When a liquid is cooled to its freezing point, it transitions to its solid state. This process involves a loss of energy in the form of heat. As this happens, the molecules of a compound bond together and the liquid becomes a solid. A number of factors influence exactly when and how this happens for different compounds.
Enthalpy and the Loss of Heat
Scientists use the term 'change in enthalpy' to describe a gain or loss in energy by a compound or reaction. If the pressure is kept constant, the change in enthalpy can be generally thought of as a change in heat. When a liquid is cooled, it has a negative change in enthalpy; it gives off heat. This results in a lower energy for the compound. Its molecules move increasingly slower, until they do not move at all.
Molecules in a compound exert an attractive force on each other. Eventually, the attractive forces overpower the energy of the fluid motion of molecules. The molecules bond together, and the substance takes on the rigid shape of a solid. The total intermolecular force between two molecules of a solid is also the energy required from outside to break this bond and change the solid into a liquid. This is called the enthalpy of fusion. The enthalpy of fusion varies based on the compound and type of bond.
The solid and liquid of a substance can coexist. At the freezing point, the substance experiences an equilibrium state. At this point, the rate of freezing is equal to the rate of melting. It takes more cooling to induce total conversion to the solid state. Once the liquid is entirely frozen, its internal temperature will stay at the freezing point; cooling the liquid further will not change the internal temperature.
Pressure and the Freezing Point
Atmospheric pressure has a negligible effect on the freezing point of a liquid. However, a liquid's internal vapor pressure influences its freezing point. As you lower the vapor pressure of a liquid, the freezing point decreases. You can lower the vapor pressure by adding a solute to a liquid, which causes a decrease in the liquid's freezing point.
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