Short Answer

Pressure affects the melting and boiling points of a substance because these phase transitions involve changes in volume and particle arrangement. Increasing pressure usually raises the boiling point because molecules need more energy to escape into the gas phase.

For most solids, increasing pressure raises the melting point, but substances like ice melt at lower temperatures under higher pressure due to their unique molecular structure. Pressure changes the conditions where solid, liquid, and gas are in equilibrium, thus affecting phase transition temperatures.

Detailed Explanation

Pressure and Phase Changes

Phase transitions, like melting (solid → liquid) and boiling (liquid → gas), occur at specific temperatures under a given pressure. These temperatures are sensitive to pressure because phase changes involve volume changes and molecular interactions. Applying external pressure alters the energy required for particles to overcome forces and rearrange into a different phase.

In thermodynamics, phase equilibrium depends on both temperature and pressure. The melting and boiling points shift when pressure is increased or decreased.

1. Effect on Boiling Point

Boiling occurs when the vapor pressure of a liquid equals the external pressure.

• Higher pressure: Molecules need more energy to escape into the gas phase.
  • Boiling point increases.
  • Example: Water boils above 100°C in a pressure cooker.
• Lower pressure: Molecules can escape more easily.
  • Boiling point decreases.
  • Example: Water boils at ~70°C on a high mountain because atmospheric pressure is lower.

Thus, boiling point is directly related to pressure.

2. Effect on Melting Point

Melting involves a solid turning into a liquid by absorbing energy. Pressure also influences melting:

• Most solids: Increasing pressure generally raises the melting point because higher pressure favors the denser phase (usually solid).
• Water and similar substances: Ice melts at a lower temperature under higher pressure because solid ice is less dense than liquid water. Pressure favors the liquid phase, reducing the melting point.

This effect explains phenomena like ice skating, where pressure from the skate blade slightly melts ice, creating a thin lubricating layer.

3. Phase Diagrams and Pressure

Phase diagrams illustrate the relationship between temperature, pressure, and phases:

• Solid–liquid line: Shows melting point variation with pressure.
• Liquid–gas line: Shows boiling point variation with pressure.
• Triple point: Pressure and temperature at which solid, liquid, and gas coexist.

Phase diagrams make it clear that pressure shifts equilibrium, changing the melting and boiling points.

4. Everyday Examples

Boiling Water

• High altitude: Lower pressure → water boils at lower temperature.
• Pressure cooker: Higher pressure → water boils at higher temperature → cooks food faster.

Ice and Pressure

• Ice melting under pressure from skates or tires lowers the melting point slightly, allowing movement on ice.

Industrial Applications

• Autoclaves and distillation columns adjust pressure to control boiling points.
• Metallurgy uses pressure to alter melting points for casting metals.

5. Scientific Explanation

The effect of pressure on melting and boiling points can be understood using Clapeyron equation:

Where:

•  = rate of change of pressure with temperature
•  = latent heat of transition
•  = volume change
• Boiling:  → increasing pressure raises boiling point.
• Melting:  may be positive or negative → effect on melting point varies by substance.

This equation quantifies how pressure influences phase transition temperatures.

Conclusion

Pressure significantly affects melting and boiling points because phase transitions involve changes in particle arrangement and volume. Increased pressure raises the boiling point of liquids and usually raises the melting point of solids, except for substances like ice. Understanding this effect is crucial in cooking, high-altitude survival, industrial processes, and physical chemistry applications.