What you'll learn
This topic examines how substances respond when thermal energy is transferred to or from them. Understanding thermal properties is essential for calculations involving temperature change, state changes, and energy transfer—question types that regularly appear in both Paper 2 (core) and Paper 4 (extended) of CIE IGCSE Physics. Mastery of specific heat capacity and latent heat calculations is particularly important, as these often carry 4-6 marks in structured questions.
Key terms and definitions
Thermal capacity (heat capacity) — the quantity of thermal energy required to raise the temperature of an object by 1°C (or 1 K), measured in joules per degree Celsius (J/°C).
Specific heat capacity — the energy required to raise the temperature of 1 kg of a substance by 1°C (or 1 K), measured in joules per kilogram per degree Celsius (J/(kg°C)).
Latent heat — the thermal energy absorbed or released when a substance changes state at constant temperature, measured in joules (J).
Specific latent heat of fusion — the energy required to change 1 kg of a substance from solid to liquid at its melting point, without change in temperature, measured in joules per kilogram (J/kg).
Specific latent heat of vaporisation — the energy required to change 1 kg of a substance from liquid to gas at its boiling point, without change in temperature, measured in joules per kilogram (J/kg).
Internal energy — the total kinetic energy and potential energy of all the particles in a substance.
Core concepts
Temperature change and specific heat capacity
When thermal energy is supplied to a substance, its temperature typically rises. The relationship between energy transferred, mass, specific heat capacity and temperature change is given by:
E = mcΔθ
Where:
- E = energy transferred (J)
- m = mass (kg)
- c = specific heat capacity (J/(kg°C))
- Δθ = change in temperature (°C or K)
Different substances have different specific heat capacities. Water has an unusually high specific heat capacity of 4200 J/(kg°C), meaning it requires substantial energy to change its temperature. This property explains why coastal regions experience less temperature variation than inland areas—large bodies of water absorb and release heat slowly.
Common specific heat capacity values tested in CIE IGCSE Physics:
- Water: 4200 J/(kg°C)
- Aluminium: 900 J/(kg°C)
- Copper: 390 J/(kg°C)
- Lead: 130 J/(kg°C)
- Ice: 2100 J/(kg°C)
Substances with low specific heat capacities heat up and cool down quickly. This explains why metal objects feel colder than wooden objects at the same temperature—the metal conducts heat away from your hand more rapidly and has a lower specific heat capacity.
Experimental determination of specific heat capacity
CIE IGCSE Physics exams frequently test understanding of the electrical method for determining specific heat capacity:
- Measure the mass of the substance (solid block or liquid) using a balance
- Record the initial temperature using a thermometer
- Heat the substance using an electrical heater of known power for a measured time
- Record the final temperature
- Calculate energy supplied: E = Pt (power × time)
- Calculate specific heat capacity: c = E/(mΔθ)
For accurate results:
- Insulate the substance to minimise heat loss to surroundings
- Stir liquids to ensure uniform temperature distribution
- Ensure the thermometer bulb is fully immersed
- Use low power heating over longer time periods to reduce percentage heat loss
Changes of state and latent heat
When a substance changes state (solid ↔ liquid ↔ gas), energy is transferred but temperature remains constant during the change. This energy changes the internal energy by altering the potential energy of particles as bonds are broken or formed, without changing their kinetic energy (which determines temperature).
During melting or boiling:
- Energy input breaks bonds between particles
- Particles move further apart
- Potential energy increases
- Kinetic energy remains constant
- Temperature remains constant
During freezing or condensing:
- Energy is released as bonds form
- Particles move closer together
- Potential energy decreases
- Kinetic energy remains constant
- Temperature remains constant
The energy equation for state changes is:
E = ml
Where:
- E = energy transferred (J)
- m = mass (kg)
- l = specific latent heat (J/kg)
Use specific latent heat of fusion (lf) for melting/freezing and specific latent heat of vaporisation (lv) for boiling/condensing.
For water:
- Specific latent heat of fusion: 334 000 J/kg (or 3.34 × 10⁵ J/kg)
- Specific latent heat of vaporisation: 2260 000 J/kg (or 2.26 × 10⁶ J/kg)
The specific latent heat of vaporisation is significantly larger than the specific latent heat of fusion because completely separating particles in the gas phase requires more energy than partially separating them in the liquid phase.
Temperature-time graphs for heating and cooling
CIE IGCSE Physics papers often include temperature-time graphs showing heating or cooling curves. Key features:
Heating curve characteristics:
- Sloped sections: temperature rising, substance in single state
- Horizontal (flat) sections: state change occurring, temperature constant
- Steeper gradients: substance has lower specific heat capacity
- Longer horizontal sections: greater latent heat or larger mass
Graph interpretation:
- First sloped section: solid heating up (E = mcsolidΔθ)
- First horizontal section: melting (E = mlf)
- Second sloped section: liquid heating up (E = mcliquidΔθ)
- Second horizontal section: boiling (E = mlv)
- Third sloped section: gas heating up (E = mcgasΔθ)
The gradient of sloped sections relates to specific heat capacity. A steeper gradient indicates a lower specific heat capacity (temperature rises faster for the same energy input rate).
Internal energy and particle model
Internal energy comprises:
- Kinetic energy of particles (related to temperature)—random motion, vibration, rotation
- Potential energy of particles (related to bonds)—energy stored in forces between particles
When temperature increases:
- Particles move faster
- Average kinetic energy increases
- Internal energy increases
When state changes occur:
- Bond arrangement changes
- Potential energy changes
- Internal energy changes
- Temperature and average kinetic energy remain constant
This particle model explains why steam at 100°C causes more severe burns than water at 100°C—the steam must release its latent heat of vaporisation (2.26 MJ/kg) when condensing on skin before it can cool down.
Evaporation versus boiling
Both are processes where liquid changes to gas, but they differ significantly:
Evaporation:
- Occurs at any temperature
- Occurs only at the liquid surface
- Particles with highest kinetic energy escape
- Average kinetic energy of remaining liquid decreases
- Temperature of liquid decreases (cooling effect)
- No bubbles form
Boiling:
- Occurs at a specific temperature (boiling point)
- Occurs throughout the liquid
- Bubbles of vapour form inside the liquid
- Temperature remains constant during boiling
- Requires continuous energy input
Factors increasing evaporation rate:
- Higher temperature (more particles have sufficient energy to escape)
- Larger surface area (more particles at surface)
- Air movement/draughts (removes vapour, maintaining concentration gradient)
- Lower humidity (greater concentration difference)
Evaporative cooling explains why sweating cools the body—the highest energy water molecules evaporate from skin, removing energy and lowering skin temperature.
Worked examples
Example 1: Specific heat capacity calculation
An aluminium block of mass 2.0 kg is heated using a 50 W electric heater for 10 minutes. The temperature rises from 20°C to 37°C. The specific heat capacity of aluminium is 900 J/(kg°C).
(a) Calculate the energy supplied by the heater. [2]
(b) Calculate the energy used to heat the aluminium block. [2]
(c) Suggest why these two values differ. [1]
Solution:
(a) E = Pt [1] E = 50 × (10 × 60) = 50 × 600 = 30 000 J [1]
(b) E = mcΔθ [1] E = 2.0 × 900 × (37 - 20) = 2.0 × 900 × 17 = 30 600 J [1]
(c) Energy is lost to the surroundings / heat losses [1]
Example 2: Latent heat calculation
A kettle supplies energy at a rate of 2000 W. Calculate how long it takes to boil away 0.15 kg of water at 100°C. The specific latent heat of vaporisation of water is 2.26 × 10⁶ J/kg. [3]
Solution:
E = ml [1] E = 0.15 × 2.26 × 10⁶ = 339 000 J
t = E/P [1] t = 339 000/2000 = 169.5 s (or 170 s or 2.8 minutes) [1]
Example 3: Combined calculation
Calculate the total energy required to change 0.50 kg of ice at -10°C into water at 30°C.
Specific heat capacity of ice = 2100 J/(kg°C) Specific heat capacity of water = 4200 J/(kg°C) Specific latent heat of fusion of ice = 3.34 × 10⁵ J/kg [4]
Solution:
Energy to heat ice from -10°C to 0°C: E₁ = mcΔθ = 0.50 × 2100 × 10 = 10 500 J [1]
Energy to melt ice at 0°C: E₂ = ml = 0.50 × 3.34 × 10⁵ = 167 000 J [1]
Energy to heat water from 0°C to 30°C: E₃ = mcΔθ = 0.50 × 4200 × 30 = 63 000 J [1]
Total energy = 10 500 + 167 000 + 63 000 = 240 500 J (or 241 kJ or 2.4 × 10⁵ J) [1]
Common mistakes and how to avoid them
Mistake: Confusing thermal capacity with specific heat capacity. Correction: Thermal capacity applies to an object (J/°C), specific heat capacity applies to a material per kilogram (J/(kg°C)). Always check whether the question asks for an object or a material property.
Mistake: Using the wrong latent heat value—fusion instead of vaporisation or vice versa. Correction: Fusion relates to melting/freezing (solid ↔ liquid), vaporisation relates to boiling/condensing (liquid ↔ gas). Identify the state change carefully.
Mistake: Stating that temperature increases during a state change. Correction: Temperature remains constant during melting, boiling, freezing or condensing. The energy changes potential energy of particles, not kinetic energy. Only kinetic energy determines temperature.
Mistake: Forgetting to convert time to seconds when using P = E/t with power in watts. Correction: Power in watts requires time in seconds. Always convert minutes to seconds by multiplying by 60.
Mistake: Writing "heat is a form of energy stored in objects". Correction: Heat is energy being transferred due to temperature difference. Objects contain internal energy, not heat. Use precise terminology.
Mistake: Using incorrect units in calculations, particularly writing kg instead of g or forgetting to convert. Correction: Standard SI units for these formulae are: mass in kg, energy in J, temperature in °C or K, power in W, time in s. Convert all measurements to SI units before calculating.
Exam technique for Thermal Properties of Matter
Calculation questions typically award 1 mark for correct formula/substitution and 1 mark for correct answer with unit. Always show working clearly—if the final answer is wrong but the method is correct, you gain partial credit. Command words include "Calculate", "Determine", "Show that".
Experimental questions may ask you to "Describe" a method for measuring specific heat capacity or "Suggest" improvements to reduce errors. Structure answers with numbered steps. For improvements, identify the error source first, then state the solution. Typical marks: 3-4 marks for a method description.
Graph interpretation questions use commands like "Explain", "State what is happening", or "Use the graph to determine". When explaining horizontal sections, explicitly state that temperature is constant AND a state change is occurring. When asked to determine energy from a graph, remember to calculate the area under the graph or use the time duration with given power.
Multi-stage calculations (e.g., heating ice to steam) require systematic breakdown. Calculate each stage separately, then sum. Examiners often award marks for each stage, so even if you make an early error, continue with subsequent steps using your values to gain method marks.
Quick revision summary
Thermal properties describe how substances respond to energy transfer. Use E = mcΔθ for temperature changes and E = ml for state changes. Specific heat capacity (J/(kg°C)) varies between materials—water has a high value at 4200 J/(kg°C). During state changes, temperature stays constant while bonds break (absorbing energy) or form (releasing energy). Latent heat of vaporisation exceeds latent heat of fusion because gas particles are completely separated. Temperature-time graphs show sloped sections during heating and horizontal sections during state changes. Always use SI units (kg, J, s, W, °C) and show clear working in calculations for maximum marks.