What Is Density And Pressure? A Visual Guide for Edexcel IGCSE Physics Students

Density and pressure are among the most commonly examined concepts in Edexcel IGCSE Physics, and the calculations are straightforward once the formulae are secure

The Pearson Edexcel IGCSE Physics specification (4PH1) treats density and pressure as foundational quantitative skills. Questions on these topics appear with notable regularity across both papers. The concepts are tested through direct calculations, practical context questions (measuring density of irregular objects, hydraulic systems) and data-interpretation problems. This guide covers every definition, formula and exam pattern you need.

Key Facts

Quantity	Formula	SI Unit
Density	rho = m / V	kg/m³
Pressure	p = F / A	Pa (pascal)
Pressure difference in a fluid	p = h x rho x g	Pa

Density: the edexcel igcse physics definition

Density is the mass per unit volume of a substance. The edexcel igcse density and pressure topic requires you to know the formula rho = m / V and to rearrange it confidently. If density is high, it means a large amount of mass is packed into a small volume. If density is low, the same volume contains less mass.

The SI unit of density is kg/m³. Water has a density of approximately 1000 kg/m³. Objects with a density greater than the fluid they are placed in will sink; objects with a density less than the fluid will float.

Worked example: A metal block has a mass of 540 g and dimensions 10 cm x 5 cm x 4 cm. Calculate its density in kg/m³.

Convert mass: 540 g = 0.54 kg
Calculate volume: 10 x 5 x 4 = 200 cm³ = 200 x 10^-6 m³ = 0.0002 m³
rho = m / V = 0.54 / 0.0002 = 2700 kg/m³

Unit conversion trap: Exam questions frequently give mass in grams and dimensions in centimetres. You must convert to kilograms and metres before substituting into the formula if you want the answer in kg/m³. Alternatively, use g/cm³ if the question asks for it, but state the unit clearly.

Measuring density experimentally

The specification requires you to describe how to investigate density using direct measurements of mass and volume. The edexcel igcse physics notes on this topic cover two main methods.

Regular solids: Measure the mass using a balance. Measure the dimensions (length, width, height) using a ruler or vernier calliper. Calculate the volume using the appropriate formula (e.g. V = l x w x h for a cuboid). Then apply rho = m / V.

Irregular solids: Measure the mass using a balance. Find the volume by displacement: fill a measuring cylinder or eureka can with water, note the initial level, lower the object in, and note the new level. The difference is the volume of the object. Then apply rho = m / V.

Pressure: force per unit area

Pressure is defined as the force acting per unit area. The formula is p = F / A, where p is pressure in pascals (Pa), F is force in newtons (N) and A is area in square metres (m²). One pascal is one newton per square metre.

Worked example: A woman wearing stiletto heels stands on one heel. Her weight is 600 N and the heel has an area of 0.0001 m². Calculate the pressure under the heel.

p = F / A = 600 / 0.0001 = 6,000,000 Pa = 6 MPa

This explains why stiletto heels can damage floors: the same force concentrated over a tiny area produces enormous pressure. The IGCSE exam regularly tests this concept by comparing objects with different base areas supporting the same weight.

Pressure in fluids

At any point in a gas or liquid that is at rest, pressure acts equally in all directions. This is a key statement from the specification. The pressure at a given depth in a fluid depends on the height of fluid above that point, the density of the fluid and the gravitational field strength.

The formula is: pressure difference = height x density x gravitational field strength, or p = h x rho x g.

Worked example: Calculate the pressure due to a column of water 5 m deep. The density of water is 1000 kg/m³ and g = 9.8 N/kg.

p = h x rho x g = 5 x 1000 x 9.8 = 49,000 Pa

This is the additional pressure caused by the water. The total pressure at the bottom includes atmospheric pressure on top of this value.

Exam question patterns for density and pressure

The what is density and pressure igcse question bank reveals several recurring patterns that the density and pressure explained approach should prepare you for.

Pattern 1: Calculate density from given mass and volume. Straightforward substitution. The challenge is unit conversion.

Pattern 2: Explain why an object floats or sinks. The answer requires you to compare the density of the object with the density of the fluid. If the object's density is less than the fluid's density, it floats. If greater, it sinks.

Pattern 3: Compare pressures under different objects. Two objects with the same weight but different base areas produce different pressures. The one with the smaller area produces greater pressure.

Pattern 4: Calculate pressure at a given depth in a fluid. Use p = h x rho x g. Watch for questions that give depth in centimetres (convert to metres) or density in g/cm³ (convert to kg/m³).

Pattern 5: Rearrange the formula. You may be given density and volume and asked for mass (m = rho x V), or given pressure and force and asked for area (A = F / p). Practise rearranging all three formulae in both directions.

Given	Find	Rearranged formula
rho and V	m	m = rho x V
m and rho	V	V = m / rho
F and p	A	A = F / p
p and A	F	F = p x A
p and rho x g	h	h = p / (rho x g)

Common mistakes

Forgetting to convert units. Centimetres to metres, grams to kilograms. One unconverted value invalidates the whole calculation.
Confusing density with pressure. Density is about mass and volume; pressure is about force and area. They only meet in the fluid pressure formula.
Using the wrong formula. p = F / A is for solid surfaces. p = h x rho x g is for pressure difference in fluids. Make sure you use the one the question requires.
Ignoring atmospheric pressure. The formula p = h x rho x g gives the pressure due to the fluid column, not the total pressure. If the question asks for total pressure at depth, add atmospheric pressure (approximately 101,325 Pa).

Linking density and pressure to other topics

Density connects to the particle model of matter. Solids are generally denser than liquids, and liquids denser than gases, because the particles in a solid are packed closely together in a regular arrangement, while gas particles are widely spaced and move randomly. This particle-level understanding is tested on both Paper 1 and Paper 2 of the Edexcel IGCSE Physics exam.

Pressure in fluids connects to the behaviour of gases. When you heat a sealed container of gas, the particles move faster, strike the walls of the container more frequently and with greater force, and the pressure increases. The gas law p₁/T₁ = p₂/T₂ (with temperature in kelvin) quantifies this relationship. While the gas laws themselves belong to the "Ideal gas molecules" topic, the pressure concepts tested there rely on the same definition of pressure covered here.

Hydraulic systems use the principle that pressure in a fluid acts equally in all directions. A small force applied to a small-area piston creates a pressure that is transmitted through the fluid to a large-area piston, producing a large output force. The pressure is the same throughout: F₁ / A₁ = F₂ / A₂. This application has appeared in exam questions requiring candidates to calculate either the output force or the area ratio needed to achieve a desired mechanical advantage.

Worked example: In a hydraulic system, a force of 50 N is applied to a piston with area 0.002 m². The output piston has an area of 0.02 m². Calculate the output force.

Pressure = F / A = 50 / 0.002 = 25,000 Pa
Output force = p x A = 25,000 x 0.02 = 500 N

The mechanical advantage is 500 / 50 = 10. The force has been multiplied by a factor of 10, which equals the ratio of the piston areas (0.02 / 0.002 = 10). This is not "creating" force from nothing; the small piston must travel a greater distance than the large piston, so the work done (force x distance) is conserved.

Self-check questions

A cube of side 0.1 m has a mass of 2.5 kg. Calculate its density.
A force of 200 N acts on an area of 0.04 m². Calculate the pressure.
A diver descends to a depth of 20 m in seawater (density 1025 kg/m³). Calculate the pressure due to the water above the diver (use g = 10 N/kg).
Explain why a ship made of steel can float, even though steel is denser than water.
An object has a volume of 0.005 m³ and a density of 800 kg/m³. Calculate its mass and state whether it would float or sink in water.

Practise these question types on the Green Bridge CBT platform using Edexcel IGCSE Physics past questions filtered by topic, and review the edexcel igcse physics explained answers to build confidence before your exam.