CIE IGCSE Physics (0625) Melting, boiling and evaporation Study Notes - New Syllabus
CIE IGCSE Physics (0625) Melting, boiling and evaporation Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Melting, boiling and evaporation
Key Concepts:
- Melting and Boiling
- Condensation and Solidification
- Evaporation
- Differences Between Boiling and Evaporation
Factors Affecting the Rate of Evaporation
Melting and Boiling
Melting
When a solid turns into a liquid (e.g. ice to water), energy is added in the form of heat. This energy is called the latent heat of fusion. During this phase change:
- The temperature remains constant.
- The added energy is used to break the intermolecular forces holding the particles in a fixed structure.
Boiling
When a liquid turns into a gas (e.g. water to steam), energy is added as the latent heat of vaporization. During boiling:
- The temperature remains constant.
- The added energy is used to overcome the attractive forces between the liquid molecules to allow them to escape into the gas phase.
Melting and Boiling Points of Water (at Standard Atmospheric Pressure)
| Property | Value |
|---|---|
| Melting Point of Water | 0°C (273.15 K) |
| Boiling Point of Water | 100°C (373.15 K) |
Note: These values are at standard atmospheric pressure (1 atm or 101.3 kPa).
Example:
A block of ice at 0°C is heated until it completely melts and then the water is boiled away to form steam at 100°C, all at standard atmospheric pressure. Describe the energy changes during the melting and boiling processes.
▶️ Answer/Explanation
Melting (Ice at 0°C → Water at 0°C):
- Heat energy is supplied to the ice at 0°C.
- Temperature does not increase during melting.
- Energy goes into breaking the strong hydrogen bonds between water molecules in the solid state.
- This energy is called the latent heat of fusion.
Heating (Water from 0°C to 100°C):
- After melting, the temperature of water rises from 0°C to 100°C.
- This part involves an increase in temperature (not a phase change).
Boiling (Water at 100°C → Steam at 100°C):
- At 100°C, water starts to boil.
- Temperature stays constant at 100°C during boiling.
- Energy is used to overcome the remaining attractive forces between water molecules to allow them to become gas particles.
- This energy is called the latent heat of vaporization.
Energy is absorbed during both melting and boiling, but temperature does not rise. Instead, this energy is used to break intermolecular forces—first to turn solid into liquid (melting), and then to turn liquid into gas (boiling).
Condensation and Solidification
Condensation
This is the process where a gas changes into a liquid.
- As the gas cools down, the particles lose energy.
- They move more slowly and come closer together.
- The attractive forces between the particles become strong enough to hold them in the liquid state.
- Condensation occurs at the boiling point (100°C for water at 1 atm), but in reverse.
Solidification (Freezing)
This is the process where a liquid turns into a solid.
- As the liquid cools further, particles lose even more energy.
- They slow down until they are held in a fixed, orderly arrangement.
- Strong intermolecular forces lock the particles into position, forming a solid.
- Solidification occurs at the melting point (0°C for water at 1 atm), but in reverse.
Summary (Particle Comparison):
| Process | State Change | Particle Behavior |
|---|---|---|
| Condensation | Gas → Liquid | Particles lose energy, move slower, come closer together |
| Solidification | Liquid → Solid | Particles lose more energy, become fixed in place |
Example:
1. A student heats a block of ice at 0°C until it completely melts.
2. The water is then heated until it boils and turns into steam at 100°C.
All this happens at standard atmospheric pressure.
Describe what happens to the energy during the melting and boiling stages.
▶️ Answer/Explanation
Melting stage (0°C):
- The temperature stays constant at 0°C.
- Energy is added as latent heat of fusion.
- This energy is used to break the strong forces between solid water (ice) molecules.
- Particles gain energy and move more freely, changing from fixed positions to sliding past each other.
Boiling stage (100°C):
- The temperature stays constant at 100°C.
- Energy is added as latent heat of vaporization.
- This energy is used to completely overcome the attractive forces between liquid molecules.
- The particles gain enough energy to escape as gas and move freely.
Important Point:
During both melting and boiling, energy is added but the temperature does not rise. This energy goes into changing the state of matter, not increasing the kinetic energy of the particles.
Final Answer: Energy is required during both melting and boiling to overcome intermolecular forces, but temperature remains constant during each process.
Evaporation
Evaporation
Evaporation is the process by which particles escape from the surface of a liquid and become gas, even below the boiling point.
- Only particles at the surface of the liquid can evaporate.
- These surface particles must have enough kinetic energy to overcome the attractive forces from neighboring particles.
- It is the most energetic (fastest-moving) particles that escape first.
Why Evaporation Causes Cooling of a Liquid
- When the most energetic particles leave the liquid, the average kinetic energy of the remaining particles decreases.
- Since temperature is a measure of average kinetic energy, the temperature of the liquid drops.
- This is why evaporation causes cooling.
Cooling of an Object in Contact with an Evaporating Liquid
- When a liquid evaporates from the surface of an object (like your skin, or a wet cloth), it absorbs energy from that object to allow particles to escape.
- This energy is taken as heat from the object, lowering its temperature.
- As a result, the object in contact with the evaporating liquid feels cooler.
Example:
A person gets out of a swimming pool on a warm, windy day. Although the air temperature is high, they feel cold as water evaporates from their skin.
Explain why the person feels cold in terms of particles and energy transfer.
▶️ Answer/Explanation
The water on the person’s skin is undergoing evaporation.
- Evaporation occurs when the most energetic particles at the surface of the water gain enough energy to escape into the air as gas.
- This process does not require the entire liquid to be at the boiling point.
- The escaping water molecules take heat energy away with them.
- This reduces the average kinetic energy of the remaining water particles.
- As a result, the water cools down, and since the water is in contact with the person’s skin, the skin also loses energy and feels cold.
The person feels cold because the most energetic water particles evaporate, removing heat energy from the skin and lowering its temperature.
Differences Between Boiling and Evaporation
Differences Between Boiling and Evaporation
| Feature | Boiling | Evaporation |
|---|---|---|
| Where it occurs | Throughout the liquid | Only at the surface |
| Temperature | Occurs at a fixed temperature (boiling point) | Occurs at any temperature below boiling point |
| Speed of process | Fast | Slow |
| Bubble formation | Bubbles form within the liquid | No bubbles; particles escape silently |
Factors Affecting the Rate of Evaporation
1. Temperature
- Evaporation happens when surface particles gain enough kinetic energy to overcome the attractive forces holding them in the liquid.
- As temperature increases, the average kinetic energy of the particles also increases.
- This means more particles have enough energy to escape into the air.
- Thus, higher temperature increases the rate of evaporation.
2. Surface Area
- Evaporation only occurs at the surface of a liquid—only surface particles can escape.
- A larger surface area means more particles are exposed to the air.
- This increases the number of particles that can escape at any moment.
- So, greater surface area leads to faster evaporation.
3. Air Movement (Wind or Airflow)
- When particles evaporate, they remain in the air just above the liquid, forming a layer of water vapor.
- If the air above is still, this layer builds up and slows further evaporation by increasing local humidity.
- Moving air (like wind) removes the water vapor layer, allowing fresh dry air to contact the surface.
- This maintains a concentration gradient, so more particles escape. Thus, faster air movement increases evaporation.
4.Humidity (Air Saturation)
- Humidity is the amount of water vapor already present in the air.
- If the air is very humid (nearly saturated), fewer liquid particles can escape into it.
- Therefore, higher humidity slows down evaporation.
Example:
Three trays (Tray A, Tray B, and Tray C) each contain the same volume of water. Tray A is placed in direct sunlight with a fan blowing over it. Tray B is placed in a shady area with no wind, but the tray is very wide and shallow. Tray C is placed in a cold room with no air movement and a deep, narrow shape.
Rank the trays in order of decreasing rate of evaporation. Explain your reasoning using particle theory and the factors affecting evaporation.
▶️ Answer/Explanation
Ranking (fastest to slowest): Tray A > Tray B > Tray C
Tray A:
- High temperature due to sunlight → particles have more kinetic energy.
- Fan increases air movement → removes humid air above the surface.
- These two factors greatly increase evaporation.
Tray B:
- Low temperature (in the shade), so particles gain less energy.
- No wind to remove vapor from the surface.
- However, the tray is wide and shallow → large surface area allows more particles to escape.
- Evaporation is moderate due to surface area.
Tray C:
- Cold environment → particles have low kinetic energy.
- No wind → water vapor builds up, reducing further evaporation.
- Small surface area (deep, narrow tray) → fewer particles exposed to air.
- All three factors reduce the rate, so evaporation is slowest.