Question

Alcohol wiped across a tabletop rapidly disappears. What happens to the temperature of the tabletop? Why?

Short answer

The temperature of the tabletop decreases due to evaporative cooling.

Step-by-step solution

Understanding Evaporation

When alcohol is wiped across the tabletop, it begins to evaporate. Evaporation is a process where a liquid turns into a vapor.

Evaporation and Energy

For evaporation to occur, the molecules of alcohol absorb energy from their surroundings to change from a liquid state to a gas state. This energy is required to overcome the intermolecular forces in the liquid state.

Effect on Temperature

The energy absorbed by the alcohol molecules is taken from the tabletop, which causes the tabletop to lose some of its thermal energy.

Resulting Temperature Change

As a result of losing thermal energy, the temperature of the tabletop decreases. This phenomenon is often referred to as evaporative cooling.

Key concepts

Evaporative Cooling

Evaporative cooling is a fascinating process that occurs when a liquid, such as alcohol, evaporates. During evaporation, molecules escape from the liquid surface into the air as vapor. This requires energy, typically drawn from the surrounding environment. When alcohol evaporates from a tabletop, it absorbs heat energy from the surface.
This energy transfer results in a decrease in temperature, which causes a cooling effect. This cooling is what we call evaporative cooling. It's the same principle that keeps our bodies cool through sweating.

• Evaporation takes heat from the surroundings.
• This decreases the surface temperature.
• It is a natural cooling mechanism.

Drinking cold water on a hot day or having a fan blow across a wet surface can enhance this effect.

Thermal Energy

Thermal energy is the energy that an object possesses due to the motion of its molecules. In simpler terms, it's what we often refer to as heat. Every object has thermal energy, and it plays a significant role in how the object interacts with its environment.
When alcohol evaporates from the tabletop, it absorbs some of this thermal energy. As the alcohol molecules gain energy, they escape into the air, leaving behind a cooler surface.

• Thermal energy is linked to temperature.
• Loss of thermal energy leads to cooling.
• It's crucial for processes like evaporation.

Understanding thermal energy helps explain why things heat up or cool down during physical changes. It acts as a bridge between energy changes and temperature changes.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together, playing a key role in defining the properties of a substance. They determine how a liquid behaves, its boiling point, and its ability to evaporate. For alcohol to evaporate, the intermolecular forces in the liquid must be overcome.
These forces, which include hydrogen bonds and van der Waals forces, require energy to break. When alcohol is on a tabletop, the energy needed to overcome these forces comes from the tabletop itself, causing it to lose heat.

• Intermolecular forces dictate liquid behavior.
• Stronger intermolecular forces mean higher energy is needed for evaporation.
• Breaking these forces is why energy is absorbed from the surroundings.

This makes intermolecular forces essential in understanding evaporative processes and why they cause a cooling effect.