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

Sweating helps cool the body during vigorous exercise by evaporating and absorbing heat from the skin, regulating body temperature. In the flask connected to a vacuum pump, the water boils and freezes due to changing pressure conditions. As the pressure inside the flask decreases, the boiling point of the water lowers, causing it to boil. This endothermic process absorbs heat, and as the temperature decreases, the remaining water freezes under the new pressure conditions.

Step 1: Understanding the cooling effect of sweating

When you exercise vigorously, your body generates heat, which increases the core temperature. Sweating is a natural process that helps regulate body temperature. When sweat is produced, it's released onto the skin's surface. As the sweat evaporates, it absorbs energy (heat) from the skin, which leads to a cooling effect. To summarize the cooling process of sweating: 1. You exercise, generating heat and increasing core body temperature. 2. To avoid overheating, your body produces sweat. 3. Sweat covers the skin surface and absorbs heat from the skin as it evaporates. 4. The skin cools as heat is absorbed by the evaporating sweat, ultimately helping to control body temperature.

Step 2: Understanding the boiling and freezing of water in a flask connected to a vacuum pump

This interesting phenomenon of water boiling and freezing in a flask connected to a vacuum pump can be explained by changing pressure conditions inside the flask and their impact on the water's boiling and freezing points. 1. When the vacuum pump is turned on, it removes air molecules (and hence pressure) from inside the flask. This lowers the boiling point of the water. 2. Due to the lowered pressure, the boiling point of water becomes lower than the temperature of the water, causing the water to start boiling. 3. As the water boils, it changes from a liquid to a gas, which is an endothermic process (absorbs heat). As the boiling process absorbs heat, the temperature of the remaining water starts to decrease. 4. If the vacuum pump continues to lower the pressure inside the flask, the temperature of the remaining water will eventually reach its freezing point under the new pressure conditions, causing the water to freeze.

Step 3: Summary

In this exercise, we explained two thermodynamic processes: the cooling effect of sweating and the boiling and freezing of water in a flask connected to a vacuum pump. Sweating cools the body by evaporating sweat and absorbing heat from the skin, while water boils and freezes in a flask connected to a vacuum pump due to changes in pressure, ultimately resulting in the lowered boiling point of water and heat absorption during the change from liquid to gas, eventually reaching the freezing point at the new pressure conditions.

Key Concepts

Cooling Effect

When you engage in vigorous physical activity, your body produces extra heat. To cool down, the body has a clever mechanism in place: sweating. Sweat glands produce moisture, which sits on the surface of your skin. This is not just any moisture, but a cooling agent. As sweat evaporates, it takes heat away from the skin, thus cooling you down.

Here’s how it works:

• Your internal body temperature rises due to exertion.
• To counteract this, your body releases sweat onto the skin.
• The sweat absorbs heat from your body as it transitions from liquid to vapor.
• This evaporation process removes heat, thus cooling the skin and helping regulate body temperature.

Nature's way of sweating is a practical example of the cooling effect caused by evaporation.

Evaporation

Evaporation is a fascinating process where a liquid turns into a gas, often observed in everyday life when water disappears from a wet surface. This process is crucial in cooling systems, including how sweating cools the body. During evaporation, molecules at the liquid's surface gain enough energy to break free and become vapor.

Here’s the breakdown:

• Molecules escape from the liquid's surface.
• Heat energy is required for this phase change.
• The environment loses heat as it is absorbed by the evaporating molecules.
• The remaining liquid becomes cooler as the faster, more energetic molecules leave.

In essence, evaporation is not just about a liquid turning into gas, but it is also about heat transfer that leads to cooling.

Pressure Changes

Pressure changes have a profound impact on phase changes of substances. When a vacuum pump is used to extract air from a container with water, it decreases the surrounding pressure. This reduction in pressure alters the traditional boiling and freezing points of water.

Consider this sequence:

• Removing air decreases pressure in the flask.
• Lower pressure means a lower boiling point, so water can boil at room temperature.
• Boiling is an endothermic process, meaning it absorbs heat, cooling the remaining water.
• This continuous pressure drop can eventually lead to freezing as the water's temperature drops below its adjusted freezing point under low pressure.

Pressure plays a key role in how substances transition between solid, liquid, and gas phases.

Boiling Point

The boiling point of water is traditionally understood as the temperature at which it turns into vapor. But did you know that this point can change? It depends on the surrounding pressure. Normally, water boils at 100°C (212°F) under standard atmospheric conditions. However, if the pressure decreases—such as inside a vacuum flask—the boiling point is lowered.

Here's why:

• Boiling occurs when vapor pressure equals the surrounding pressure.
• Less atmospheric pressure requires less heat for the vapor pressure to equalize.
• Water can boil at much lower temperatures under reduced pressures.

Adjusting pressure shows us that boiling points are not fixed, but adaptable to environmental conditions.

Freezing Point

Freezing point refers to the temperature at which a liquid becomes a solid. Much like boiling point, pressure can influence when this occurs. Normally, water freezes at 0°C (32°F), but under specific conditions such as low pressure inside a vacuum, the process changes.

Here’s how:

• In lower pressure environments, boiling may initially cool the water.
• Continual heat loss allows the water's temperature to drop further.
• Once the reduced temperature reaches the adjusted freezing point, water solidifies.

The freezing point, therefore, is also subject to change with varying pressure, illustrating the dynamic nature of phase changes in thermodynamics.