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Energy - Condensation and Evaporation

Grade 9IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Evaporation is the process by which a liquid turns into a gas at its surface, occurring at temperatures below the boiling point.

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According to Kinetic Theory, particles in a liquid have a distribution of kinetic energies (KEKE). Only the fastest-moving particles near the surface have enough energy to overcome intermolecular forces and escape as vapor.

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The Cooling Effect: When high-energy particles escape during evaporation, the average KEKE of the remaining particles decreases. Since temperature is proportional to average KEKE (T∝KEavgT \propto KE_{avg}), the temperature of the liquid falls.

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Factors increasing the rate of evaporation include: Increased surface area, higher temperature, increased wind speed (airflow), and decreased humidity.

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Condensation is the process where a gas changes into a liquid. This occurs when gas particles lose thermal energy and their KEKE decreases, allowing intermolecular bonds to reform.

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Energy is conserved during phase changes. The energy required to change a liquid to a gas is called the Latent Heat of Vaporization (LvL_v), and this same amount of energy is released during condensation.

📐Formulae

Q=mLvQ = m L_v

KE=12mv2KE = \frac{1}{2} m v^2

ΔQ=P×t\Delta Q = P \times t

💡Examples

Problem 1:

Calculate the amount of thermal energy required to evaporate 0.25 kg0.25 \text{ kg} of water at 100∘C100^{\circ}\text{C}. (Specific latent heat of vaporization of water Lv=2.26×106 J/kgL_v = 2.26 \times 10^6 \text{ J/kg})

Solution:

Q=mLvQ = m L_v Q=0.25 kg×2.26×106 J/kgQ = 0.25 \text{ kg} \times 2.26 \times 10^6 \text{ J/kg} Q=5.65×105 JQ = 5.65 \times 10^5 \text{ J}

Explanation:

To find the energy required for a phase change without a temperature change, we use the latent heat formula where mm is the mass and LvL_v is the specific latent heat of vaporization.

Problem 2:

A person feels cold when they step out of a swimming pool on a windy day. Explain this phenomenon using the kinetic theory of matter.

Solution:

The water on the person's skin begins to evaporate. The most energetic water molecules escape into the air. This reduces the average KEKE of the remaining water molecules, lowering the temperature. The wind increases the rate of evaporation by moving vapor away from the skin, leading to rapid heat loss from the body.

Explanation:

This demonstrates the cooling effect of evaporation. The body provides the latent heat required for the water to change phase, resulting in a sensation of coldness.