IGCSE Physics 0625

2.2.3 Melting, Boiling and Evaporation

Objective: Explain the cooling of an object in contact with an evaporating liquid

When a liquid evaporates, it absorbs energy from its surroundings. This is because the molecules in the liquid gain enough kinetic energy to overcome the intermolecular forces holding them together and escape into the gaseous phase. This energy is taken from the cooler object in contact with the evaporating liquid, causing a decrease in its temperature.

Explanation

The cooling effect is a direct consequence of the latent heat of evaporation. Latent heat is the energy absorbed or released during a phase change without a change in temperature. In the case of evaporation, the energy absorbed is the latent heat of vaporization.

Consider an object at a temperature above the boiling point of a liquid. When the liquid evaporates in contact with the object, the water molecules gain energy. This energy comes from the object itself. As the object loses energy, its temperature decreases. This process continues until the object's temperature reaches the boiling point of the liquid, at which point the object will start to boil.

Key Concepts

• Evaporation: The process where molecules at the surface of a liquid gain enough kinetic energy to escape into the gaseous phase.
• Latent Heat of Vaporization: The amount of energy required to change a substance from a liquid to a gas at a constant temperature.
• Heat Transfer: The flow of thermal energy from a hotter object to a colder object. In this case, heat is transferred from the object to the evaporating liquid.

Factors Affecting the Rate of Evaporation

1. Surface Area: A larger surface area allows for a faster rate of evaporation.
2. Temperature: Higher temperatures increase the rate of evaporation.
3. Humidity: Lower humidity increases the rate of evaporation.
4. Airflow: Airflow removes water vapour from the surface, increasing the rate of evaporation.

Quantitative Relationships

The amount of heat absorbed by the evaporating liquid is related to the mass of the liquid and its latent heat of vaporization. The equation is:

$$Q = m \times L_v$$

Where:

• Q is the amount of heat absorbed (in Joules or Calories).
• m is the mass of the evaporating liquid (in kg or grams).
• L_v is the latent heat of vaporization of the liquid (in J/kg or cal/g).

Example

Imagine a cold metal block placed in a dish of boiling water. The water evaporates, absorbing heat from the metal block. This causes the metal block to cool down.

Concept	Description
Evaporation	Process of liquid turning into gas.
Latent Heat	Energy absorbed or released during a phase change.
Cooling Effect	Decrease in temperature of an object due to heat absorption by evaporating liquid.