A substance’s physical form, or state of matter, is determined by the energy it contains. The three familiar states—solid, liquid, and gas—represent different internal energy levels within the material’s molecules. Altering this internal energy through heating or cooling drives a change in physical state. When thermal energy is removed from a substance, its molecules lose energy, causing the material to cool and transition into a more ordered, lower-energy form. This principle governs the transformation of liquid water.
The Resulting Physical State
Removing thermal energy from liquid water causes it to undergo a phase change into its solid state, commonly known as ice. This transition from a liquid to a solid is referred to as freezing or solidification. The solid state represents the lowest energy level for water under standard atmospheric conditions, possessing less internal energy than its liquid counterpart. The extraction of heat is necessary for the molecules to settle into this more stable, structured arrangement.
How Molecular Motion Changes
Thermal energy in a liquid corresponds directly to the kinetic energy, or speed, of its molecules. As energy is removed from liquid water, the molecules decelerate, decreasing their random motion and vibration. This reduction allows the attractive forces between the water molecules, primarily hydrogen bonds, to exert a greater influence. In the liquid state, the molecules are too energetic to maintain fixed attractions and constantly slide past one another.
Once the motion slows sufficiently, hydrogen bonds force the molecules to align into a fixed, highly ordered, three-dimensional crystalline structure. This arrangement is a hexagonal lattice, characterized by open spaces between the molecules. This rigid structure causes the substance to become a solid. This unique lattice structure holds the molecules further apart than they were in the denser liquid water, which is why ice floats.
The Role of Latent Heat and the Freezing Point
The transition from liquid water to solid ice occurs precisely at the freezing point, defined as 0° Celsius (32° Fahrenheit) at standard atmospheric pressure. Before this temperature is reached, the removal of thermal energy causes a measurable drop in the water’s temperature. Once the liquid is cooled to 0°C, the temperature remains constant even though cooling continues.
This phenomenon is explained by the concept of “Latent Heat of Fusion,” which refers to the large amount of energy that must be transferred out of the substance to complete the phase change. For water, this value is approximately 334 kilojoules for every kilogram undergoing the change. During this phase of constant temperature, the energy being removed is utilized to rearrange the liquid molecules into the fixed, lower-energy crystal lattice of the solid. Only once all the water has solidified will the continued removal of thermal energy cause the temperature of the ice to drop below 0°C.