Freezing is the fundamental physical process where a substance transforms from its liquid state into a solid state at a constant temperature. This concept is a core topic in physical sciences, commonly introduced in educational curricula like Class 11 to explain states of matter and phase changes.
The provided reference defines freezing as:
"The change in the liquid to the solid phase at a constant temperature is called freezing. The temperature at which liquid changes to solid is called the freezing point. Example: water-filled in an ice cube tray, when kept in the freezer, changes into solid, so here the water freezes to form ice cubes."
Understanding Freezing
Freezing is a specific type of phase transition that occurs when a liquid loses enough thermal energy (heat) for its particles to settle into a more ordered, rigid structure, characteristic of a solid.
Key Characteristics of Freezing:
- Phase Transformation: It involves a direct change from the liquid state to the solid state.
- Constant Temperature: The process occurs at a specific, constant temperature known as the freezing point. While the substance is freezing, its temperature does not change, even though heat is being removed. This removed heat is often referred to as latent heat of fusion.
- Energy Removal: For freezing to occur, heat energy must be removed from the liquid. As the kinetic energy of the particles decreases, they move less freely and can form fixed positions.
Freezing Point Explained
The freezing point is the precise temperature at which a liquid solidifies under standard conditions. For pure substances, the freezing point is typically the same as their melting point.
For instance, the freezing point of pure water at standard atmospheric pressure is 0°C (32°F). When water in an ice cube tray is placed in a freezer, it cools down to 0°C, and then it remains at 0°C while it changes from liquid water to solid ice. Only after all the water has become ice will its temperature drop further below 0°C.
Practical Example
Let's consider the classic example provided in the reference:
| Substance | Initial State | Final State | Process | Key Temperature |
|---|---|---|---|---|
| Water | Liquid | Solid (Ice) | Freezing | 0°C (Freezing Point) |
When a water-filled ice cube tray is placed in a freezer:
- The water gradually cools down.
- Upon reaching its freezing point (0°C), the water begins to transform into ice.
- During this transformation, the temperature of the water/ice mixture remains constant at 0°C until all the water has solidified.
- Once entirely frozen into ice, the temperature of the ice will continue to drop to match the freezer's temperature.
This everyday observation perfectly illustrates the concept of freezing as a phase change occurring at a constant temperature.
