Question

What are the three colligative properties that involve phase changes?

Short answer

The three colligative properties involving phase changes are freezing point depression, boiling point elevation, and osmotic pressure.

Step-by-step solution

Understanding Colligative Properties

Colligative properties are properties of solutions that depend on the number of solute particles in a solution, not on the type of particles. They are influenced by the presence of a solute but not the identity of the solute itself.

Identifying Phase Change Related Properties

There are several colligative properties, but only three of them directly involve phase changes. These are freezing point depression, boiling point elevation, and osmotic pressure.

Detailing the Phase Change Properties

1. **Freezing Point Depression**: This occurs when the addition of a solute lowers the freezing point of a solvent. 2. **Boiling Point Elevation**: This property describes the increase in boiling point of a solvent when a solute is dissolved in it. 3. **Osmotic Pressure**: Although more complex, this involves the movement of solvent molecules through a semipermeable membrane from a less concentrated to a more concentrated solution, typically leading to a physical separation.

Key concepts

Freezing Point Depression

Freezing point depression is a fascinating phenomenon observed when a solute is added to a solvent. Essentially, as solute particles are introduced, they disrupt the orderly lattice structure that the liquid solvent forms when it freezes. This disruption means more energy must be removed from the solution for it to solidify, thus lowering the freezing point.
A classic example of freezing point depression is the use of salt on icy roads in winter. The salt, dissolving in the ice water, lowers the freezing point, preventing further freezing at temperatures where pure water would already have turned to ice.
Mathematically, freezing point depression can be calculated using the formula:
\[\Delta T_f = i \cdot K_f \cdot m\]where:

• \( \Delta T_f \) is the freezing point depression
• \( i \) is the van't Hoff factor (number of particles the solute splits into)
• \( K_f \) is the freezing point depression constant of the solvent
• \( m \) is the molality of the solution

Understanding this concept is essential for explaining why solutions behave differently than pure solvents in colder temperatures.

Boiling Point Elevation

Boiling point elevation is another crucial concept in colligative properties that occurs when a solute is dissolved in a solvent. The presence of solute particles interferes with the evaporation process that turns the liquid into gas. As a result, the solution requires a higher temperature to reach the boiling point compared to the pure solvent.
This colligative property is significant in cooking and chemical industries where changing the boiling point can have practical applications.
The boiling point elevation can be determined using the formula:
\[\Delta T_b = i \cdot K_b \cdot m\]where:

• \( \Delta T_b \) is the boiling point elevation
• \( i \) is the van't Hoff factor
• \( K_b \) is the boiling point elevation constant of the solvent
• \( m \) is the molality of the solution

These principles highlight how the addition of a solute can have substantial effects on the boiling characteristics of a solvent.

Osmotic Pressure

Osmotic pressure is a unique colligative property that involves the behavior of solutions separated by a semipermeable membrane. It's a process where solvent molecules cross the membrane from a less concentrated solution to a more concentrated one.
This movement continues until equilibrium is reached, meaning the concentration of solutes on both sides of the membrane becomes equal.
Osmotic pressure is fundamentally important in biological systems, where it plays a critical role in maintaining cell structure and nutrient transport.
The mathematical expression for osmotic pressure \( \pi \) is given by:
\[\pi = i \cdot M \cdot R \cdot T\]where:

• \( \pi \) is the osmotic pressure
• \( i \) is the van't Hoff factor
• \( M \) is the molarity of the solution
• \( R \) is the ideal gas constant
• \( T \) is the temperature in Kelvin

Through understanding osmotic pressure, we gain insights into essential processes such as the absorption of water by plant roots and the filtration of blood by our kidneys.