Question

The melting points of most of the solid substances, increases with increase in pressure. However, ice melts at a lower temperature lower than its usual melting point when the pressure is increased. This is because (1) ice is less denser than water (2) pressure generates heat (3) the chemical bonds break under pressure (4) ice is not a solid

Short answer

Option (1): ice is less dense than water.

Step-by-step solution

Understanding the relationship between pressure and melting points

Normally, the melting points of solid substances increase as the pressure increases. This happens because most materials are denser in their solid form than in their liquid form. Hence, increasing the pressure usually stabilizes the solid phase, requiring higher temperatures to melt.

Identify the unique property of ice

Ice is an exception to the general rule. Unlike most substances, ice is less dense than water. This unique characteristic is essential to understanding why increased pressure leads to a lower melting point for ice.

Application of pressure on ice

When pressure is applied to ice, it encourages the ice to transition to its denser phase (liquid water). To facilitate this phase change under pressure, the melting point of ice decreases.

Analyzing the given options

Review the options provided: (1) ice is less dense than water, (2) pressure generates heat, (3) the chemical bonds break under pressure, (4) ice is not a solid. Based on the steps above, we determine the reason is specifically related to the lower density of ice compared to water.

Final Step: Selecting the correct answer

The correct reason why ice melts at a lower temperature under increased pressure is that ice is less dense than water. This matches option (1).

Key concepts

density and phase change

The concept of density and phase change is crucial in understanding how substances transition from one state to another under varying conditions. Density refers to the mass per unit volume of a substance. For most materials, the solid phase is denser than the liquid phase. This means that when you increase the pressure on a solid, it becomes more stable, and its melting point rises.
For instance, if you keep pressing down on a block of metal, it will require more heat (higher temperature) to melt because the atoms are packed tighter together in the solid form.
However, water and its solid form, ice, behave differently due to their unique molecular structure.

unique properties of ice

Ice has some unique properties that set it apart from most other solid substances. One of the most important features is that ice is less dense than water. This is why ice floats on water.
This unique property is due to the arrangement of water molecules in a hexagonal structure when they freeze. This structure creates more space between the molecules compared to their arrangement in liquid water, making ice less dense.
Because of this, when pressure is applied to ice, it tends to transition into its more dense phase, which is liquid water. This phase change necessitates a lowering of the melting point.

pressure effects on solids

Pressure can significantly impact the melting points and phase changes of solids, including ice. For most solids, increasing pressure makes the solid phase more stable and thus higher temperatures are needed to induce melting.
In contrast, due to ice's unique low density relative to liquid water, applying pressure actually lowers its melting point. This effect can be observed in phenomena like ice skating, where the pressure from the skater's blade on the ice surface causes a thin layer of water to form, reducing friction and helping the blade glide smoothly.
As a result, understanding pressure effects on different solids is essential for explaining various natural and human-made phenomena.