Solids, Liquids and Gases

IGCSE Edexcel Physics
5.1–5.22 Density, Pressure and Gas Laws
Key Concepts: Density relates mass and volume. Pressure is force per area and also depends on depth in fluids. Gas pressure depends on temperature and volume.

Section A: Units

1. State the SI units for temperature, energy, mass, density, distance, area, volume, speed, acceleration, force and pressure. [11]

Section B: Density and Pressure

2. A block has mass 2.4 kg and volume $0.003\,m^3$. Calculate its density. [2]
3. Describe a practical method to determine the density of an irregular solid. [3]
4. A force of 200 N acts on an area of $0.5\,m^2$. Calculate pressure. [2]

Section C: Pressure in Fluids

5. Explain why pressure in a liquid acts equally in all directions. [2]
6. Calculate pressure at 3 m depth in water ($\rho = 1000\,kg/m^3$, $g = 9.8\,N/kg$). [3]

Section D: Gas Laws and Temperature

7. Describe the motion of gas molecules and explain why there is an absolute zero temperature. [3]
8. Convert $25^\circ C$ to Kelvin and 300 K to $^\circ C$. [2]
9. Explain why Kelvin temperature is proportional to the average kinetic energy of gas molecules. [2]
10. Describe the relationship between pressure and volume at constant temperature, and between pressure and Kelvin temperature at constant volume. [4]
11. A gas has pressure 100 kPa at 300 K. What is the pressure at 360 K (constant volume)? [2]
12. A gas has volume $2.0\,dm^3$ at 100 kPa. What volume at 200 kPa (constant temperature)? [2]
Total marks: 36

Mark Scheme

1. $^\circ C$, K, J, kg, $kg/m^3$, m, $m^2$, $m^3$, m/s, $m/s^2$, N, Pa [11]
2. $\rho = m/V = 2.4/0.003 = 800\,kg/m^3$ [2]
3. Measure mass; measure volume by displacement in measuring cylinder; density = m/V [3]
4. $p = F/A = 200/0.5 = 400\,Pa$ [2]
5. Fluid pressure acts in all directions because particles move randomly and collisions are equal [2]
6. $p = h \rho g = 3 \times 1000 \times 9.8 = 29{,}400\,Pa$ [3]
7. Gas molecules move randomly; pressure from collisions; absolute zero is when average kinetic energy is minimum [3]
8. $25^\circ C = 298\,K$; $300\,K = 27^\circ C$ [2]
9. Higher temperature means higher average kinetic energy of molecules [2]
10. At constant temperature, pressure inversely proportional to volume; at constant volume, pressure proportional to Kelvin temperature [4]
11. $p_2 = p_1 T_2/T_1 = 100 \times 360/300 = 120\,kPa$ [2]
12. $p_1V_1 = p_2V_2$, $V_2 = (100 \times 2.0)/200 = 1.0\,dm^3$ [2]