(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.

During vigorous exercise, the body's temperature rises due to the increased energy production in the muscle cells. To maintain thermal balance and prevent overheating, the body releases sweat - a mixture of water, minerals, and other compounds - which comes to the surface of the skin through the sweat glands.

When sweat is released onto the skin, it evaporates into the surrounding air. Evaporation is a process in which the liquid (sweat) turns into a vapor, which requires energy in the form of heat to occur. The heat needed for evaporation is taken from the body, thus cooling it down. This is how sweating helps to regulate the body's temperature during vigorous exercise.

When the flask of water is connected to a vacuum pump, the pump starts to remove the air molecules from the flask, creating a decrease in pressure inside the flask.

As the pressure inside the flask decreases, the boiling point of water also decreases. Eventually, the pressure becomes low enough that the boiling point of the water reaches room temperature, causing the water to boil. As the vacuum pump continues to decrease the pressure, the boiling point drops further below the freezing point. At this point, the water loses more energy in the form of heat than it receives due to boiling and begins to freeze. To summarize: Steps for (a): 1. Understand the role of sweating in maintaining thermal balance during vigorous exercise. 2. Explain how evaporation of sweat helps to cool down the body. Steps for (b): 1. Describe the process of pressure reduction inside the flask when connected to the vacuum pump. 2. Explain the relationship between pressure, boiling point, and freezing point. 3. Discuss the reasons behind the observed boiling and freezing of water in the flask due to a decrease in pressure.