B.1.2 Phase changes and energy transfer

Phase Changes: Particle Behavior and Energy Transfer

What Happens to Particles During Phase Changes?

During a phase change, a substance transitions between solid, liquid, and gas states.
These changes occur at a constant temperature, where energy is used to alter the arrangement of particles rather than their speed.

Tip

Temperature measures the average kinetic energy of particles, while phase changes involve changes in potential energy.

Melting and Freezing

Melting occurs when a solid becomes a liquid.
- As energy is added, particles vibrate more vigorously until they overcome the forces holding them in fixed positions.
Conversely, freezing is the process where a liquid becomes a solid.
- Energy is removed, causing particles to slow down and settle into a structured arrangement.

Example

When ice melts, the temperature remains at 0°C until all the ice has turned into water. During this time, the energy added increases the potential energy of the particles, not their kinetic energy.

Boiling and Condensation

Boiling is the transition from liquid to gas.
- At the boiling point, energy is used to break the intermolecular forces, allowing particles to move freely.
Condensation is the reverse process, where gas particles lose energy and form a liquid.

Note

Boiling occurs throughout the liquid at a specific temperature, while evaporation happens only at the surface and can occur at any temperature.

Evaporation

Evaporation is a surface phenomenon where the fastest-moving particles escape into the gas phase.
- This process cools the remaining liquid because the average kinetic energy of the particles decreases.

Tip

Evaporation is why sweating cools you down. As sweat evaporates, it absorbs energy from your skin, reducing your body temperature.

Quantitative Analysis of Heat Transfer

Specific Heat Capacity

Definition

Specific heat capacity

The specific heat capacity $c$ of a substance is the energy required to raise the temperature of 1 kg of the substance by 1 K.

The formula for calculating the energy transferred is:

$Q = m c Δ T$

where:

$Q$ is the energy transferred (in joules, J)
$m$ is the mass (in kilograms, kg)
$c$ is the specific heat capacity (in $J k g^{- 1} K^{- 1}$ )
$Δ T$ is the change in temperature (in kelvin, K)

Example question

How much energy is needed to raise the temperature of 2 kg of water from 20°C to 80°C? (Specific heat capacity of water = $4200 J k g^{- 1} K^{- 1}$ )

Solution

Calculate the temperature change: $Δ T = 80 ° C - 20 ° C = 60 K$
Use the formula: $Q = m c Δ T = 2 \times 4200 \times 60 = 504, 000, J$

Specific Latent Heat

Definition

Specific latent heat

Specific latent heat $L$ is the energy required to change the phase of 1 kg of a substance at constant temperature.

The formula is:

$Q = m L$

where:

$Q$ is the energy transferred (in joules, J)
$m$ is the mass (in kilograms, kg)
$L$ is the specific latent heat (in $J k g^{- 1}$ )

There are two types of specific latent heat:

Latent heat of fusion ( $L_{F}$ ): Energy required to change 1 kg of a substance from solid to liquid (or vice versa).
Latent heat of vaporization ( $L_{V}$ ): Energy required to change 1 kg of a substance from liquid to gas (or vice versa).

Note

The latent heat of vaporization is typically much larger than the latent heat of fusion because breaking the bonds between liquid molecules requires more energy than loosening the bonds in a solid.

Example question

How much energy is needed to melt 0.5 kg of ice at 0°C? (Latent heat of fusion of ice = $334, 000 J k g^{- 1}$ )

Solution

Use the formula: $Q = m L_{F}$

$= 0.5 \times 334, 000 = 167, 000 J$

Why Does Temperature Stay Constant During Phase Changes?

During a phase change, the energy added or removed changes the potential energy of the particles, not their kinetic energy.

This is why the temperature remains constant.

Example

When boiling water, the temperature stays at 100°C until all the water has turned into steam. The energy supplied is used to break the intermolecular forces, not to increase the speed of the particles.

Self-Assessment

Theory of Knowledge

How does our understanding of phase changes and heat transfer influence the design of energy-efficient technologies?
Could this knowledge help address global challenges like climate change?

Self review

What happens to the temperature of a substance during a phase change?
How is the energy required to heat a substance calculated?
Why is the latent heat of vaporization greater than the latent heat of fusion for most substances?

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