By Kevin Beck | Updated Mar 24, 2022
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Everyone has witnessed the transformation of a liquid into solid ice or the reverse. Understanding when this phase change occurs is essential for everyday life, from driving safely in winter to cooking and scientific experiments.
Most substances exist in three phases—solid, liquid, or gas—depending on temperature and pressure. Water is unique in that it can be solid and liquid at comfortably low temperatures, and it turns to gas at 100 °C (212 °F), a temperature that is approachable for most people. The transition from solid to liquid is called melting; the opposite, from liquid to solid, is freezing or solidification. When a liquid becomes a gas, it is boiling, while the reverse process is condensation. In rare cases, a solid can bypass the liquid phase and turn directly into gas through sublimation, or a gas can deposit directly onto a solid surface.
The freezing point of a substance depends on its molecular structure. For instance, water has a relatively high freezing point of 0 °C because its hydrogen bonds readily “lock” together, whereas ethyl alcohol (CH₃CH₂OH) freezes at –114 °C (–174 °F). When different liquids are mixed or when a solid is dissolved in a liquid, the resulting mixture’s freezing point shifts predictably based on the components involved.
In a solution, the dissolved solid is called the solute and the liquid that carries it is the solvent. Adding a solute lowers the solvent’s freezing point—a phenomenon known as freezing‑point depression, a classic colligative property. The effect depends on the number of particles produced by the solute, not on its chemical identity.
Table salt (sodium chloride) dissolves in water and reduces its freezing point, which is why road crews spread salt in winter to melt ice. NaCl dissociates into two ions (Na⁺ and Cl⁻), so each mole of salt yields two particles that lower the freezing point. A solute that produces more particles per formula unit would depress the freezing point even further.
Use an online freezing‑point calculator to look up the temperatures of common liquids in Celsius, Fahrenheit, and Kelvin. Comparing the freezing points of different substances can reveal patterns linked to their molecular structures.
