Question

A length of wire is placed on top of a block of ice. The ends of the wire extend over the edges of the ice, and a heavy weight is attached to each end. It is found that the ice under the wire gradually melts, so the wire slowly moves through the ice block. At the same time, the water above the wire refreezes. Explain the phase changes that accompany this phenomenon.

Short answer

The wire melts the ice under pressure, causing it to refreeze above, thereby moving through the ice. This is known as pressure melting.

Step-by-step solution

Understanding Pressure Melting

The pressure from the weights at the ends of the wire increases the pressure on the ice directly beneath it. According to the phase diagram of water, increased pressure at temperatures close to 0°C causes ice to melt into water.

Pressure-Induced Melting of Ice

As the increased pressure from the wire causes the ice to melt, the wire sinks downwards as it remains suspended by the weights. This results in the wire moving gradually through the ice.

Refreezing Process

Once the pressure is relieved on the melted ice (as the wire moves down), the water quickly refreezes into ice above the wire. This happens because the pressure that initially caused the melting is no longer present.

Continuous Cycle

The combination of melting due to high pressure below the wire and refreezing once the pressure decreases creates a continuous cycle. This is why the wire is able to gradually move through the ice block.

Key concepts

Phase Diagram of Water

The phase diagram of water is a valuable tool for understanding the different states of water—solid, liquid, and gas—and how they change with varying temperatures and pressures. It is essentially a map that shows the conditions under which water exists in each phase. On this diagram, temperature is typically on the x-axis, while pressure is on the y-axis. The lines on the diagram represent boundaries where phase changes occur, such as melting or boiling.

For example, at 1 atmosphere of pressure, ice melts at 0°C. However, as pressure increases, the melting point of ice decreases. This peculiar behavior is crucial for phenomena like pressure-induced melting. Unlike many other substances, water has a negative slope on its solid-liquid boundary, highlighting its unique ability to transform from solid to liquid with increased pressure even below its standard melting point of 0°C.

• The phase diagram reveals critical points like the triple point where all phases coexist.
• Water's behavior is atypical due to the hydrogen bonding between molecules.
• Pressure not only affects melting but also freezing conditions as seen in high pressure environments.

Phase Changes

Phase changes refer to the transformation of matter from one state to another and are an essential concept in understanding how substances react to different conditions. These include processes such as melting, freezing, evaporation, and condensation. Each of these changes involves either absorbing or releasing energy, usually in the form of heat. For example, when ice melts to form water, it absorbs heat, causing the molecules to move more freely.

In our specific context, understanding phase changes helps clarify how ice turns into water under pressure and then reverts to ice once the pressure is removed. This dynamic behavior is of great importance in natural and engineered systems where pressure variations induce phase shifts.

• Melting occurs when a solid absorbs enough energy to change into a liquid.
• Freezing involves cooling a liquid until it forms a solid.
• Phase changes are not instantaneous and require a consistent input or removal of energy.

Pressure-Induced Melting

Pressure-induced melting is a fascinating phenomenon where the application of pressure can cause a solid to transition into a liquid state. In the case of water, this results from its unusual phase diagram characteristics. When pressure increases on ice, especially near the melting point, it may cause ice to temporarily become water despite temperatures remaining under 0°C.

This occurrence is most famously demonstrated with ice skates or, as in the exercise, a wire with weights. Here, the wire's weight concentrates force on the ice beneath it, increasing the pressure and causing it to melt. As the wire passes through, it leaves melted water in its wake, showcasing how pressure alters physical states without raising temperature.

• This type of melting is distinct because it relies on pressure, not heat.
• It's an example of how external conditions beyond temperature can affect phase transitions.
• Such phenomena have implications in glaciology and materials science.

Refreezing Process

The refreezing process occurs when conditions change and revert water back to its solid state—ice, in our scenario. After the pressure has caused the ice beneath the wire to melt, the removal of that pressure as the wire passes allows the water to refreeze. The melted ice above the wire refreezes because the temperature remains at or below the freezing point once the pressure is relieved.

This refreezing is as critical to the wire's journey through the ice block as the initial melting. It effectively seals the path the wire has traveled, allowing for a continuous progression through the block. The pendulum between melting and refreezing forms a closed cycle, exemplifying how external forces can direct phase changes without altering overall mass.

• Refreezing occurs rapidly as the system rebalances to ambient pressure conditions.
• This process is critical in maintaining the structural integrity of the block during the wire's passage.
• It highlights the reversible nature of pressure-induced phase changes under stable temperature conditions.