Increase of pressure: (a) always increases the boiling point...

Increase of pressure:
(a) always increases the boiling point of a liquid
(b) always increases the melting point of a solid
(c) increases the melting point of solid which expand on melting
(d) always decreases the melting point of solid

Key Concepts

Clapeyron Equation

The Clapeyron equation provides a quantitative description of the relationship between pressure and temperature during phase transitions. It relates the slope of the phase boundary in a pressure-temperature diagram to the latent heat and volume change associated with the transition. This equation forms the theoretical underpinning for analyzing how pressure variations affect boiling and melting points.

Melting Point Dependence on Pressure

The melting point of a solid can change with varying pressure, but the direction of this change depends on the relative densities of the solid and its liquid phase. For most substances where the solid is denser than the liquid, an increase in pressure raises the melting point. Conversely, if the solid expands upon melting (making the liquid denser), increased pressure can lower the melting point.

Boiling Point Dependence on Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding pressure. When the external pressure increases, the liquid must be heated to a higher temperature to reach the required vapor pressure for boiling. This concept is central to understanding everyday phenomena such as pressure cooking, where higher pressures increase the boiling point of water.

Pressure Effects on Phase Transitions

This concept involves understanding how changes in external pressure influence the phase boundaries between solid, liquid, and gas states. An increase in pressure typically affects the temperature at which phase changes occur, such as boiling or melting. The relationship is governed by thermodynamic principles that determine whether a phase transition temperature will rise or fall under increased pressure.

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