Melting, boiling and evaporation

When a substance is melting or boiling, the energy input is used to overcome the forces of attraction between the particles (atoms or molecules) of the substance. This energy increases the potential energy of the particles, allowing them to move more freely and change state. Because the energy is being used to change the state, it does not increase the kinetic energy of the particles. Therefore the temperature remains constant during the melting or boiling process.

The energy supplied is used to break the bonds between the water molecules in the ice. This allows them to move more freely and transition from a solid to a liquid state. This is also called the latent heat of fusion.

The temperature remains constant at 0°C (the melting point of ice) because the thermal energy (heat) from the room is used to break the bonds between the water molecules in the ice, allowing it to change state from solid to liquid. This energy is absorbed as latent heat of fusion and does not raise the temperature of the mixture until all the ice has melted.

During condensation, water particles move closer together. The particles lose kinetic energy and slow down. The arrangement of the particles becomes more ordered as they transition from a more random liquid state to a more closely packed liquid state at the same temperature.

As water vapour cools to liquid and then solidifies to ice, the forces of attraction *between* the water molecules increase significantly. In water vapour, the forces are weak; in liquid water, the forces are stronger; and in ice, they are at their strongest, holding the particles in a fixed lattice structure.

Evaporation occurs when more-energetic particles at the surface of a liquid gain sufficient kinetic energy to overcome the attractive forces holding them in the liquid. These particles then escape into the gas phase. These energetic particles leave, so the average energy of the remaining particles decreases.

1. Increasing the temperature of the water.
2. Increasing the surface area of the water.

Explanation: Higher temperature provides more particles with sufficient energy to escape. Larger surface area provides more locations for energetic particles to break free.

Evaporation is the escape of the most energetic (fastest moving) molecules from the surface of a liquid.

When these energetic molecules leave, the average kinetic energy of the remaining molecules in the liquid decreases.

Since temperature is a measure of the average kinetic energy of the molecules, the temperature of the liquid decreases, resulting in cooling.

1. Sweating: When sweat evaporates from the skin, it cools the body.
2. Alcohol on the skin: When alcohol is rubbed on the skin, it evaporates quickly and the skin feels cooler.

- Boiling occurs at a specific temperature (boiling point), while evaporation occurs at any temperature below the boiling point. (1 mark)
- Boiling occurs throughout the entire liquid, while evaporation occurs only at the surface. (1 mark)
- Boiling is a much faster process than evaporation. (1 mark)

- Temperature: Higher temperature increases the kinetic energy of the molecules, allowing more of them to overcome the intermolecular forces and escape into the gaseous phase, increasing the rate of evaporation. (1 mark)
- Surface Area: A larger surface area provides more space for molecules to escape from the liquid, thereby increasing the rate of evaporation. (1 mark)
- Airflow/Wind: Airflow sweeps away the vapor molecules above the liquid, reducing the vapor pressure and allowing more molecules to evaporate. (1 mark)

Increasing the temperature increases the rate of evaporation. (1 mark)

Explanation: Higher temperature means water molecules have more kinetic energy. (1 mark) This allows more molecules to overcome the intermolecular forces holding them in the liquid phase and escape into the gas phase as vapour.

1. Increase the surface area of the clothes. (1 mark)
Explanation: Larger surface area allows more water molecules to be exposed to the air, increasing the opportunity for evaporation.
2. Increase the air movement around the clothes (

1. Alcohol molecules require energy to overcome the intermolecular forces holding them in the liquid state and transition into the gaseous state during evaporation.
2. This energy is taken from the skin in the form of thermal energy (heat).
3. As the skin loses thermal energy, its temperature decreases, resulting in the sensation of cooling.

1. Temperature of the alcohol/metal block.
2. Surface area of the alcohol drop.
3. Airflow over the alcohol drop (wind/draughts).