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How does the particle model explain solids, liquids and gases, and how do we calculate density?

The particle model of the three states of matter, the arrangement and motion of particles in solids, liquids and gases, and density as mass per unit volume, including measuring the density of regular and irregular objects.

A focused answer to OCR Gateway GCSE Physics A topic P1 on the particle model and density, covering the arrangement and motion of particles in solids, liquids and gases, the density equation, and how to measure the density of regular and irregular objects.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. The particle model of the three states
  3. Density
  4. Measuring density (the P1 practical)
  5. Try this

What this topic is asking

OCR wants you to use the particle model to describe and explain the three states of matter, and to define and calculate density as mass per unit volume, including measuring the density of regular and irregular objects in the practical activity. This is topic P1.1 of the OCR Gateway Physics A (J249) specification, examined on the Paper 1 or Paper 3 side.

The particle model of the three states

The three states differ in how the particles are arranged and how they move:

  • Solid. The particles are packed closely together in a regular arrangement and vibrate about fixed positions. The strong forces between them hold a fixed shape and a fixed volume.
  • Liquid. The particles are still close together but arranged randomly, and they can move past one another. A liquid keeps a fixed volume but flows to take the shape of its container.
  • Gas. The particles are far apart, move quickly in random directions, and have only very weak forces between them. A gas has no fixed shape or volume and spreads out to fill its container.

This explains everyday behaviour. A gas is easily compressed because of the large gaps between particles, while a solid and a liquid are nearly incompressible because their particles are already in contact.

Density

You must recall the density equation for OCR Gateway Physics A; it is not given on the data sheet. For the same substance, the solid is usually denser than the liquid, and both are far denser than the gas, because the particle spacing increases from solid to liquid to gas. Water is an important exception: ice is slightly less dense than liquid water, which is why ice floats.

To convert, note that 1 g/cm3=1000 kg/m31\,\text{g/cm}^3 = 1000\,\text{kg/m}^3, because there are 1000 g1000\,\text{g} in a kilogram and 106 cm310^6\,\text{cm}^3 in a cubic metre.

Measuring density (the P1 practical)

The density practical asks you to find the density of regular and irregular objects.

For a regular solid (such as a cube or cylinder), measure its dimensions with a ruler or vernier callipers, calculate the volume from the appropriate formula, find the mass on a balance, and divide. For a liquid, measure the mass of an empty measuring cylinder, add a known volume of the liquid, measure the new mass, and divide the mass of liquid by its volume.

For an irregular solid (such as a pebble), you cannot use a formula for the volume, so you use displacement: lower the object into a measuring cylinder of water (or a displacement can) and measure the volume of water it pushes aside, which equals the object's volume. Then divide the object's mass by that volume.

Try this

Q1. A block has a mass of 240 g240\,\text{g} and a volume of 30 cm330\,\text{cm}^3. Calculate its density. [2 marks]

  • Cue. ρ=mV=24030=8.0 g/cm3\rho = \dfrac{m}{V} = \dfrac{240}{30} = 8.0\,\text{g/cm}^3.

Q2. Explain, using the particle model, why a gas can be squashed into a smaller volume but a liquid cannot. [2 marks]

  • Cue. Gas particles are far apart with large gaps to be pushed into; liquid particles are already close together and touching.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksA student measures a metal cube of side 2.02.0 cm and finds its mass is 63 g63\,\text{g}. Calculate the density of the metal in g/cm3\text{g/cm}^3, and state how you would find the density of an irregularly shaped pebble.
Show worked answer →

A P1 Calculate question on the recall equation ρ=mV\rho = \dfrac{m}{V}. The volume of the cube is V=2.0×2.0×2.0=8.0 cm3V = 2.0 \times 2.0 \times 2.0 = 8.0\,\text{cm}^3 (1 mark). The density is ρ=mV=638.0=7.9 g/cm3\rho = \dfrac{m}{V} = \dfrac{63}{8.0} = 7.9\,\text{g/cm}^3 to two significant figures (2 marks for substitution and answer with units). For the pebble, lower it into a measuring cylinder (or displacement can) partly filled with water and measure the volume of water displaced, which equals the volume of the pebble; then divide its mass by that volume (1 mark). Markers reward the correct cube volume, the density with units, and a valid displacement method.

OCR 20213 marksDescribe the arrangement and motion of the particles in a solid, a liquid and a gas, and explain why a gas can be compressed much more easily than a solid.
Show worked answer →

A Describe and Explain question worth three marks. In a solid the particles are packed closely in a regular arrangement and vibrate about fixed positions (1 mark). In a liquid the particles are still close together but are arranged randomly and can move past one another (1 mark). In a gas the particles are far apart, move quickly in random directions, and there are large gaps between them, which is why a gas can be compressed much more easily, because there is empty space for the particles to be pushed into, whereas a solid's particles are already touching (1 mark). Markers reward a clear contrast in spacing and motion across the three states and the link from large gaps to compressibility.

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