Particle Model of Matter
The particle model of matter represents all substances, solid, liquid and gas, as tiny particles whose arrangement, movement and energy differ between the three states. GCSE physics uses it to explain density, changes of state, and the specific heat capacity and specific latent heat equations for calculating energy changes during heating.
Method
- Decide whether the question is about density, a change of state, or a heating and energy calculation, since each uses a different equation.
- For density questions, use density = mass / volume, converting units so mass is in kg or g and volume is in m^3 or cm^3 consistently.
- For heating a substance without changing state, use change in thermal energy = mass x specific heat capacity x change in temperature.
- For a change of state, such as melting or boiling, use thermal energy = mass x specific latent heat instead, since temperature does not change.
- Explain particle behaviour using the model: particles gain kinetic energy when heated, or gain potential energy as bonds are stretched or broken during a change of state.
- Check whether the question wants a numerical value, a unit, or an explanation in terms of particles, and answer in that form.
Worked example
A 0.40 kg block of copper is heated from 15 degrees C to 65 degrees C. The specific heat capacity of copper is 385 J/(kg degrees C). Calculate the change in thermal energy of the block.
- Find the temperature change: 65 - 15 = 50 degrees C.
- Write the equation: change in thermal energy = mass x specific heat capacity x temperature change.
- Substitute the values: 0.40 x 385 x 50.
- Calculate: 0.40 x 385 = 154, then 154 x 50 = 7700.
- State the final answer: change in thermal energy = 7700 J.
Practice questions
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Q1State the correct SI unit for density.Show answer
Answer: kg/m^3
Q2Name the change of state when a liquid becomes a gas throughout the liquid, at its boiling point.Show answer
Answer: Boiling (vaporisation)
Q3A block has a volume of 50 cm^3 and a mass of 135 g. Calculate its density.Show answer
Answer: 2.7 g/cm^3 (135/50)
Q4State what happens to the internal energy of a substance as it is heated, in terms of the kinetic and potential energy of its particles.Show answer
Answer: The kinetic energy of the particles increases (they move or vibrate faster), and/or the potential energy of the particles increases if bonds are stretched or broken
Q5Calculate the energy needed to melt 0.50 kg of a substance with a specific latent heat of fusion of 210000 J/kg.Show answer
Answer: 105000 J (0.50 x 210000)
Q6A 1.5 kg block of iron is heated from 18 degrees C to 98 degrees C. The specific heat capacity of iron is 450 J/(kg degrees C). Calculate the change in thermal energy.Show answer
Answer: 54000 J (1.5 x 450 x 80, since 98 - 18 = 80)
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A metal cube has sides of length 5.0 cm and a mass of 675 g. Calculate the density of the cube in g/cm^3.
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Explain, in terms of the particle model, why the temperature of a pure substance stays constant while it is melting, even though it is still being heated.
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A student carries out the required practical to determine the specific heat capacity of a metal block, using an electrical heater inserted into the block together with a thermometer, with the block wrapped in an insulating jacket. (a) Name one piece of apparatus, other than a thermometer, needed to measure the energy transferred to the block, and state what quantity it measures. (2 marks) (b) The student records: mass of block = 2.0 kg, initial temperature = 16 degrees C, final temperature = 36 degrees C, energy supplied = 18000 J. Calculate the specific heat capacity of the metal. (3 marks) (c) State one reason why the student's calculated value might be higher than the accepted value for the metal. (1 mark)
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Free printable worksheet
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This topic is chapter 3 of GCSE Physics Workbook, the whole course as one free printable PDF.
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