Temperature is a fundamental physical property that quantifies the degree of hotness or coldness of an object. It is defined as the measure of the average kinetic energy of its constituent atoms or molecules.
When an object is cooled, thermal energy is transferred away from the system into the cooler surroundings, such as through conduction or radiation. This energy removal drives all subsequent physical changes within the object, starting at the atomic level.
The Loss of Kinetic Energy
The most fundamental change that occurs when an object’s temperature decreases is a reduction in the motion of its particles. Atoms and molecules are always in a state of random motion, which can include vibration, rotation, and translation, depending on the state of matter. When the object is cooled, the average speed and intensity of these movements decrease.
This slowing of microscopic movement corresponds directly to the definition of a temperature decrease, as temperature is proportional to the average kinetic energy. If cooling continued indefinitely, the particles would theoretically cease all motion at absolute zero. The energy leaving the system is the energy of this motion, which is transferred to the environment.
The particles lose energy through collisions, where faster-moving particles transfer their momentum to slower-moving ones in the surroundings. This continuous transfer of energy away from the object is necessary for all macroscopic observations of cooling. The reduction in kinetic energy alters the balance between motion and the attractive forces that hold the material together.
Changes in Volume and Density
As particles within a substance slow down due to cooling, the attractive forces between them become stronger. This allows the forces to pull the molecules closer, resulting in a decrease in the overall volume of the object. This phenomenon is known as thermal contraction, and it occurs across solids, liquids, and gases.
Because the mass remains unchanged while its volume shrinks, the material’s density increases. Engineers account for this predictable change in structures that experience wide temperature swings, such as bridges and concrete sidewalks. They incorporate small gaps, known as expansion joints, to accommodate the size changes that occur between the heat of the day and the cold of the night.
A notable exception is water, which exhibits anomalous behavior near its freezing point. As liquid water cools, its density initially increases, reaching a maximum density at approximately 4°C. Below this temperature, water begins to expand instead of contracting as it approaches freezing.
This expansion occurs because water molecules form strong hydrogen bonds that force them into an open, hexagonal crystalline structure as they solidify into ice. This arrangement occupies more space than the disorganized, closely packed liquid state at 4°C. Consequently, ice is less dense than liquid water, which is why it floats.
The expansion of water has implications, including the bursting of uninsulated water pipes due to the pressure exerted by the expanding ice. Importantly, it ensures that bodies of water freeze from the surface downward, creating an insulating layer of ice that protects aquatic life beneath. This molecular quirk is fundamental to the survival of organisms in cold climates.
Phase Transitions
If cooling continues and the object reaches a sufficiently low temperature, it may undergo a phase transition, or a change in its state of matter. For a gas, continued energy loss causes particles to slow down enough that attractive intermolecular forces draw them into a liquid state (condensation). For a liquid, cooling results in solidification (freezing), as particles lose enough kinetic energy to lock into fixed, ordered positions.
During the phase change, the temperature of the substance remains constant, even though energy continues to be removed. This observation is explained by the concept of latent heat. The energy being removed is instead used to facilitate the change in physical state.
Latent heat is the energy released as the new, stronger bonds of the lower-energy phase are formed, such as the bonds of a solid or the closer attractive forces of a liquid. As molecules arrange themselves into a more ordered structure, the energy associated with their previous state is released into the surroundings. This release must be completed before the substance can continue cooling within its new phase.