Solids, liquids and gases form the bridge between the particle model you learned in chemistry and the quantitative physics of pressure, density and temperature. This section of the Edexcel IGCSE Science Single Award formalises that bridge with equations, units and testable predictions.
The edexcel igcse science single award physics: solids, liquids and gases section of the 4SS0 specification covers three areas: units, density and pressure, and ideal gas molecules. These edexcel igcse science single award revision notes work through every specification point, with worked examples, common pitfalls and self-check questions to anchor your understanding. The Science Single Award is a single IGCSE covering biology, chemistry and physics in a focused format, and the depth of the physics content within it is substantial.
Units
| Quantity | Unit | Symbol |
|---|---|---|
| Temperature (Celsius) | degree Celsius | °C |
| Temperature (Kelvin) | kelvin | K |
| Energy | joule | J |
| Mass | kilogram | kg |
| Length | metre | m |
| Area | metre squared | m2 |
| Volume | metre cubed | m3 |
| Speed | metre per second | m/s |
| Acceleration | metre per second squared | m/s2 |
| Force | newton | N |
| Pressure | pascal | Pa |
The pascal is the SI unit of pressure: 1 Pa = 1 N/m2. This definition connects the force and area concepts you already know from mechanics to the new context of fluids and gases. One pascal is a very small amount of pressure. Standard atmospheric pressure is approximately 101,325 Pa, often written as about 100 kPa.
Density and pressure
Pressure, force and area
The relationship between pressure, force and area is:
p = F / A
Pressure (in pascals) equals force (in newtons) divided by area (in metres squared). This equation explains everyday observations: a sharp knife cuts more easily than a blunt one because the same force is concentrated over a smaller area, producing a higher pressure. Snowshoes spread the wearer's weight over a larger area, reducing the pressure on the snow surface and preventing the person from sinking.
p = F / A = 600 / 0.2 = 3,000 Pa
The box exerts a pressure of 3,000 Pa (3 kPa) on the surface.
Rearranging the equation is straightforward and frequently tested. If you know the pressure and the area, you can find the force: F = p × A. If you know the pressure and the force, you can find the area: A = F / p. Practise rearranging until it is automatic.
F = p × A = 50,000 × 0.004 = 200 N
Pressure in fluids
The specification requires you to understand that pressure at a point in a gas or liquid at rest acts equally in all directions. This means that a balloon inflated with air experiences the same pressure pushing outward at every point on its inner surface, which is why it expands into a roughly spherical shape rather than stretching in just one direction. In a liquid, the pressure at any given depth pushes equally upward, downward and sideways. This property is fundamental to how hydraulic systems work: pressure applied at one point in a fluid is transmitted equally to all other points.
Ideal gas molecules
Random motion and pressure
Gas molecules move randomly in all directions. They collide with each other and with the walls of their container. Each collision with a wall exerts a tiny force on the wall. The combined effect of billions of these molecular collisions per second produces a measurable force on the container wall, and that force per unit area is the gas pressure. This molecular-level explanation of pressure is central to understanding all gas behaviour in this section.
Absolute zero and the Kelvin scale
Temperature is a measure of the average kinetic energy of the particles in a substance. As temperature decreases, the average kinetic energy decreases and the particles move more slowly. There is a theoretical lower limit: the temperature at which particles have zero kinetic energy and cease all motion. This is absolute zero, -273 degrees Celsius (more precisely, -273.15 °C).
The Kelvin scale starts at absolute zero. To convert between the two scales:
- Celsius to Kelvin: T(K) = T(°C) + 273
- Kelvin to Celsius: T(°C) = T(K) - 273
| Temperature | Celsius | Kelvin |
|---|---|---|
| Absolute zero | -273 °C | 0 K |
| Melting point of ice | 0 °C | 273 K |
| Room temperature | 20 °C | 293 K |
| Boiling point of water | 100 °C | 373 K |
Temperature and molecular speed
An increase in temperature results in an increase in the average speed of gas molecules. This is because temperature is proportional to the average kinetic energy, and kinetic energy depends on the speed of the molecules (kinetic energy = 0.5 × mass × speed2). Faster molecules collide with the container walls more frequently and with greater force, which is why increasing the temperature of a gas at constant volume increases its pressure.
The Kelvin temperature of a gas is proportional to the average kinetic energy of its molecules. This is a fundamental relationship that underpins all the gas behaviour described in this section. It is worth memorising as a direct statement because it appears frequently in exam questions.
Gas laws: qualitative relationships
The specification requires you to explain the qualitative relationships for a fixed amount of gas:
Pressure and volume at constant temperature (Boyle's law): At constant temperature, when the volume of a gas decreases, the pressure increases. This happens because the molecules have less space to move in, so they collide with the walls more frequently, increasing the pressure. Conversely, increasing the volume gives the molecules more space, reduces the collision rate, and the pressure drops. The relationship is inversely proportional: if you halve the volume, the pressure doubles (provided the temperature stays constant).
Pressure and Kelvin temperature at constant volume (pressure law): At constant volume, when the Kelvin temperature of a gas increases, the pressure increases. Higher temperature means faster molecules, which produce more forceful and more frequent collisions with the container walls. The relationship is directly proportional: doubling the Kelvin temperature doubles the pressure. This is why a sealed container should not be heated without a safety mechanism: the rising pressure could cause it to burst.
Common mistakes
- Using Celsius in gas law proportionalities: Gas law relationships only hold when temperature is measured in Kelvin. A student who writes "the temperature doubled from 100 °C to 200 °C, so the pressure doubled" has made an error. In Kelvin, 100 °C is 373 K and 200 °C is 473 K, which is a ratio of about 1.27, not 2.
- Confusing pressure with force: Pressure is not force. It is force per unit area. A large force spread over a large area can produce a low pressure. A small force concentrated on a tiny area can produce a very high pressure. Always specify both the force and the area when explaining pressure.
- Forgetting to explain gas pressure in terms of molecules: When the exam asks you to "explain" why gas pressure increases, you must mention molecules (or particles), their collisions with the container walls, and the frequency or force of those collisions. Simply saying "the pressure increases because the temperature increases" is description, not explanation.
Self-check questions
Use these edexcel igcse science single award practice questions to test your understanding of physics: solids, liquids and gases edexcel igcse content. Write your answers before checking against the edexcel igcse science single award notes above.
- A force of 450 N acts on an area of 0.15 m2. Calculate the pressure.
- Convert 350 K to degrees Celsius. Convert 25 °C to Kelvin.
- Explain, in terms of molecules, why the pressure of a gas increases when its volume is decreased at constant temperature.
- Explain why the Kelvin scale, rather than the Celsius scale, must be used when relating temperature to the average kinetic energy of gas molecules.
- A sealed container of gas is heated from 300 K to 600 K at constant volume. Describe what happens to the pressure and explain why in terms of molecular behaviour.
For question 1: p = F / A = 450 / 0.15 = 3,000 Pa. For question 2: 350 K = 350 - 273 = 77 °C; 25 °C = 25 + 273 = 298 K. For question 5: the pressure doubles because the Kelvin temperature has doubled; the molecules have twice the average kinetic energy, so they move faster and collide with the container walls more frequently and more forcefully.
These edexcel igcse science single award notes on igcse 4ss0 physics: solids, liquids and gases cover every specification point in the section. The exam will test your ability to apply p = F / A, convert between temperature scales, and explain gas behaviour in terms of molecular motion. If you can do those three things confidently, you are well prepared for this part of the edexcel igcse science single award explained material on the Green Bridge CBT platform.
Revision notes for edexcel igcse science single award physics: solids, liquids and gases covering pressure, gas laws and the Kelvin scale.
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