Question

\- You have a shallow dish of alcohol set into a tray of water. If you blow across the tray, the alcohol evaporates, while the water cools significantly and eventually freezes. Explain why.

Short answer

Alcohol evaporation absorbs heat, cooling the water until it eventually freezes.

Step-by-step solution

Understand Alcohol Evaporation

Alcohol has a lower boiling point compared to water, meaning it evaporates more quickly when exposed to air.

Concept of Evaporative Cooling

When alcohol evaporates, it requires energy in the form of heat. This heat is taken from the surroundings, leading to a cooling effect on the surface.

Heat Transfer Mechanism

As alcohol evaporates, it absorbs heat from the tray and the water underneath. This heat is used to change the alcohol from liquid to gas.

Temperature Drop of Water

Due to the absorbed heat, the water loses heat and its temperature drops significantly.

Freezing Point of Water

If enough heat is removed from the water, its temperature can drop below 0°C, causing it to freeze.

Key concepts

Alcohol Evaporation

Alcohol has a fascinating characteristic: it evaporates much faster than water. This is because alcohol has a lower boiling point, which means it turns into gas at a lower temperature. Imagine you have a shallow dish filled with alcohol and it’s sitting in a tray of water. When you blow across the surface of the tray, the heat from the surroundings enables the alcohol to vaporize quickly.

• Lower boiling point of alcohol means quicker evaporation.
• Evaporation process is sped up when air blows over the alcohol.

This rapid evaporation process is essential for understanding how the system cools. Since alcohol takes up less heat to switch from liquid to gas, it rises and goes into the air much faster than water, leading us to the next important concept: evaporative cooling.

Heat Transfer

The principle of heat transfer plays a pivotal role in this setup. As alcohol evaporates, it needs energy to transform from liquid to gas. Where does this energy come from? It’s taken from the surroundings, specifically from the tray and the water in the tray. This process is known as evaporative cooling.

• Heat energy from water and tray is used for alcohol evaporation.
• The loss of heat results in a temperature drop of the remaining liquid.

As the alcohol absorbs the heat from the water, the temperature of the water starts to decrease. The heat initially present in the water is now being used to carry away the alcohol molecules into the air, significantly dropping the water’s temperature.

Freezing Point of Water

So, what happens when the water continues to lose heat? If the temperature of the water drops down to 0°C or below, it reaches its freezing point. This is the temperature at which water molecules start to form ice. It’s because the water has lost so much heat due to the evaporating alcohol that it can no longer maintain its liquid state.

• Water freezes when its temperature reaches 0°C.
• Continuous heat loss due to alcohol evaporation leads to freezing.

Eventually, the water in the tray turns into ice. To summarize, alcohol's rapid evaporation causes a significant heat transfer from the water, resulting in the water cooling down and freezing if enough heat is removed.