The most familiar examples of temperature plateaus involve water, which exhibits plateaus at both its freezing/melting point and its boiling/condensation point. When ice melts, a mixture of ice and water will maintain a temperature of 0°C (32°F) until the last piece of ice has converted to liquid water. The heat absorbed from the environment is used exclusively to break the hydrogen bonds in the ice, preventing the liquid water from getting warmer.
Another common plateau is observed when water boils at 100°C (212°F) at standard atmospheric pressure. Even though a stove continues to supply heat to a pot of boiling water, the temperature will not exceed this point; the energy is instead used to turn the liquid into steam. The large amount of energy required to vaporize water means that steam releases substantial latent heat upon condensing back to a liquid.
Beyond Water: Unique Plateaus for Different Materials
While water’s plateaus are well-known, every pure substance has its own unique set of fixed temperatures for melting and boiling, each corresponding to a specific latent heat requirement. For instance, gold melts at 1,064°C, and iron melts at 1,538°C. These distinct temperatures are directly related to the strength of the intermolecular forces holding the substance together, meaning more energy is needed to break the stronger bonds in metals than in water.
The precise temperature of these plateaus acts as a signature property, allowing scientists and engineers to identify and characterize materials. In industries like metallurgy, the exact melting plateau of a metal is a factor in manufacturing and quality control. The amount of latent heat needed also varies significantly; for example, the energy required to melt a kilogram of ice is much less than the energy needed to melt a kilogram of certain rocks or metals.