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Chapter 11: Problem 83

(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.

Short Answer

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(a) When exercising vigorously, the body generates heat due to increased metabolism. Sweating helps to cool the body by releasing sweat onto the skin's surface, where it evaporates. This evaporation process requires heat, which is absorbed from the body, resulting in a cooling effect. (b) When a vacuum pump is connected to a flask of water, it reduces the pressure inside the flask. This decrease in pressure lowers the boiling point of water, causing it to boil at room temperature. Boiling is an endothermic process that absorbs heat from the surroundings, including the remaining water in the flask. As the water evaporates and loses heat, the temperature of the remaining water decreases, eventually reaching its freezing point under low-pressure conditions, resulting in the water freezing.

Step by step solution

01

Part (a): Explaining how sweating helps cool the body

When we exercise, our body generates heat as a result of increased metabolic activities. Sweating is a natural mechanism to maintain a stable body temperature. Sweat mainly consists of water, salts, and other minerals. As we sweat, water evaporates from the surface of our skin, which requires heat. This heat is absorbed from our body, resulting in a cooling effect. Let's break the process down into steps: 1)
02

Exercise increases body temperature

During vigorous exercise, our body generates more heat due to increased metabolism and muscle activity. This causes an increase in body temperature. 2)
03

Sweating as a response to increased body temperature

As our body temperature increases, the thermoregulatory center in our brain (hypothalamus) detects the change and initiates the sweating mechanism. 3)
04

Evaporation of sweat from the skin surface

Sweat is released onto the skin's surface, where it evaporates. Evaporation is an endothermic process, meaning that it requires energy in the form of heat. The heat is taken from our body, which helps to cool us down. 4)
05

Cooling effect of sweat evaporation

As the heat is absorbed from our body to evaporate the sweat, our body temperature decreases, resulting in the cooling effect.
06

Part (b): Explaining the boiling and freezing of water in a flask connected to a vacuum pump

When a vacuum pump is connected to a flask of water, the pressure inside the flask decreases. The phase changes of water (boiling and freezing) are dependent on both temperature and pressure conditions. 1)
07

Vacuum pump reduces pressure

When the vacuum pump is turned on, it removes air molecules from the flask, thereby lowering the pressure inside the flask. 2)
08

Boiling of water at lower pressure

Under lower pressure conditions, the boiling point of water decreases. As the pressure inside the flask decreases, the boiling point of water decreases to a point where it starts to boil at room temperature. This is why the water begins to boil once the vacuum pump is turned on. 3)
09

Evaporation of water absorbs heat

Boiling of water is an endothermic process, meaning that it requires energy in the form of heat. As the water evaporates and boils, it absorbs heat from the surroundings, including the remaining water in the flask. 4)
10

Freezing of water due to heat loss

As the water continues to boil and lose heat, the temperature of the remaining water in the flask decreases. Eventually, the temperature drops to the freezing point of water under the low-pressure condition. Consequently, the remaining water in the flask starts to freeze.

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