Temperature measures the thermal energy contained within matter, which is the average kinetic energy of its constituent atoms and molecules. When a substance is cooled, this thermal energy is removed from the system. Cooling reduces the average speed and intensity of movement of the particles that make up any material. This removal of energy initiates a cascade of physical changes, altering the motion of individual particles and the overall structure and properties of the material.
The Slowdown of Molecular Movement
The most immediate physical effect of cooling is the loss of kinetic energy, which causes the particles to slow down. In a gas or a liquid, particles move more sluggishly as the temperature drops. Since kinetic energy is directly proportional to temperature, a lower temperature means less energetic motion.
In a solid material, atoms are fixed in a crystal lattice, and motion is primarily vibrational. Cooling a solid reduces the amplitude and intensity of these atomic vibrations around their fixed positions. This decrease in vibrational energy explains why materials feel colder to the touch. This reduced activity precedes every other macroscopic change observed during cooling.
State Changes and Phase Transitions
As cooling continues, the attractive forces between molecules begin to overcome the diminishing kinetic energy. This imbalance causes the matter to undergo a phase transition, reorganizing into a more ordered state. A gas transitions into a liquid (condensation) when its molecules slow enough for intermolecular forces to hold them loosely together.
Further cooling causes a liquid to solidify (freezing) when the remaining kinetic energy falls below the threshold needed to maintain a fluid state. These phase changes occur at specific, constant temperatures for any given substance, such as water freezing at \(0\,^{\circ}\text{C}\). During this transition, the substance must release internal energy, known as latent heat, even though the temperature remains constant until the process is complete. This latent heat is extracted before the material can proceed to a lower temperature state.
Thermal Contraction and Density Shifts
For most substances, a reduction in the space occupied by slower-moving molecules causes thermal contraction, shrinking the material’s volume. Since mass remains the same, this decrease in volume results in an increase in density. This effect is observed in all states of matter; for instance, a metal ring shrinks when cooled with liquid nitrogen.
Water, however, exhibits a unique behavior known as the density anomaly. Liquid water reaches its maximum density at approximately \(4\,^{\circ}\text{C}\), just before freezing. As water cools from \(4\,^{\circ}\text{C}\) to \(0\,^{\circ}\text{C}\), it expands and becomes less dense. This is due to the unique shape of the water molecule and the hydrogen bonds that form between them.
When water freezes, these hydrogen bonds force the molecules into a highly organized, open hexagonal crystal lattice. This structured arrangement contains more empty space than the configuration of liquid water, causing the volume to increase by about nine percent upon solidification. This expansion explains why ice floats and why water pipes can burst in freezing temperatures.
Approaching Absolute Zero
The conceptual lower limit of temperature is absolute zero, defined as \(0\,\text{K}\) or \(-273.15\,^{\circ}\text{C}\). While scientists have achieved temperatures below 100 picokelvin, this theoretical limit can never be fully reached. This unattainability is a consequence of the third law of thermodynamics, which states that complete removal of all thermal energy is physically impossible.
Even near absolute zero, particles never truly stop moving due to a quantum mechanical phenomenon called zero-point energy. The Heisenberg uncertainty principle prevents a particle from having both a perfectly defined position and zero momentum simultaneously, meaning atoms must retain some residual vibrational energy.
Exotic States of Matter
At these ultracold temperatures, matter can enter exotic states, such as a Bose-Einstein Condensate (BEC). In a BEC, a large fraction of atoms occupy the lowest quantum state and behave as a single, collective matter wave. Other phenomena, like superconductivity—the complete loss of electrical resistance—also emerge when certain materials are cooled below a characteristic temperature.