Question

Imagine that you take an ice cube out of the freezer and place it on a counter at room temperature. a. Write a balanced equation for the process that occurs. b. Describe what you would do to reverse the process.

Short answer

The balanced equation for ice melting is: \( \text{H}_2\text{O(s)} \rightarrow \text{H}_2\text{O(l)} \). To reverse, cool the liquid water below 0°C to freeze it back into ice.

Step-by-step solution

Identify the Process

The process we're observing is the melting of an ice cube. This involves ice, which is solid water (H₂O), changing from a solid state to a liquid state as it absorbs heat from the surrounding environment.

Write the Balanced Equation

The melting of ice involves the phase change of water from solid to liquid. The balanced equation for this physical change can be written as: \[ \text{H}_2\text{O(s)} \rightarrow \text{H}_2\text{O(l)} \]This represents solid water (ice) transforming into liquid water as it absorbs heat.

Describe the Reverse Process

To reverse the process of melting ice, one needs to convert liquid water back into solid ice. This can be achieved by removing heat from the liquid water. Practically, you would place the water in a freezer to reduce its temperature below the freezing point (0°C or 32°F), causing it to solidify back into ice.

Key concepts

Melting

Melting is one of the most familiar phase changes we encounter in our daily lives. When a solid substance turns into a liquid, it melts. In the case of an ice cube, when you take it out of the freezer and place it on a counter at room temperature, it starts to melt. This happens because the solid ice absorbs heat energy from the warmer surroundings.

This energy causes the water molecules in the ice to break free from their rigid structure and move more freely, transforming the solid into a liquid.

• The melting point is the specific temperature at which a solid becomes a liquid.
• For ice, this occurs at 0°C (32°F).

Understanding melting helps explain why our ice cubes disappear on a hot day or how snow turns into slushy water.

Freezing

Freezing is the opposite of melting, where a liquid becomes a solid. When you want to reverse the melting of an ice cube, you need to freeze the water. To achieve this, the temperature of the liquid must be lowered to the freezing point, hence removing thermal energy from the liquid water.

When the water is placed in a freezer, its temperature drops, causing its molecules to slow down and arrange themselves into a solid structure. Once the temperature reaches the freezing point, the liquid becomes solid ice.

• Freezing is an exothermic process, which means it releases heat energy to the surroundings.
• Both the melting and freezing point for water is 0°C (32°F).

Temperature Changes

Temperature changes play a crucial role in phase changes such as melting and freezing. The transfer of heat energy is what causes substances to reach their melting or freezing points.

During the phase change, the temperature of a substance remains constant as it absorbs or releases energy. For example, when ice melts into water, the temperature stays at 0°C until all the ice has melted.

• In the process of melting, heat is absorbed but there is no increase in temperature until the change is complete.
• During freezing, heat is released but the temperature stays constant until the entire liquid has solidified.

This is because the energy is being used to change the phase, rather than changing the temperature.

Enthalpy

Enthalpy is a concept used to describe the heat content of a system. It is especially useful in understanding phase changes like melting and freezing.

The enthalpy change of melting, also known as the enthalpy of fusion, is the amount of energy required to turn a solid into a liquid at constant pressure. When a substance melts, it absorbs heat, increasing its enthalpy. Conversely, during freezing, heat is released, and the enthalpy decreases.

• The enthalpy of fusion for water is about 334 J/g, meaning each gram of ice requires 334 joules to melt.
• For freezing, the same amount of energy is released as the liquid becomes a solid.

Understanding enthalpy helps us calculate and predict the energy changes involved in these phase transitions.