Question

We can represent the freezing of \(\mathrm{H}_{2} \mathrm{O}(1)\) at \(0^{\circ} \mathrm{C}\ \mathrm{as} \mathrm{H}_{2} \mathrm{O}\) \(\left(1, d=1.00 \mathrm{g} / \mathrm{cm}^{3}\right) \rightleftharpoons \mathrm{H}_{2} \mathrm{O}\left(\mathrm{s}, d=0.92 \mathrm{g} / \mathrm{cm}^{3}\right) . \quad \mathrm{Ex}\) plain why increasing the pressure on ice causes it to melt. Is this the behavior you expect for solids in general? Explain.

Short answer

Increasing the pressure on ice causes it to melt because the increased pressure favors the phase with the higher density. Water is unique in that its solid form (ice) is less dense than its liquid form because of the way its molecules are arranged. This characteristic causes it to behave differently than most solid substances under pressure.

Step-by-step solution

Understanding Phase Transitions

First, consider the phase diagram of water which shows the changes in states of matter (solid, liquid, gas) under varying temperature and pressure. At 0°C, where the freezing point of water is, we can observe a transition from liquid water (1.00 g/cm³) to ice (0.92 g/cm³). This transition is unique due to the density difference between water and ice.

Understanding the Density Difference

The density of liquid water is higher than that of ice. This is because the molecules in ice are arranged in a crystal lattice that leaves relatively large gaps between them. These gaps are not present in the structure of liquid water, which allows more molecules to occupy a given volume, increasing its density.

Understanding the Effect of Pressure

If pressure is applied to this system, it will favour the phase with the higher density. As liquid water has a higher density than ice, increasing the pressure will cause the ice to transition back into a liquid hence melting the ice. This is due to Le Chatelier's Principle, which states that a system in equilibrium will adjust to counteract the change in conditions (in this case, the increase in pressure).

Comparing to Other Solids

This behavior is not common in most solids. Most solids have a higher density than their liquid forms, so increasing pressure typically favours the solid phase. Water is a unique exception due to its lower density when solid versus liquid.

Key concepts

Density differences between solid and liquid

In general, when we think of turning a liquid into a solid, we expect the solid to be denser. This means that the molecules in the solid state are packed more tightly together compared to the liquid state. However, water exhibits unusual behavior. The density of liquid water is about 1.00 g/cm³, while the density of ice is only 0.92 g/cm³. This means that ice is actually less dense than liquid water.
This density difference arises because water molecules in ice form a crystal lattice structure. This structure has spaces between molecules, making ice less dense. In the liquid state, these water molecules are more closely packed, eliminating the gaps, and increasing its overall density. This is why ice floats in water, an unusual trait among solids and liquids that has significant implications for aquatic life and Earth's climate.

Le Chatelier's Principle

Le Chatelier's Principle is a central concept in chemistry that helps us understand how systems in equilibrium respond to changes. It states that if a system at equilibrium experiences a change in pressure, temperature, or concentration, it will adjust to counteract that change and regain balance.
Applied to the freezing or melting of water, an increase in pressure on ice actually encourages it to melt. Remember, liquid water is denser than ice. According to Le Chatelier's Principle, if we apply pressure, the equilibrium will shift to favor the phase with the higher density to counteract this pressure change. Since liquid water is denser than ice, the application of pressure causes the system to shift toward producing more liquid water, resulting in the melting of ice.
This is a unique scenario specific to water because for most substances, increasing pressure would favor the solid phase due to their higher density. Water’s behavior under pressure is an excellent illustration of Le Chatelier’s Principle in action.

Phase diagram of water

The phase diagram of water is a graphical representation that shows how water behaves under different temperature and pressure conditions. This diagram is crucial to understand why and how water transitions between solid, liquid, and gas states.
At the freezing point, which is 0°C, the diagram highlights a critical point where water can exist as both liquid and ice. At this point, even a slight increase in pressure can push the balance towards the liquid phase due to the increased density of liquid water.
Unlike most substances, water's phase diagram has a unique negative slope for the solid-liquid equilibrium line. This means that by increasing pressure, you can actually encourage ice to melt into water. This peculiar slope is due to the lower density of ice compared to liquid water, defying the usual expectation for phase transitions.

• Normal solids: Solid phase lines have a positive slope, where increased pressure favors solid formation.
• Water: Solid phase line has a negative slope, leading to melting when pressure is applied.

Understanding water's phase diagram is critical to explaining its behavior under pressure and its broader implications for natural phenomena.