Problem 72 - Chemistry The Science in Context

Question

Can ice be melted merely by applying pressure? Explain your answer.

Step-by-Step Solution

Answer

Answer: In theory, ice can be melted by applying pressure alone, as the melting point of ice lowers under high pressure. However, in practice, it is difficult to achieve this without supplying any external heat, as some heat is required to change the state of ice from solid to liquid.

Step 1: Phase Diagram for Water

The phase diagram for water shows the different states of water (solid, liquid, and gas) at varying levels of temperature and pressure. Using this diagram, we can understand the effects of temperature and pressure on water and ice.

Step 2: Effect of Pressure on Ice

The phase diagram for water indicates that applying pressure while maintaining a constant temperature can cause ice to melt. This occurs because, as pressure increases, the melting point of ice decreases. However, the effect of pressure on the melting point of ice is relatively small compared to the effect of temperature.

Step 3: Principle and Practicality

In principle, ice can be melted merely by applying pressure, as the melting point of ice lowers under high pressure. But in practice, it is difficult to achieve this without supplying any external heat, as some heat is required to change the state of ice from solid to liquid.

Step 4: Conclusion

In summary, while theoretically ice can be melted by applying pressure alone, in reality, it is challenging to achieve this without changes in temperature or supplying external heat.

Key Concepts

Phase Diagram for Water

Understanding the unique properties of water is critical, and one tool scientists use to comprehend these characteristics is the phase diagram. A phase diagram for water graphically represents the different states—solid (ice), liquid (water), and gas (vapor)—and the conditions under which these states exist relative to temperature and pressure.

The diagram typically has pressure on the y-axis and temperature on the x-axis. The lines that divide the various phases indicate the conditions where two states of matter coexist in equilibrium. For instance, the line separating solid and liquid shows the melting point at different pressures. As most students know from daily experience, water usually melts at 0°C at standard atmospheric pressure, but the diagram highlights that this melting point can change with pressure: an increase in pressure can actually lower the melting point, allowing ice to melt at temperatures below 0°C.

Moreover, the phase diagram reveals a point called the triple point, where all three states of water coexist. This and other insights from the phase diagram are essential for scientists and engineers who deal with water in various environmental conditions and industrial processes.

States of Matter

Water can exist in three states of matter: solid, liquid, and gas. Students are most familiar with solid ice, liquid water, and water vapor. Each state is characterized by the arrangement and energy levels of the water molecules. In a solid state, the molecules are tightly packed in a regular pattern and vibrate in fixed positions. When water is in its liquid state, the molecules are still close together but can move around and slide past one another. As a gas, the molecules are far apart and move freely.

Transitions between these states are common and occur through processes such as melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), and condensation (gas to liquid). The fascinating aspect of water is how sensitive these transitions are to changes not only in temperature but also in pressure, which is less intuitive and often a point of confusion for students. That's why unraveling the complexity of this state change through exercises such as understanding how pressure affects the melting of ice is so instructive.

Melting Point

The melting point is a fundamental concept in chemistry and physics, defined as the temperature at which a solid turns into a liquid. For pure water, this temperature is typically 0°C (32°F) under standard atmospheric pressure. But, as students will learn through exercises, the melting point is not a fixed number engraved in stone; it can be influenced by pressure.

Why does this happen? The explanation lies in the molecular structure of ice. Under pressure, the rigid structure of ice is disrupted, and molecules are forced closer together. Since liquid water is denser than ice, this increased pressure favors the transition to a liquid state, effectively lowering the melting point. However, it should be emphasized to students that the pressure required to significantly alter the melting point is considerable and not commonly encountered in everyday situations.

Through classroom demonstrations, experiments, and thought exercises asking to melt ice with pressure, students not only explore this concept but also develop a deeper appreciation for the delicate balance of intermolecular forces that govern the state changes of matter.