Problem 83 - Chemistry: The Central Science

Question

(a) When you exercise vigorously, you sweat. How does this help your body cool? (b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

Step-by-Step Solution

Answer

(a) Sweating helps our body cool during vigorous exercise by evaporative cooling. As we exercise, excess heat is transferred to the water in our sweat. The heat is removed from our body when the liquid sweat evaporates into a gaseous state, thus lowering our body temperature. (b) When a vacuum pump is connected to a flask of water, the pressure inside the flask decreases. This causes the water to boil at a lower temperature. As the water continues to boil and evaporate, it loses energy, leading to a decrease in temperature. With sufficient time and pressure reduction, the water's temperature drops below its freezing point and thus begins to freeze.

Step 1: 1. Evaporative cooling from sweat

When we exercise vigorously, our body produces sweat to help lower its temperature. Sweat is mostly composed of water, which has a high heat capacity and can absorb a significant amount of heat. As we continue to exercise, excess heat produced by our body is transferred to the water in the sweat.

Step 2: 2. Evaporation of sweat

As the water in our sweat absorbs heat, it starts to evaporate. Evaporation is the process in which the liquid sweat turns into a gaseous state, which requires energy. This energy is taken from our body in the form of heat.

Step 3: 3. The body cools down

As the heat is removed from our body through the evaporation of sweat, our body temperature decreases. This cooling effect helps maintain our body core temperature within the optimal range, preventing overheating and allowing us to continue exercising efficiently.

Step 4: 4. Connection between temperature, pressure, and states of matter

For part (b) of the exercise, it is crucial to understand the relationships between temperature, pressure, and the states of matter. The boiling point of a substance is the temperature at which the vapor pressure of the substance equals the ambient pressure. In a vacuum, the ambient pressure is considerably lower than at standard atmospheric pressure, which causes the water to boil at a lower temperature. Similarly, the freezing point of a substance depends on the surrounding pressure. When the pressure is significantly reduced, the freezing point of water can decrease. In this context, the vacuum pump helps reduce the pressure.

Step 5: 5. Water boiling in a vacuum

When the vacuum pump is turned on, it removes air from the flask decreasing the pressure inside. The lower pressure means that water will start boiling at a lower temperature. Thus, the water begins to boil even if it was initially at room temperature, as the conditions inside the flask change.

Step 6: 6. Water freezing in a vacuum

As the water continues to boil in the vacuum, it loses energy through the conversion of liquid water into water vapor. This loss of energy results in a decrease in the water's temperature. With continued pumping and sufficient time, the temperature of the water will drop below its freezing point (under the low pressure condition), causing the water to freeze.