The Freezing Point: Where Liquids Turn to Solids
The Molecular Dance: Energy and State Changes
Everything around us is made of tiny particles called atoms and molecules. These particles are constantly moving. The state of matter—solid, liquid, or gas—depends on how much energy these particles have and how strongly they are attracted to each other, a force called cohesion.
In a liquid, molecules have enough energy to slide past one another. They are close together but not locked in place. Think of a crowded dance floor where people can move around freely. As the liquid cools, it loses thermal energy to its surroundings. The molecules move slower and slower. At the specific freezing point, the molecules have lost enough energy that the attractive forces between them can pull them into a fixed, organized arrangement. This organized structure is a solid. The dance floor becomes a frozen grid where everyone stands still in a specific spot.
This process releases energy, called the latent heat of fusion[1]. This is why the temperature of a pure substance stays constant at its freezing point until all of the liquid has turned to solid, even though heat is continuously being removed. The energy being removed is this latent heat, not the energy that would lower the temperature.
Freezing Point Versus Melting Point
For a pure substance under the same conditions, the freezing point and melting point are the same temperature. However, they describe processes from opposite directions.
- Freezing Point: The temperature at which a liquid turns into a solid.
- Melting Point: The temperature at which a solid turns into a liquid.
Consider an ice cube. At exactly 0°C, it can coexist with liquid water. If you add a tiny bit of heat, some ice melts (solid to liquid). If you remove a tiny bit of heat, some water freezes (liquid to solid). The temperature remains constant during this two-phase equilibrium.
Factors That Change the Freezing Point
The freezing point is not an absolute constant. Several factors can raise or lower it.
1. Purity of the Substance: Adding another substance (a solute) to a pure liquid (solvent) lowers its freezing point. This is called freezing point depression. For example, adding salt to water disrupts the orderly formation of ice crystals. The water molecules now have to get even colder (lose more energy) to arrange themselves into a solid around the salt ions. This is why we salt icy roads and why ocean water freezes at a lower temperature (about -2°C) than fresh water.
2. Pressure: For most substances, increasing pressure lowers the freezing point slightly because pressure tends to favor the denser state. Water is a famous exception! Increasing pressure on ice actually lowers its melting point (which is the same as the freezing point). This is why ice skates work: the pressure from the thin blade melts the ice beneath, creating a slippery layer of water. For most other materials, like wax or metals, higher pressure makes it harder to melt them.
| Substance | Chemical Formula | Freezing/Melting Point (°C) | Notes |
|---|---|---|---|
| Water | $H_2O$ | 0.0 | Defines the Celsius scale. |
| Table Salt (Sodium Chloride) | $NaCl$ | 801 | A very high melting point for an ionic solid. |
| Ethanol (Alcohol) | $C_2H_5OH$ | -114 | Used in antifreeze and thermometers. |
| Mercury | $Hg$ | -38.83 | A liquid metal at room temperature. |
| Oxygen | $O_2$ | -218.79 | Freezes into a bluish solid. |
Supercooling: The Liquid That Won't Freeze
Sometimes, a pure liquid can be cooled below its normal freezing point without turning into a solid. This unstable state is called supercooling. It happens because the molecules need a starting point—a nucleation site[2]—to begin forming the orderly crystal structure. This could be a tiny impurity, a scratch on the container, or even a vibration.
A classic example is very pure water in a very smooth container, which can be cooled to around -40°C without freezing. The moment it is disturbed or a seed crystal is added, it freezes instantly and its temperature jumps back up to 0°C as the latent heat is released. This is a dramatic demonstration of the energy dynamics at the freezing point.
From Kitchen to Highway: Practical Applications
The science of freezing point is not just theoretical; it's used every day in many technologies.
Food Preservation: Your freezer is set below 0°C to ensure all water in food turns to ice. This drastically slows down the growth of bacteria and chemical reactions that cause spoilage. Making ice cream relies on freezing point depression: salt is mixed with ice around the cream mixture, creating a bath colder than 0°C, which allows the creamy mixture to freeze solid.
Automotive Antifreeze: A car's radiator contains a mixture of water and ethylene glycol. This mixture has a much lower freezing point than pure water (as low as -37°C), preventing the engine coolant from freezing and cracking the engine block in winter.
De-icing Roads: As mentioned, salt lowers the freezing point of water. When spread on icy roads, it causes the ice to melt even if the air temperature is below 0°C, creating a brine solution that prevents re-freezing until the temperature gets very low.
Climate Science: The freezing point of seawater is critical for understanding polar ice formation, ocean currents, and global climate models. The difference in freezing point between fresh and salt water drives important ocean circulation patterns.
Important Questions
Q1: Why does the temperature stay constant during freezing, even though the freezer is still running?
Q2: If I add sugar to water, will it freeze at 0°C?
Q3: Can a substance have more than one freezing point?
Footnote
[1] Latent Heat of Fusion (Lf): The amount of heat energy required to change a unit mass of a substance from solid to liquid at its melting point (or released during liquid to solid change) without a change in temperature. Measured in Joules per kilogram (J/kg).
[2] Nucleation Site: A location where atoms or molecules begin to arrange themselves into a new, stable solid phase (a crystal) during a phase change like freezing or condensation. It can be an impurity, a surface irregularity, or a dust particle.