Freezing is a physical process in which a liquid changes into a solid after losing sufficient heat energy. It is one of the key phase changes of matter and occurs when the temperature of the liquid reaches or drops below a specific value called the freezing point. ^[1-4]

A common example of freezing is water turning into ice. The freezing point of water is 0°C (32°F) at standard atmospheric pressure.

Examples of Freezing ^[1-5]

Natural Examples

• Water freezes into ice at 0°C (32°F).
• Snow or frost forms when water vapor directly freezes onto cold surfaces.
• Candle wax hardens upon cooling.
• Lava solidifies into igneous rock after a volcanic eruption.

Industrial and Scientific Examples

• Molten metals like aluminum solidify into molds during casting.
• Scientists freeze biological materials such as cells and tissues for research, storage, or medical use.
• Frozen foods like vegetables or meat are preserved at sub-zero temperatures in food industries and supermarkets.

What Happens During Freezing

According to the kinetic theory of matter, particles such as atoms, ions, or molecules in a liquid have enough kinetic energy to move past one another. As the liquid cools, the particles lose energy and slow down. Eventually, they no longer have enough energy to overcome the attractive forces between them. They lock into a fixed, orderly arrangement known as a crystalline lattice, which gives solids their definite shape. In this solid state, the particles can only vibrate in place and cannot move freely. ^[1-5]

Freezing is an exothermic process, meaning that it releases heat to the surroundings during the phase change.

Freezing Point Values

The table below lists the freezing points of a few substances in both Celsius (°C) and Fahrenheit (°F) scales. ^[5]

Note: Values are approximate and measured at standard pressure (1 atm).

Factors That Influence Freezing ^[1-5]

1. Intermolecular Forces

The strength of the intermolecular attraction varies among different substances, resulting in different freezing points. For example, polar substances like water and ammonia have strong intermolecular forces due to hydrogen bonding and freeze at relatively higher temperatures. In contrast, nonpolar substances like methane and propane have weaker forces and require much lower temperatures to freeze. 

2. Impurities or Solutes

Adding impurities or dissolved substances to a liquid can lower its freezing point. This phenomenon is known as freezing point depression. A common example is the use of salt on icy roads in winter. The salt lowers the freezing point of water, making it harder for ice to form. This effect explains why ocean water does not freeze at 0°C like pure water.

3. Pressure

Pressure has a minor but noticeable effect on the freezing point of most substances, which can be understood by examining a phase diagram. In general, increasing pressure slightly raises the freezing point for most liquids because it makes the particles come closer together, encouraging solid formation. However, water behaves differently due to its unique molecular structure. Increasing pressure on water can lower its freezing point. This is the principle behind ice skating. Ice melts under the pressure of a skate blade, creating a thin layer of water that reduces friction. When the pressure is removed, the water refreezes.

4. Rate of Cooling

The rate at which a substance is cooled also influences how it freezes. If a liquid is cooled slowly, its particles have enough time to arrange themselves into large, well-ordered crystals. Rapid freezing can lead to smaller or even disordered crystal structures, as the particles do not have enough time to arrange themselves properly. This concept is important in applications such as producing smooth-textured ice cream or preserving biological samples without damaging their structure.