Question

The melting points of most of the solid substances increase with an increase of pressure acting on them. However, ice melts at a temperature lower than its usual melting point, when the pressure increases. This is because

• A. Ice is less dense water
• B. Pressure generates heat
• C. The bonds break under pressure
• D. Ice is not a true solid

Answer 1

Answer: (A)

Ice is less dense than water

Answer 2

The phenomenon described is an exception to the general rule that the melting point of solids increases with pressure. This anomaly in ice is due to its unique property of being less dense than its liquid form (water).

1. General Principle: For most substances, the solid phase is denser than the liquid phase. When pressure is applied, it favors the more compact (denser) phase. Therefore, to melt such a solid, a higher temperature is required to overcome the increased pressure, meaning the melting point increases with pressure. This is quantitatively described by the Clausius-Clapeyron equation, where dP/dT = ΔH_fus / (T * ΔV_fus). For most substances, ΔV_fus = V_liquid - V_solid is positive (since V_liquid > V_solid), and ΔH_fus is positive, leading to dP/dT > 0.

2. Anomalous Behavior of Ice: Ice is an exception because it is less dense than liquid water at its melting point. This means that the volume of ice is greater than the volume of an equal mass of water (V_ice > V_water). When pressure is applied to ice, it favors the more compact phase, which in this case is liquid water. Therefore, increasing pressure causes ice to melt at a lower temperature. In terms of the Clausius-Clapeyron equation, ΔV_fus = V_water - V_ice is negative. Since ΔH_fus and T are positive, a negative ΔV_fus results in dP/dT < 0. This means an increase in pressure (dP > 0) leads to a decrease in melting temperature (dT < 0). This phenomenon is known as regelation.

3. Evaluating the options:

   • A) Ice is less dense than water: This is the correct reason, as explained above. The lower density of ice compared to water means that increasing pressure favors the formation of the denser liquid phase, thus lowering the melting point.
   • B) Pressure generates heat: While compression can generate heat, this is not the fundamental reason for the change in melting point itself. The melting point is a thermodynamic property influenced by pressure and volume changes, not merely heat generation.
   • C) The bonds break under pressure: While pressure can affect intermolecular forces, the primary reason for the anomalous behavior compared to other solids is the density difference, not just general bond breaking.
   • D) Ice is not a true solid: This is incorrect. Ice is a crystalline solid with a definite structure.

The final answer is $\boxed{A}$

Explanation of the solution:

Ice is less dense than water. When pressure increases, it favors the formation of the denser phase, which is liquid water. Consequently, ice melts at a lower temperature to transition to the denser liquid state. This is described by the Clausius-Clapeyron equation where the change in volume upon melting (ΔV_fus) is negative for ice, leading to a negative slope for the pressure-temperature melting curve.