Ch 5 · 2.2.3 Melting, Boiling and Evaporation · O Level Physics 5054
Topic 5 of 9 in Thermal Physics — O Level Physics 5054 — 79 lessons.
Introduction
Put a pan of water on the stove and its temperature climbs to 100 °C, then stops, even though the flame keeps heating it. Where is that energy going? It is being used to change the water from liquid to gas. This lesson explains changes of state in terms of energy and particles, and why sweating cools you down. (Syllabus section 2.2.3.)
Key ideas — Changes of state happen at a constant temperature
During melting and boiling, energy is transferred into the substance but its temperature does not change until the change of state is complete.
🧑🏫 Teacher's explanation
Imagine you are heating a pot of water on a stove. As the water gets hotter, the thermometer reading rises. However, once the water reaches 100 °C, the temperature stops rising, even though the flame is still on! The energy isn't being used to make the water 'hotter'; instead, it is being used to break the bonds holding the water molecules together so they can turn into steam.
📖 Definitions
🔍 Worked example — Easy
A substance is heated at a constant rate. It starts as a solid at 20 °C. It melts at 80 °C and boils at 150 °C. How many temperature plateaus (flat sections) will be observed on a temperature-time graph during the transition from solid to gas?
🔍 Worked example — Standard
A 0.50 kg block of ice at 0 °C absorbs 1.7 imes 10^5 J of energy to melt completely. Calculate the specific latent heat of fusion, L_f, of the ice.
🔍 Worked example — Exam-style
A liquid is evaporating from a surface. The temperature of the liquid is 30 °C. If the liquid absorbs 2000 J of energy from the surroundings to allow 0.05 kg of molecules to escape, calculate the latent heat of vaporization, L_v.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe the shape of the graph when the water is boiling at 100 °C at standard atmospheric pressure. [1] (b) Explain, in terms of particle behavior, why the temperature does not rise during this boiling process. [2] [3 marks]
Key ideas — Water's fixed points
At standard atmospheric pressure:
🧑🏫 Teacher's explanation
Imagine you are heating a pot of water on a stove. As the water reaches 100 ° C, you might notice that even if you turn the flame up higher, the water stays at exactly 100 ° C while it is boiling. This is because the extra energy isn't being used to make the water 'hotter' (increase its temperature); instead, it is being used to break the attractive forces holding the liquid particles together so they can escape as a gas. This 'hidden' energy is called latent heat.
📖 Definitions
🔍 Worked example — Easy
A student observes a beaker of pure water at standard atmospheric pressure. At what temperature (in ° C) does the water begin to boil?
🔍 Worked example — Standard
Ice is melting in a glass of water. What is the temperature of the ice-water mixture during this phase change at standard atmospheric pressure?
🔍 Worked example — Exam-style
A liquid is evaporating from a surface. If the average kinetic energy of the particles remaining in the liquid decreases, what happens to the temperature of the liquid?
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Latent heat: where the energy goes
The energy needed to change the state of a substance, without changing its temperature, is called latent heat ("latent" means hidden: the thermometer does not show it).
🧑🏫 Teacher's explanation
Imagine you are heating a pot of water. As you turn up the flame, the temperature rises steadily until it hits 100 °C. At that exact moment, even if you turn the flame much higher, the thermometer stays stuck at 100 °C until every single drop has turned to steam. Where is that extra heat going? It isn't making the water 'hotter'; it is being used as a 'crowbar' to pry the water molecules apart.
📖 Definitions
🔍 Worked example — Easy
A 50 g block of ice at 0 °C absorbs 16,700 J of energy. Calculate the mass of ice that has melted. (Specific latent heat of fusion of ice = 3.34 × 10^5 J/kg)
🔍 Worked example — Standard
Calculate the energy required to turn 0.2 kg of water at 100 °C into steam at 100 °C. (Specific latent heat of vaporization of water = 2.26 × 10^6 J/kg)
🔍 Worked example — Exam-style
A liquid undergoes evaporation. If the most energetic particles leave the surface, explain why the temperature of the remaining liquid decreases. (Explain in terms of average kinetic energy)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the appearance of the graph at the moment the water begins to boil. [1] (b) Explain, in terms of particle behavior, why the temperature does not rise while the water is boiling. [2] [3 marks]
Key ideas — Boiling and evaporation compared
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. In a liquid, people (particles) are close together but moving around. When you add energy, they move faster. If you add enough energy to boil the liquid, the particles gain so much energy that they break free from the 'social group' entirely, forming bubbles of gas inside the liquid.
📖 Definitions
🔍 Worked example — Easy
A beaker of water is heated at 100 °C at standard atmospheric pressure. While the water is boiling, what happens to its temperature?
🔍 Worked example — Standard
A liquid is evaporating. If the most energetic particles escape the surface, what happens to the average kinetic energy and the temperature of the remaining liquid?
🔍 Worked example — Exam-style
A student observes that a liquid evaporates faster when a fan is placed above it. Explain why using the three factors of evaporation.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State how the rate of evaporation would change if the water were placed in a deep beaker instead of a shallow dish. [1] (b) Explain, in terms of particles, why the water in the shallow dish evaporates faster than the water in the deep beaker. [2] [3 marks]
Key ideas — Evaporation: the more energetic particles escape
Particles in a liquid move at a range of speeds. Some near the surface move fast enough, with enough kinetic energy, to escape the attraction of the other particles and leave as gas. That is evaporation.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where most people are swaying in place, but a few energetic dancers suddenly break away and sprint toward the exit. Even though the crowd as a whole hasn't sped up, the ones who left were the fastest. This is exactly what happens in a liquid during evaporation.
📖 Definitions
🔍 Worked example — Easy
A student notices that a puddle of water on a sunny day disappears faster when the wind blows. Identify the factor affecting the rate of evaporation and explain why.
🔍 Worked example — Standard
A liquid is heated. At 100 ^circC at standard atmospheric pressure, it begins to boil. How does the temperature of the liquid change during the boiling process?
🔍 Worked example — Exam-style
A 500 g sample of water at 25 ^circC evaporates. If the evaporation causes the temperature of the remaining water to drop to 20 ^circC, explain the energy transfer in terms of particle behavior.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe how the fan affects the rate of evaporation. [1] (b) Explain why the temperature of the water decreases as it evaporates. [2] [3 marks]
Key ideas — Faster evaporation
Evaporation is faster when:
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where most people are moving slowly, but a few energetic dancers are sprinting toward the exit. Evaporation is like those energetic dancers escaping the crowd. In a liquid, particles are constantly colliding and exchanging energy. Even if the liquid isn't boiling, some particles at the surface gain enough kinetic energy from these collisions to overcome the attractive forces holding them to their neighbors.
📖 Definitions
🔍 Worked example — Easy
A student notices that a puddle of water evaporates faster on a sunny day than on a cloudy day. Explain why the temperature increase leads to faster evaporation.
🔍 Worked example — Standard
A liquid is placed in a container with a surface area of 0.02 m^2. If the surface area is doubled by using a wider tray, how does this affect the rate of evaporation?
🔍 Worked example — Exam-style
A person is sweating. If they stand in front of a fan, the cooling effect is enhanced. Explain the role of air movement in this process.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe how the rate of evaporation would change if the water were moved from the shallow dish to a deep, narrow beaker. [2] (b) Explain why the temperature of the water decreases during this process. [2] [4 marks]
Key ideas — Evaporation causes cooling
The particles that escape are the most energetic ones. The particles left behind have a lower average kinetic energy, so the temperature of the liquid falls. The liquid then takes energy from anything it touches.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is moving at different speeds. Some people are dancing wildly (high energy), while others are just swaying slowly (low energy). If the fastest, most energetic dancers suddenly burst through the exit doors, the people left on the floor are, on average, much calmer and moving more slowly.
📖 Definitions
🔍 Worked example — Easy
A liquid has a surface area of 0.05 m^2. If the air movement over the surface is doubled, how does the rate of evaporation change?
🔍 Worked example — Standard
A beaker of water is placed in a room. The water evaporates, and the temperature of the water drops from 25 ° C to 22 ° C. Explain why the temperature of the water decreases using particle theory.
🔍 Worked example — Exam-style
A student notices that a wet cloth dries faster when placed in front of a fan compared to a still room. Using the factors affecting evaporation, explain why the fan increases the rate of drying.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe, in terms of particle behavior, how the evaporation of sweat cools the skin. [2] (b) State two ways the person could increase the rate of evaporation of the sweat. [2] [4 marks]
Worked example
The energy from the heater is being used as latent heat to overcome the forces between the water particles and turn the water into steam. It does not increase the particles' average kinetic energy, so the temperature stays constant at the boiling point, 100 °C.
🔍 Worked example — Easy
A student observes a beaker of ice melting at 0 ° C. Even though the beaker is placed on a hot plate, the thermometer reads 0 ° C until all the ice has turned to water. Explain why the temperature does not rise during the melting process.
🔍 Worked example — Standard
A liquid is boiling at 100 ° C. If 2000 J of energy is added to the liquid, what is the change in temperature of the liquid during the boiling process?
🔍 Worked example — Exam-style
A container of water is evaporating. Which of the following would increase the rate of evaporation: (a) increasing the surface area, (b) decreasing the air movement, or (c) decreasing the temperature? Explain why evaporation causes the remaining liquid to cool.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) 0 °C and 100 °C. (2) Any two: boiling happens at one temperature, evaporation at any temperature; boiling happens throughout the liquid, evaporation only at the surface; boiling needs a heater, evaporation cools the liquid. (3) The most energetic particles escape from the surface; the remaining particles have a lower average kinetic energy, so the temperature falls. (4) A larger surface area means more particles are at the surface and can escape. (5) The energy required to change the state of a substance without a change in temperature.