Problem 83 - Chemistry The Central Science In Si Units

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 cools the body through evaporative cooling. In a vacuum, reduced pressure lowers the boiling point, causing water to boil and eventually freeze due to energy loss during evaporation.

Step 1: Understanding Sweat and Cooling

When you exercise vigorously, your body generates heat. Sweating helps cool the body because when sweat evaporates from the skin, it absorbs heat energy from the body. This process, known as evaporative cooling, helps maintain a stable body temperature by removing heat from the body surface.

Step 2: Boiling Point and Pressure Relationship

In the flask connected to a vacuum pump, reducing pressure causes the boiling point of the water to decrease. When the boiling point drops below the water's temperature, it begins to boil at room temperature due to reduced pressure. This is because boiling occurs when the vapor pressure of the liquid equals the surrounding pressure, which is lowered by the vacuum.

Step 3: Freezing during Evaporation

As the water in the flask boils, it undergoes a phase change from liquid to gas. This phase change requires energy, which is absorbed from the remaining liquid water, causing its temperature to drop. If the temperature falls below the water's freezing point, the water can freeze despite the presence of a vacuum, as energy loss due to continuous evaporation outweighs heat generation.

Key Concepts

Boiling Point and Pressure

The boiling point of a liquid is the temperature at which it changes from a liquid to a gas. Normally, this occurs when the vapor pressure inside the liquid equals the surrounding atmospheric pressure. However, when the pressure around a liquid decreases, such as in a vacuum, the boiling point also decreases. This means that the liquid can begin to boil at a much lower temperature than it would under normal atmospheric conditions.

In the context of a vacuum pump and a flask of water, the reduced pressure leads to an interesting phenomenon: water can boil at room temperature. Under these conditions, the surrounding pressure is much lower due to the vacuum, allowing the water to boil without needing to be heated. This is a perfect demonstration of the relationship between boiling point and pressure.

• Decreased pressure: Lowers boiling point
• Vapor pressure: Must equal surrounding pressure for boiling to occur

Understanding this relationship helps explain how changes in pressure affect the phase stability of liquids.

Phase Change

Phase changes are transitions between different states of matter: solid, liquid, and gas. Common phase changes include melting, freezing, boiling, and condensation. During these changes, a substance can absorb or release energy without changing temperature. For instance, in the process of boiling, a liquid absorbs energy to overcome intermolecular forces and transition into vapor.

When a liquid, like water, boils in a vacuum, it goes through a phase change from liquid to gas as it vaporizes. This process utilizes energy from the surrounding liquid, causing the temperature to drop. If the energy absorbed leads to a decrease in temperature below the freezing point, the water can start to freeze, even while boiling.

• Energy absorption: Necessary for phase changes
• Temperature drop: Can lead to freezing during boiling

This paradoxical behavior highlights the complex nature of phase changes, where multiple transformations can occur simultaneously under unique conditions.

Thermodynamics

Thermodynamics is the branch of physics that deals with heat, work, and the internal energy of systems. One key aspect is the understanding of how energy is transferred and transformed. In the case of sweating and boiling water in a vacuum, thermodynamic principles are at work.

When you sweat, the evaporation process is a cooling mechanism that relies on energy transfer. The body releases heat energy as sweat evaporates, thus cooling the skin's surface. Similarly, boiling and subsequent freezing of water in a vacuum implement the principles of energy absorption and conservation. As water vaporizes, it absorbs energy, leading to a decrease in temperature. Eventually, this cooling effect can lead the remaining liquid water to freeze.

• Energy transfer: Central to evaporative cooling
• Heat absorption: Leads to phase transitions

These examples underscore the importance of thermodynamics in everyday processes and illustrate how energy changes can affect the state and temperature of a substance.