Molecules and Matter

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infoWhy this? Particle behaviour explains density, changes of state, internal energy, gas pressure, and whether objects float or sink. Combining particle models with measurements, force diagrams, and heating curves enables us to connect microscopic behaviour with observable properties quantitatively.

scheduleWhy now? This unit revisits the Year 7 particle model with the greater mathematical and conceptual precision appropriate in Year 9. It integrates earlier learning about pressure, upthrust, energy, and changes of state, preparing us to apply these models confidently in later physical science.

Density is defined as mass per unit volume, with the equation `\rho = \frac{m}{V}`.
The SI unit of density is kilograms per cubic metre (kg/m³), but grams per cubic centimetre (g/cm³) is also commonly used.
Mass is measured in kilograms (kg) or grams (g); volume is measured in cubic metres (m³) or cubic centimetres (cm³).
An object will float in water if its density is less than that of water, approximately 1.0 g/cm³ or 1000 kg/m³, and it will sink if its density is greater.
Floating and sinking are determined by the balance of forces: upthrust, also called buoyant force, and weight.
The particles in a substance may be atoms, molecules, or ions, depending on the substance.
Solids have a fixed shape and volume, with particles arranged closely in a regular pattern and vibrating in place.
Liquids have a fixed volume but take the shape of their container, with particles close together but able to move past each other.
Gases have neither fixed shape nor fixed volume, with particles far apart and moving rapidly in all directions.
Gases are less dense than solids and liquids because their particles are much further apart.
The mass of a substance remains the same when it changes state because mass is conserved.
Physical changes are changes in state or form that do not create new substances and are usually reversible.
When a substance changes state, its particles move further apart or closer together, but the number of particles stays the same.
Melting point is the temperature at which a solid turns into a liquid; boiling point is the temperature at which a liquid turns into a gas.
On a temperature–time graph, the temperature remains constant during melting or boiling, even though energy is still being supplied.
Internal energy is the total energy stored in a substance due to the kinetic and potential energy of its particles.
Heating a substance increases its internal energy, causing particles to move faster, increasing their kinetic energy, and/or move further apart, increasing their potential energy.
Pressure is defined as force per unit area, using the equation `P = \frac{F}{A}`, and is measured in pascals (Pa).
In gases, particles move randomly and collide with the walls of their container, creating pressure.
Increasing the temperature of a gas increases the speed of its particles, leading to more frequent and forceful collisions and therefore increasing pressure.
Compressing a gas by reducing its volume increases its density and can cause it to change state without changing temperature.

rocket_launchYou must be able to

Measure the mass of a solid or liquid using a balance.
Measure the volume of a regular solid using a ruler and the appropriate volume formula, such as `V = \text{length} \times \text{width} \times \text{height}`.
Measure the volume of an irregular solid using the displacement method, such as by using a measuring cylinder and water.
Measure the volume of a liquid using a measuring cylinder.
Convert between units of mass, including `\text{g} \leftrightarrow \text{kg}`, and units of volume, including `\text{cm}^3 \leftrightarrow \text{m}^3`, as required for density calculations.
Use the density equation `\rho = \frac{m}{V}` to calculate density, mass, or volume, including when unit conversions are necessary.
Draw and interpret force diagrams to explain floating and sinking, showing weight and upthrust.
Draw and interpret particle diagrams to show the arrangement and movement of particles in solids, liquids, and gases.
Explain, using particle diagrams, why gases are less dense than solids and liquids.
Use counters or models to represent conservation of mass during changes of state.
Interpret and annotate temperature–time graphs to identify melting and boiling points and explain plateaus in terms of energy changes.
Plot a cooling or heating curve from experimental data.
Describe, using particle theory, what happens to particles during melting, boiling, condensation, and freezing.
Explain changes in internal energy during heating and changes of state by relating them to the kinetic and potential energy of particles.
Draw and interpret particle diagrams to explain changes in gas pressure with temperature and volume.
Apply knowledge of gas pressure to explain phenomena such as the collapsing can or imploding bottle, using particle theory and force diagrams.

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