🧰 You need: pen, pencil, ruler, calculator, graph paper or squared paper. Time: 45 minutes. Course: Combined and Separate.
Information – read this first
| State | How to draw the particles | How they move |
|---|---|---|
| Solid | Circles touching, in neat rows | Vibrate about fixed positions |
| Liquid | Circles mostly touching, but jumbled with no pattern | Move around and slide past each other |
| Gas | Circles far apart, randomly spread | Move quickly in random directions |
- Changes of state (melting, freezing, boiling, condensing, evaporating, sublimating) are physical changes. They can be reversed, and mass is conserved.
- When a substance is changing state, its temperature stays the same even though it is still being heated. The energy is used to break the bonds between particles. This shows up as a flat section on a heating curve.
- Heating a substance increases its internal energy – the total kinetic and potential energy of all its particles.
- Density = mass ÷ volume (ρ = m ÷ V). Units: kg/m³ or g/cm³.
- Solids and liquids are dense because their particles are close together. Gases have a low density because their particles are far apart.
The data: heating ice
Some ice at −20 °C was heated steadily and its temperature recorded every minute.
| Time (min) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Temperature (°C) | −20 | −10 | 0 | 0 | 0 | 0 | 20 | 40 | 60 | 80 | 100 | 100 | 100 | 100 |
What to do
- (2 min) Write the title States of Matter and today’s date. Underline both with a ruler.
- (8 min) Draw three boxes side by side, each about 4 cm × 4 cm. Label them Solid, Liquid and Gas. Draw the particles in each box using the table above. Use the same size circles in every box. Under each box, write how the particles move.
- (3 min) Between the boxes, draw arrows and label them: solid → liquid melting, liquid → solid freezing, liquid → gas boiling/evaporating, gas → liquid condensing.
- (12 min) Plot the heating curve:
• Draw axes with a ruler. Time (min) goes along the bottom from 0 to 13. Temperature (°C) goes up the side from −20 to 100.
• Plot each point with a small neat cross.
• Join the points with straight lines using a ruler.
• Label each section: solid ice warming, melting, liquid water warming, boiling.
• Label the melting point (0 °C) and boiling point (100 °C). - (5 min) Under your graph, explain in one or two sentences why the temperature stays the same between 2 and 5 minutes.
- (12 min) Write the heading Density calculations. Copy the worked example, then do Q1–Q5.
- (3 min) Check your answers and correct in a different colour.
Calculations
Worked example: A stone has a mass of 200 g and a volume of 50 cm³. Find its density.
ρ = m ÷ V = 200 ÷ 50 = 4 g/cm³
Q1 A block has a mass of 500 kg and a volume of 0.25 m³. Calculate its density.
Show answer
ρ = 500 ÷ 0.25 = 2000 kg/m³
Q2 An aluminium block measures 2 cm × 3 cm × 5 cm and has a mass of 81 g. Calculate its volume, then its density.
Show answer
V = 2 × 3 × 5 = 30 cm³. ρ = 81 ÷ 30 = 2.7 g/cm³
Q3 Water has a density of 1000 kg/m³. What is the mass of 0.002 m³ of water?
Show answer
m = ρ × V = 1000 × 0.002 = 2 kg
Q4 1 kg of ice melts into water. What is the mass of the water? Explain your answer.
Show answer
1 kg. Mass is conserved in a change of state – the same particles are still there, just arranged differently.
Q5 Steam has a much lower density than water. Use the particle model to explain why.
Show answer
In steam (a gas) the particles are much further apart, so the same mass takes up a much bigger volume. Bigger volume for the same mass means lower density.
⭐ Challenge
A key has a mass of 120 g. When it is lowered into a measuring cylinder, the water level rises from 40 cm³ to 55 cm³. Calculate the density of the key.
Show answer
Volume = 55 − 40 = 15 cm³. ρ = 120 ÷ 15 = 8 g/cm³
