Freezing Point Depression

Colligative Property – Freezing Point Depression

Freezing point depression is simply the process of LOWERING THE FREEZING POINT OF A LIQUID by adding a solute to it.

Ordinarily, water freezes at 32°F (0°C), but can you add salt to lower its freezing point to 20°F (-6°C). That’s why we use salt to melt ice on the road in the winter!

    • Adding non-volatile substance to volatile substance lowers the freezing point
    • Non-volatile substances do not readily evaporate into a gas
The solute preventing the molecules from getting into an organized manner
The solute preventing the molecules from getting into an organized manner
  • During freezing, the molecules instead of being able to bounce around randomly settle to a more organized structure called the SOLID STATE.

    organized structure of solid state
    Molecules in a solid state
  • The freezing point of a solution (solute + solvent) is always lower than that of a pure solvent. That is because the solute prevents the molecules from getting together into an organized structure and the temperature has to fall a lot lower to drastically reduce the kinetic energy that causes the molecules to bounce around.
  • Example, salt solution has lower freezing point than water.
The freezing point of a solution (solute + solvent) is always lower than that of a pure solvent.

 

Freezing-point depression describes the process in which adding a solute to a solvent decreases the freezing point of the solvent.

Examples include salt in water, alcohol in water, or the mixing of two solids such as impurities in a finely powdered drug. In the last case, the added compound is the solute, and the original solid is thought of as the solvent. The resulting solution or solid-solid mixture has a lower freezing point than the pure solvent or solid. This phenomenon is what causes sea water, (a mixture of salt (and other things) in water) to remain liquid at temperatures below 0 °C (32 °F), the freezing point of pure water.

Non-volatile substances

 

 

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