The freezing point of a substance is the temperature at which its liquid phase turns into a solid phase, as the temperature decreases and the substance loses heat. This occurs when the molecules in the liquid slow down enough to form a solid structure.
Just like the boiling point, the freezing point can vary depending on the pressure. However, at standard atmospheric pressure, the freezing point is often fixed for most pure substances.
Examples:
- Water:
- Freezing Point: 0°C (32°F) When water is cooled down to 0°C or below, it transitions from liquid water to solid ice.
- Ethanol (Alcohol):
- Freezing Point: -114.1°C (-173.4°F) Ethanol has a much lower freezing point than water, which is why it remains liquid at colder temperatures.
- Mercury:
- Freezing Point: -38.83°C (-37.9°F) Mercury, a metal, remains liquid at much lower temperatures than water, which is why it’s used in thermometers for measuring lower temperatures.
- Saltwater (Sea Water):
- Freezing Point: Around -2°C (28°F) Salt lowers the freezing point of water, which is why seawater freezes at a temperature lower than the freezing point of pure water.
Key Point:
- Freezing Point Depression: Adding substances like salt to a liquid (such as water) can lower the freezing point. This is why salt is spread on roads in winter to prevent ice from forming.
In summary, the freezing point marks the temperature at which a liquid turns into a solid, and it varies depending on the substance and external conditions.