Particle model of matter

1. Zoom in: solids, liquids and gases

A particle model helps us explain what we cannot see directly. The particles might be atoms or molecules. Start with two questions: How are they arranged? How do they move?

2. Density: how much mass fits into a volume?

Density is mass per unit volume. Compare equal volumes: the one with the greater mass has the greater density.

For the same substance, gas particles are much further apart than liquid or solid particles. A fixed volume of gas therefore contains fewer particles and less mass. Gases usually have much lower densities. Solids and liquids both have close particles; their densities are often similar.

Useful exception: ice is less dense than liquid water. Its particles form a more open arrangement. The water molecules do not become heavier when ice melts.

3. Choose matching units before calculating

1. Read the units requested in the answer.
2. Convert the measurements if needed.
3. Substitute into the equation and include the final unit.

4. Required practical 17: a regular solid

A rectangular metal block has a shape we can measure directly. Measure mass and volume separately.

1. Zero the balance and measure the block’s mass.
2. Measure length, width and height. Use a ruler, Vernier callipers or a micrometer as appropriate. Check for a zero error.
3. For a cuboid, calculate V = length × width × height.
4. Calculate density from mass ÷ volume. Repeat measurements and compare values.

Improve the result: measure small dimensions with finer-resolution equipment. Measure at several positions if the block is not perfectly even. Repeats reveal variation; they do not automatically remove a zero error.

5. Required practical 17: an irregular solid

A stone has no simple length × width × height volume. Instead, measure the water it displaces when submerged.

1. Measure the dry object’s mass on a balance.
2. Record the starting water volume.
3. Lower the object carefully until fully submerged. Remove trapped air bubbles and record the new volume.
4. Subtract the two readings, then calculate density.

Use a solid that does not dissolve or react with the liquid. Avoid splashing. Trapped bubbles make the measured volume too large, giving a density that is too small.

6. Required practical 17: a liquid

The balance also measures the container. We must subtract its mass to find the liquid’s mass.

1. Measure the mass of a clean, dry measuring cylinder.
2. Add the liquid; read its volume at eye level.
3. Measure the cylinder plus liquid.
4. Subtract to find liquid mass, then calculate density.

A balance can be tared with the empty cylinder on it, so it displays liquid mass directly. Use a suitable cylinder scale, wipe liquid from the outside, and keep the balance dry.

7. Changing state: same substance, same mass

Melting ice makes liquid water, not a new substance. If the water freezes, it recovers the properties of ice. This makes it a physical change.

Mass is conserved: the same particles are still present during a change of state. In an open beaker, the balance reading may fall when gas escapes. The total mass of beaker contents plus escaped gas is unchanged.

8. Internal energy is more than temperature

Internal energy is the total kinetic energy and potential energy of all the particles in a system.

• Kinetic energy: particles have energy because they move or vibrate.
• Potential energy: energy is associated with particle positions and interactions.
• Heating: transfers energy into the system. It may raise temperature, cause a change of state, or both in successive stages.

9. Warming a substance: specific heat capacity

Specific heat capacity, c, is the energy needed to raise the temperature of 1 kg of a substance by 1 °C, without a change of state.

For the same energy input, a smaller mass or a smaller c gives a bigger temperature rise. This is the same relationship used in the Energy worksheet; required practical 14 is covered there.

10. Changing state: specific latent heat

Specific latent heat, L, is the energy needed to change the state of 1 kg of a substance without changing its temperature.

During melting or boiling, energy changes particle arrangement and potential energy. Average kinetic energy, and therefore temperature, stays constant during the change of state of a pure substance at constant pressure.

11. Read a heating or cooling graph

Read the axes first. A rising line means temperature increases. A flat section can show a change of state while energy is still being transferred.

For cooling, follow the changes in reverse: temperature falls between changes of state; energy is released during condensation or freezing at constant temperature.

12. Gas pressure: tiny collisions, a measurable force

Gas particles are in constant random motion. When they hit a container wall, they exert a force. The combined effects of many collisions produce gas pressure.

1. The gas temperature increases.
2. Particles have greater average kinetic energy and move faster.
3. They hit the walls more often and each collision produces a larger change in momentum.
4. The average force on the walls increases, so pressure increases.

Cooling a fixed amount of gas at constant volume has the opposite effect. Not every particle has the same speed: temperature is related to the average kinetic energy.

Required practical 17: interpret the measurements

An irregular metal sample has mass 135 g. It raises the water level from 40.0 to 65.0 cm³. A second student leaves air bubbles on the sample.

Draw, then explain

1. On paper, draw three equal-size boxes for solid, liquid and gas. Use particle spacing and movement to show each state.
2. Sketch a cooling graph for water vapour at constant pressure. Include condensation and freezing plateaus, with labelled axes.
3. Compare with the diagrams in Learn, explaining what each model shows and leaves out.

These drawing activities are completed on paper. Written explanations can be saved in the answer boxes.

Your answer sheet

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One final thought

Use a scientific idea and a specific example in your reflection.