CIE iGCSE Co-ordinated Sciences-P2.2.2 Melting, boiling and evaporation- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-P2.2.2 Melting, boiling and evaporation – Study Notes
CIE iGCSE Co-ordinated Sciences-P2.2.2 Melting, boiling and evaporation – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Know the melting and boiling temperatures for water at standard atmospheric pressure (limited to Celsius only)
- Describe condensation and solidification (freezing) in terms of particles
- Describe evaporation in terms of the escape of the more energetic particles from the surface of a liquid
- Know that evaporation causes cooling of a liquid
- Describe melting and boiling in terms of energy input without a change in temperature
Supplement
- Describe the differences between boiling and evaporation
- Describe how temperature, surface area and air movement over a surface affect evaporation
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Melting and Boiling Temperatures of Water
Melting Point:
The temperature at which solid water (ice) changes to liquid water at standard atmospheric pressure.
- Melting temperature of water: 0°C
- At this temperature, ice and water coexist in equilibrium.
Boiling Point:
The temperature at which liquid water changes to water vapor (gas) at standard atmospheric pressure.
- Boiling temperature of water: 100°C
- At this temperature, water and steam coexist in equilibrium.
Key Concept: These temperatures are fixed physical properties of water at 1 atm pressure, and represent phase changes where particle arrangement and motion change dramatically:
- At 0°C, particles in ice gain enough energy to break some bonds, allowing movement → liquid water forms.
- At 100°C, water molecules gain enough kinetic energy to overcome intermolecular forces completely → vapor forms.
Example :
Why does ice at 0°C melt very slowly even when placed in a warmer room?
▶️ Answer/Explanation
At 0°C, ice and water are in equilibrium. Heat energy from the room is used to break bonds between ice particles rather than raising temperature → ice melts slowly at its melting point.
Condensation and Solidification (Freezing)
Condensation
Condensation is the change of state from gas to liquid.
Particle Explanation:
- Gas particles lose kinetic energy when cooled.
- Slower-moving particles experience stronger attractive forces between them.
- Particles come closer together and form a liquid, moving past each other but remaining close.
Example: Water droplets forming on a cold glass of water.
Solidification (Freezing)
Solidification or freezing is the change of state from liquid to solid.
Particle Explanation:
- Liquid particles lose kinetic energy as temperature decreases.
- Particles slow down and cannot overcome attractive forces fully.
- Particles settle into a fixed lattice arrangement, forming a solid.
Example: Water freezing to form ice in a freezer.
Example :
Explain why water vapor forms droplets on a cold window pane using the particle model.
▶️ Answer/Explanation
Water vapor particles lose kinetic energy when in contact with the cold surface → move slower → attractive forces pull them closer → particles form liquid droplets (condensation).
Evaporation
Evaporation is the process where particles at the surface of a liquid gain enough energy to escape into the gas phase, even when the liquid is below its boiling point.
Particle Explanation:
- In a liquid, particles move randomly with a range of kinetic energies.
- Some surface particles have higher kinetic energy than others.
- These particles can overcome the intermolecular forces holding them in the liquid.
- They then escape into the air as gas particles.
Why Evaporation Causes Cooling:
- The escaping particles are the most energetic ones.
- The particles left behind have a lower average kinetic energy.
- Since temperature depends on average kinetic energy, the liquid’s temperature decreases.
Key Points:
- Evaporation happens at all temperatures, not just at boiling point.
- It is a surface process (only surface particles escape).
- It is affected by: – Temperature (higher → faster evaporation) – Surface area (larger → faster evaporation) – Air movement (wind removes vapor, speeding evaporation) – Humidity (lower humidity → faster evaporation)
Example :
Explain why your skin feels cool when sweat evaporates.
▶️ Answer/Explanation
Step 1: The fastest, most energetic particles in sweat escape into the air.
Step 2: The remaining sweat particles have lower average kinetic energy.
Step 3: This lowers the temperature of the sweat and your skin.
Final Answer: Evaporation of sweat causes cooling because the most energetic particles leave, reducing the liquid’s average energy.
Melting and Boiling in Terms of Energy Input
During melting and boiling, heat energy is supplied to a substance, but its temperature does not change until the process is complete.
Melting (Solid → Liquid)
- When a solid (e.g., ice) is heated to its melting point (0°C for water), the temperature stays constant while melting occurs.
- The energy supplied is not used to raise particle temperature, but to break the strong intermolecular forces that hold particles in a fixed lattice.
- Particles gain freedom to move past each other, forming a liquid.
Boiling (Liquid → Gas)
- When a liquid (e.g., water) is heated to its boiling point (100°C for water at 1 atm), the temperature remains constant during boiling.
- The energy supplied is used to overcome intermolecular forces completely, allowing particles to separate widely and form a gas.
- Particles escape from within the liquid as well as from the surface (unlike evaporation, which occurs only at the surface).
Why Temperature Does Not Change:
- Temperature measures the average kinetic energy of particles.
- During melting and boiling, the input energy is used as latent heat to change the arrangement of particles, not to increase their speed.
- Only after the change of state is complete does further heating increase temperature again.
Example :
Why does the temperature of boiling water remain at 100°C even though heat is continuously supplied?
▶️ Answer/Explanation
Step 1: At 100°C, water is at its boiling point.
Step 2: The heat energy supplied does not increase particle speed, but is used to break intermolecular bonds.
Step 3: This energy is called latent heat of vaporization.
Final Answer: The temperature stays constant because the energy is used for the change of state, not for raising kinetic energy.
Differences Between Boiling and Evaporation
Boiling:
- Occurs at a fixed temperature (the boiling point).
- Happens throughout the whole liquid, not just at the surface.
- Requires a continuous supply of heat energy.
- Produces bubbles of vapor inside the liquid.
Evaporation:
- Occurs at any temperature below the boiling point.
- Happens only at the surface of the liquid.
- No continuous heating is required (it can happen naturally at room temperature).
- No bubbles form inside the liquid — particles escape directly from the surface.
Comparison Table
| Feature | Boiling | Evaporation |
|---|---|---|
| Temperature | Occurs only at the boiling point | Occurs at any temperature below boiling point |
| Where It Happens | Throughout the liquid | Only at the surface |
| Energy Supply | Requires continuous heating | Can occur naturally without heating |
| Bubbles | Bubbles of vapor form inside the liquid | No bubbles, particles escape individually |
Example :
Why does water in an open dish slowly disappear at room temperature, even though it is not boiling?
▶️ Answer/Explanation
Step 1: At room temperature, some surface particles have enough kinetic energy to escape into the air.
Step 2: This is evaporation, which occurs at all temperatures below boiling.
Final Answer: Water evaporates slowly because energetic particles escape from the surface, even though the liquid is not at boiling point.
Factors Affecting Evaporation
Evaporation is influenced by conditions that affect the energy of particles and the escape of particles from the surface.
1. Temperature
- Higher temperature → particles have more kinetic energy.
- More particles at the surface can overcome intermolecular forces and escape.
- Therefore, evaporation is faster at higher temperatures.
2. Surface Area
- Evaporation occurs only at the surface of a liquid.
- A larger surface area exposes more particles to escape.
- Therefore, increasing surface area makes evaporation faster.
3. Air Movement (Wind)
- After particles escape, they remain close to the surface if the surrounding air is still.
- Air movement (wind) carries these vapor particles away, preventing them from returning.
- This keeps the concentration of vapor above the liquid low, so more particles can escape.
- Therefore, evaporation is faster when air moves over the surface.
Summary Table
| Factor | Effect on Evaporation | Particle Model Explanation |
|---|---|---|
| Temperature ↑ | Evaporation increases | Particles have more energy → more escape |
| Surface Area ↑ | Evaporation increases | More particles exposed at surface → more escape |
| Air Movement ↑ | Evaporation increases | Escaped particles removed → more can leave |
Example :
Clothes dry faster on a hot, windy day than on a cold, still day. Explain why using the particle model.
▶️ Answer/Explanation
Step 1: Higher temperature → water particles in clothes have more kinetic energy → more can escape.
Step 2: Wind moves vapor particles away from the surface → prevents saturation → continuous evaporation.
Step 3: Large exposed surface of clothes increases the rate further.
Final Answer: Clothes dry faster in hot, windy conditions because temperature, surface area, and air movement all increase evaporation.