Problem 88 - 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) Vigorous exercise increases body temperature due to a higher metabolic rate. To cool down, the thermoregulation system activates sweat glands to produce sweat. As sweat evaporates from the skin's surface, heat is removed from the body and transferred to the environment, lowering body temperature. (b) In the vacuum pump setup, the air pressure inside the flask decreases, causing the boiling point of water to decrease. Water starts to boil at room temperature. As the water boils, it evaporates and removes heat from the remaining water, thus lowering the temperature. When the temperature of the water drops below its freezing point under low-pressure conditions, the water begins to freeze. This demonstrates the effect of vacuum conditions on boiling and freezing points of a substance.

Step 1: Understanding how sweating cools the human body

Sweating is the body's natural way of cooling itself down by producing sweat, which is mostly water, from the sweat glands. When the body temperature rises, the sweat glands produce sweat which is released onto the skin's surface. As the sweat evaporates, the heat is removed from the body and transferred to the environment, ultimately lowering the body temperature.

Step 2: Explaining why vigorous exercise leads to sweating

Vigorous exercise increases the body's metabolic rate, which generates more heat as a byproduct. This excess heat causes the body temperature to rise. In response, the thermoregulation system of the body activates the sweat glands to produce more sweat, which helps to cool the body down as the sweat evaporates from the skin's surface.

Step 3: Understanding the vacuum pump setup

A vacuum pump setup consists of a flask of water connected to a vacuum pump, which removes air from the flask. As the vacuum pump operates, air molecules present inside the flask are removed and the air pressure decreases.

Step 4: Explaining why water starts to boil

Boiling is the process of liquid converting into vapor at a specific temperature and pressure. The boiling point of water is determined by the ambient pressure. As the vacuum pump decreases the air pressure inside the flask, the boiling point of water also decreases. Eventually, the boiling point of water becomes lower than the room temperature, and water starts to boil at room temperature.

Step 5: Explaining why water freezes after boiling

As the water boils inside the flask, it evaporates and removes heat from the remaining water, similar to the process of sweating discussed in part (a). This continuous evaporation and heat removal cause the temperature of the remaining water to drop. When the temperature of the water goes below its freezing point under the given low-pressure conditions, the water begins to freeze. This process demonstrates that under vacuum conditions, the boiling and freezing points of a substance can be significantly affected.