Problem 1066 - JEE Physics

Question

Melting point of ice (A) Increases with increasing pressure (B) Decreases with increasing pressure (C) Is independent of pressure (D) is proportional of pressure

Step-by-Step Solution

Answer

The melting point of ice decreases with increasing pressure. Therefore, the correct answer is (B) Decreases with increasing pressure.

Step 1: Understanding Melting Point, Pressure, and Phase Diagrams

A melting point is the temperature at which a substance changes from its solid phase to its liquid phase. Pressure is the force applied per unit area on a surface, typically affecting a substance's phase changes. A phase diagram is a graphical representation of the equilibrium conditions (i.e., temperature and pressure) for various phases of a substance.

Step 2: Examining the Phase Diagram for Water

For this question, we are interested in the solid-liquid equilibrium line because melting occurs at this boundary. For water, the slope of the solid-liquid equilibrium line is negative. This means when pressure increases, melting point decreases, and vice-versa.

Step 3: Relating the Melting Point of Ice to Pressure

Based on our understanding of the phase diagram for water and the concept of melting point, we can observe the following relationship: As pressure increases, the melting point of ice decreases.

Step 4: Comparing the Relationship with the Given Options

Now that we know the relationship between pressure and the melting point of ice, we need to compare this relationship to the given options: (A) Increases with increasing pressure - This is not correct. As pressure increases, the melting point of ice decreases. (B) Decreases with increasing pressure - This is the correct answer. As pressure increases, the melting point of ice decreases. (C) Is independent of pressure - This is not correct. Pressure has an effect on the melting point of ice. (D) Is proportional to pressure - This is not correct. The melting point and pressure have a negative relationship, not a direct proportionality. So, based on our analysis, the correct answer is (B) Decreases with increasing pressure.

Key Concepts

Pressure and Phase Diagrams

Phase diagrams are essential tools in chemistry and physics, illustrating how substances behave under different temperature and pressure conditions. A phase diagram provides a visual map showing the boundaries between different states of matter—solid, liquid, and gas for a specific substance.
A typical phase diagram has temperature on the x-axis and pressure on the y-axis. Different regions on the diagram represent different phases. The lines separating these regions are called phase boundaries. These lines show the conditions under which two phases can coexist in equilibrium.

• The triple point is where all three states coexist.
• The critical point denotes the end of the liquid-gas boundary.
• The solid-liquid line is particularly interesting when studying melting points.

These diagrams help visualize how pressure and temperature influence phase changes, such as melting and boiling.

Solid-Liquid Equilibrium

The solid-liquid equilibrium line is vital in understanding the melting behavior of a substance.
This line in a phase diagram represents the conditions at which solid and liquid phases coexist.
When a substance is at this equilibrium line, any change in temperature or pressure can shift the balance, leading to melting or freezing. For water, this equilibrium line has a unique characteristic:

• It has a negative slope, meaning that as pressure increases, the melting point decreases.
• This is contrary to most substances where the melting point increases with pressure due to their positive slope lines.

Understanding this principle is key to answering questions about the melting point and how it is affected by pressure changes.

Effect of Pressure on Melting Point

Pressure can have a profound effect on the melting point of substances. For most substances, increasing pressure typically raises the melting point. However, water behaves unusually due to its molecular structure. For water:

• The hydrogen bonding between molecules creates a slightly open, hexagonal structure in solid form (ice).
• This open structure makes ice less dense than liquid water, causing ice to float.
• As pressure increases, this structure is destabilized, making it easier for ice to melt, hence lowering the melting point.

This unique behavior of water under pressure is why, under high-pressure conditions, ice melts at a lower temperature.

Phase Diagram of Water

The phase diagram of water is a special case in the landscape of phase diagrams due to the peculiar properties of water.
One of its most unusual features is the negative slope of the solid-liquid equilibrium line.
This indicates that when pressure increases, the melting point of ice decreases. The key features of water's phase diagram include:

• A pronounced triple point where ice, liquid water, and water vapor coexist.
• A critical point at high temperatures and pressures where liquid and vapor become indistinguishable.
• The negative slope of the solid-liquid line, making it distinctive from most substances.

These features exemplify the unique behavior of water, which is critical in various scientific and practical scenarios, from understanding Earth's climate to industrial applications.