1. Definition: Freezing is the phase transition in which a substance changes from a liquid to a solid state when its temperature is lowered below its freezing point.
2. Freezing Point: Each substance has a specific freezing point, which is the temperature at which it freezes. For example, water freezes at 0 degrees Celsius (32 degrees Fahrenheit) at standard atmospheric pressure.
3. Formation of Crystals: During freezing, the particles (atoms, ions, or molecules) in the liquid slow down and arrange themselves in a regular, repeating pattern to form a solid. This arrangement is called a crystal lattice.
4. Heat Transfer: Heat is released during the freezing process. This heat is known as the heat of fusion and is the energy required to change a substance from a liquid to a solid at its freezing point. The reverse process, melting, requires the same amount of heat energy.
5. Importance of Freezing: Freezing is a crucial process in many industries, such as food preservation. It helps to extend the shelf life of perishable foods by slowing down the growth of bacteria and other microorganisms.
6. Effect of Impurities: Impurities or additives can lower the freezing point of a substance. For example, adding salt to water lowers its freezing point, which is why salt is often used to melt ice on roads in cold climates.
7. Supercooling: Sometimes, a liquid can be cooled below its freezing point without actually freezing. This is known as supercooling. It occurs when there are no nucleation sites for crystal formation. Disturbing the liquid or adding a seed crystal can trigger freezing in supercooled liquids.
8. Phase Diagram: A phase diagram is a graphical representation of how a substance behaves under different temperature and pressure conditions. It shows the boundaries between the solid, liquid, and gas phases and indicates the conditions at which freezing occurs.
9. Frost Formation: Frost is a common example of freezing in our daily lives. It forms when the moisture in the air condenses and freezes on surfaces like windows, cars, and grass, especially on cold winter mornings.
10. Expansion During Freezing: Most substances expand when they freeze, except for water. Water is unique in that it contracts and becomes denser as it freezes, which is why ice floats on liquid water.

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