Spec links: 4.3.1.2 Changes of state; 4.3.2.1 Internal energy; 4.3.2.2 Temperature changes in a system and specific heat capacity; 4.3.2.3 Changes of heat and specific latent heat · Reading time: about 4 minutes
Putting the kettle on
Millions of cups of tea are drunk in the UK every day. Each one starts with a kettle. Flick the switch and energy is transferred to the water. But what is actually happening to the water particles?
Internal energy and temperature
The particles in water are always moving. They also have forces between them. The total kinetic energy and potential energy of all the particles is called the internal energy.
Heating the water increases its internal energy. At first, the particles move faster, so the temperature rises. How much energy is needed depends on the mass, the temperature change and the material. Each material has a specific heat capacity. This is the energy needed to raise the temperature of 1 kg of the material by 1 °C. For water it is about 4200 J/kg °C, which is high compared with most materials.
The equation is change in thermal energy = mass × specific heat capacity × temperature change, or ΔE = m c Δθ. Heating 0.25 kg of water from 20 °C to 100 °C needs ΔE = 0.25 × 4200 × 80 = 84 000 J.
Boiling without getting hotter
Once the water reaches 100 °C, something odd happens. The kettle keeps heating, but the temperature stops rising. The energy is now being used to break the bonds between particles, so the water can turn into steam. This is a change of state. It is a physical change, not a chemical one, and mass is conserved. If the steam is cooled, it turns back into water.
The energy needed to change the state of 1 kg of a substance without changing its temperature is the specific latent heat. For melting or freezing it is the specific latent heat of fusion. For boiling or condensing it is the specific latent heat of vaporisation.
The equation is energy for a change of state = mass × specific latent heat, or E = m L. For water, L for vaporisation is about 2 260 000 J/kg. Boiling away just 0.01 kg of water needs E = 0.01 × 2 260 000 = 22 600 J.
Why steam scalds are worse
When steam touches your skin, it condenses back into water. As it condenses, it releases all of its latent heat into your skin. Then the hot water releases more energy as it cools. That is why a scald from steam is usually much worse than a scald from water at the same temperature.
Blowing on your tea
Your tea is too hot, so you blow on it. This speeds up evaporation. The fastest-moving particles escape from the surface. They carry energy away, so the average kinetic energy of the particles left behind falls. The tea cools down.
Adding cold milk cools it too. Energy is transferred from the hot tea to the cooler milk until they reach the same temperature. Sweating works the same way as blowing: as sweat evaporates, it cools your skin.
Key words
- Internal energy: the total kinetic energy and potential energy of all the particles that make up a system.
- Specific heat capacity: the amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius.
- Specific latent heat: the amount of energy required to change the state of one kilogram of a substance with no change in temperature.
- Specific latent heat of fusion: the energy needed to change 1 kg of a substance from solid to liquid without a change in temperature.
- Specific latent heat of vaporisation: the energy needed to change 1 kg of a substance from liquid to vapour without a change in temperature.
- Change of state: a physical change, such as melting or boiling, in which the material can recover its original properties if reversed and mass is conserved.
- Evaporation: a change from liquid to gas that happens at the surface of a liquid at any temperature.
Questions
- What is meant by the internal energy of a system? [1 mark]
- Give the specific heat capacity of water stated in the text. Give the unit. [1 mark]
- What happens to the temperature of the water while it is boiling? [1 mark]
- Explain why blowing on a hot drink cools it down. [3 marks]
- Explain why a scald from steam at 100 °C is more serious than a scald from water at 100 °C. [2 marks]
- A kettle heats 0.5 kg of water from 15 °C to 100 °C. The specific heat capacity of water is 4200 J/kg °C. Calculate the energy transferred to the water. Give the unit. [3 marks]
- An ice cube of mass 0.02 kg melts at 0 °C. The specific latent heat of fusion of ice is 334 000 J/kg.
a) Calculate the energy needed to melt the ice cube. Give the unit. [3 marks]
b) State what happens to the temperature of the ice while it is melting. [1 mark] - Describe what happens to the particles and the energy of some water as it is heated from 20 °C until it has all boiled away. Refer to internal energy, temperature and latent heat in your answer. [6 marks]
Show answers
- The total kinetic energy and potential energy of all the particles (in the system) (1)
- 4200 J/kg °C (1)
- It stays the same / stays at 100 °C (1)
- Blowing increases the rate of evaporation (1); the fastest-moving / most energetic particles escape from the surface (1); so the average kinetic energy of the remaining particles decreases so the temperature falls (1)
- Steam condenses on the skin and releases latent heat (1); then the water formed also releases energy as it cools, so more energy is transferred to the skin in total (1)
- ΔE = m c Δθ (1); ΔE = 0.5 × 4200 × 85 (1); ΔE = 178 500 J (1)
- a) E = m L (1); E = 0.02 × 334 000 (1); E = 6680 J (1)
b) It stays the same / stays at 0 °C (1) - Indicative content:
- energy is transferred to the water, increasing its internal energy
- from 20 °C to 100 °C the particles move faster / kinetic energy increases
- so the temperature rises
- amount of energy depends on mass, specific heat capacity and temperature change (ΔE = m c Δθ)
- at 100 °C the water boils and the temperature stays constant
- energy (latent heat) is used to break bonds / overcome forces between particles, increasing potential energy, not kinetic energy
- particles move far apart and escape as a gas / steam
- energy needed = m L, where L is the specific latent heat of vaporisation; mass is conserved
- Level 3 (5–6 marks): a detailed, logical description of both the heating stage and the boiling stage, linking energy to particle kinetic and potential energy, temperature and latent heat; Level 2 (3–4 marks): describes both stages with some correct links to particles or energy; Level 1 (1–2 marks): simple relevant statements, e.g. the particles move faster or the water turns to steam
More reading: See all 30 GCSE Physics comprehension articles