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Colligative Properties

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A series of free High School Chemistry Video Lessons.

In this lesson, we will learn

  • Colligative Properties
  • Vapor Pressure
  • Boiling Point Elevation
  • Freezing Point Depression

Colligative Properties
Colligative properties are the properties of a solution as a whole and depend on the concentration. The colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering and osmotic pressure.
An overview of the colligative properties.
The first part of this lesson on colligative properties introduces freezing point depression and boiling point elevation. We discuss common examples of each and begin to look at the relationship between amount of solute and degree of freezing point depression-boiling point elevation.
Part 2 of this sequence discusses vapor pressure lowering and how to use molality to calculate the change in colligative properties.This lesson discusses vapor pressure as a colligative property and its connection to boiling point elevation. We also look at how to use molality to calculate the degree of freezing point depression.

Vapor Pressure
Lowering Vapor pressure lowering is a colligative property of solutions. The vapor pressure of a pure solvent is greater than the vapor pressure of a solution containing a non volatile liquid. This lowered vapor pressure leads to boiling point elevation.
The pressure caused by the evaporation of liquids.
Vapor Pressure, Volatility, and Evaporation
Boiling Point Elevation
Boiling point elevation is a colligative property of solutions. Solutions' boiling points are higher than that of the solute or solvent because the vapor pressure of solutions is lower. A boiling point is when the vapor pressure of the solution becomes equal to the external pressure, so when the initial vapor pressure is lower, it takes more heat to elevate the vapor pressure to the same point.
Understanding the colligative property of solutions that elevates boiling point.
This video describes boiling point elevation and presents a worked problem calculating molar mass from boiling point depression data.

Freezing Point Depression
Freezing point depression is a colligative property of solutions. Solutions' freezing points are lower than that of the pure solvent or solute because freezing, or becoming solid, creates order and decreases entropy. Solutions have high entropy because of the mix of solvent and solute, so it takes more energy to decrease their entropy to the same point.
Understanding the colligative property of solutions that decreases the freezing point.
This video shows you how to calculate the Freezing Point Depression of a solution. The covers the use of the van't Hoff factor using the electrolyte CaCl2.
Determine the freezing point of a solution made from 3.0 kg of water and 2.5 mol of CaCl2. The freezing point constant for water is -1.86 °C/m.

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