What is meant by the normal freezing point of a substance?

The freezing point is a key concept in understanding how substances transition from one phase to another. It is defined as the temperature at which a liquid turns into a solid. When a liquid is cooled, its molecules slow down. Eventually, they move at a pace slow enough to form a rigid structure, or a solid.

For example, the freezing point of water is 0°C (32°F). At this temperature, water molecules form a structured lattice, turning the water into ice. The freezing point can vary for different substances.

It is crucial to remember that the freezing point can be affected by various factors such as pressure and the presence of impurities in the substance.

A phase transition refers to the process in which a substance changes from one state of matter to another, like from a liquid to a solid. This involves either absorption or release of energy. In the case of freezing, energy (in the form of heat) is released as the substance transitions from a liquid to a solid.

There are many types of phase transitions including:

• Freezing (liquid to solid)
• Melting (solid to liquid)
• Evaporation (liquid to gas)
• Condensation (gas to liquid)
• Sublimation (solid to gas)
• Deposition (gas to solid)

Each transition occurs at specific temperatures and conditions and involves changes in energy and molecular arrangement.

Atmospheric pressure is the force exerted onto a surface by the weight of the air above that surface in the atmosphere of Earth. It affects many physical properties, including the boiling and freezing points of substances.

At sea level, the standard atmospheric pressure is 1 atmosphere (atm). When we refer to the 'normal freezing point' of a substance, we are talking about the freezing point at this standard pressure of 1 atm.

Changes in atmospheric pressure can alter the freezing point. Higher pressure can raise the freezing point, while lower pressure can lower it. This is why scientists specify the 'normal' freezing point as the freezing point measured at 1 atm of pressure.