Question

Use your knowledge of changes of state to explain the following: a. How does perspiration during heavy exercise cool the body? b. Why do towels dry more quickly on a hot summer day than on a cold winter day? c. Why do wet clothes stay wet in a plastic bag?

Short answer

Sweat cools the body through evaporation, towels dry faster on hot days due to increased thermal energy, and wet clothes stay wet in a plastic bag because it is impermeable.

Step-by-step solution

Understanding Perspiration and Body Cooling

When exercising heavily, the body produces sweat. This sweat, which is primarily water, evaporates from the surface of the skin. Evaporation is a cooling process because it requires heat energy to change the state from liquid to gas. This heat energy is taken from the body, cooling it down.

Drying Towels on Hot vs Cold Days

On a hot summer day, the air temperature is higher. Higher temperature means more thermal energy is available for water molecules in the towel. This energy allows the water molecules to overcome the hydrogen bonding and transition into a gas phase more rapidly, thereby drying the towel faster. On a cold winter day, there is less thermal energy, so this process happens much slower.

Wet Clothes in a Plastic Bag

Plastic bags are not permeable, meaning they do not allow the passage of air. For water to evaporate, it needs to escape into the air. In a plastic bag, because there is no airflow, the water cannot evaporate into the surrounding environment, and the clothes stay wet.

Key concepts

Evaporation

Evaporation is the process where liquid water changes into water vapor, a gas. This happens when water molecules gain enough energy to break free from the liquid surface.
In evaporation, the molecules transition from liquid to gas at temperatures below boiling. This process is always occurring, whether you see it or not.
For example, when you sweat during heavy exercise, your body uses evaporation to cool down. The sweat (liquid) on your skin absorbs heat from your body and becomes vapor (gas), thus cooling you off.
Evaporation is crucial because it helps regulate body temperature and also explains phenomena like drying towels or wet clothes in open air.

• Requires heat energy
• Converts liquid to gas
• Occurs at temperatures below boiling point

Thermal Energy

Thermal energy refers to the total kinetic energy of particles in a substance. It's all about how much energy particles have while they move around.
Higher temperatures mean higher thermal energy because particles move faster. For example, on a hot summer day, water in your towel gets more thermal energy, which helps it evaporate faster.
On a cold winter day, the water molecules in your towel have less thermal energy, so they move slower. As a result, evaporation takes longer.

• Higher temperatures provide more thermal energy
• Helps in faster evaporation
• Essential for changes of state

Hydrogen Bonding

Hydrogen bonding is a type of attractive force between molecules, particularly in water. It occurs when hydrogen atoms bond with oxygen, nitrogen, or fluorine atoms.
In water, hydrogen bonds make the molecules stick together. This is why water has a relatively high boiling point.
During evaporation, water molecules need enough energy to break these hydrogen bonds. On hot days, the energy available is higher, making it easier for water molecules to escape into the air.

• Strong attractive forces
• Makes water molecules stick together
• Requires energy to break during evaporation

Cooling Process

The cooling process often involves reducing the thermal energy of a substance. When you sweat, your body loses heat through the cooling process of evaporation.
When water changes from liquid to gas, it absorbs heat from the surface it evaporates from. This absorption of heat results in cooling.
On a hot day, you might notice how sweat dries faster and keeps you cooler. This is due to faster evaporation.

• Involves heat loss
• Key mechanism for temperature regulation
• Relies on evaporation

Permeability

Permeability refers to how easily substances (like air or liquids) can pass through a material. Plastic bags, for instance, are impermeable—they block air and water.
When you put wet clothes in a plastic bag, the lack of air circulation means water cannot evaporate. This is why the clothes stay wet.
On the other hand, a breathable fabric allows water vapor to escape, leading to drying. Understanding permeability is important for practical reasons, such as how to store wet items.

• Measures air/liquid passage
• Plastic bags are impermeable
• Airflow is necessary for evaporation