Density
Cambridge IGCSE Physics (0625) · Unit 1: Motion, forces and energy · 8 flashcards
Density is topic 1.4 in the Cambridge IGCSE Physics (0625) syllabus , positioned in Unit 1 — Motion, forces and energy , alongside Physical quantities and measurement techniques, Motion and Mass and weight.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 show this topic across undefined questions worth 81 marks (around 1.2% of all Physics marks in those years).
The deck below contains 8 flashcards — covering the precise wording mark schemes reward.
What the Cambridge 0625 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- Define Define density as mass per unit volume; recall and use the equation m P=V
- Describe Describe how to determine the density of a liquid, of a regularly shaped solid and of an irregularly shaped solid which sinks in a liquid (volume by displacement), including appropriate calculations
- Determine Determine whether an object floats based on density data
- Determine Determine whether one liquid will float on another liquid based on density data given that the liquids do not mix Supplement
A metal block has a mass of 250g and a volume of 100 cm³. Calculate the density of the metal block in kg/m³.
Density (ρ) = mass (m) / volume (V)
m = 250g = 0.25 kg
V = 100 cm³ = 100 x 10⁻⁶ m³ = 1 x 10⁻⁴ m³
ρ = 0.25 kg / (1 x 10⁻⁴ m³)
ρ = 2500 kg/m³
Density is calculated by dividing the mass by the volume. We converted the given mass and volume to SI units (kg and m³ respectively) before calculation.
Explain why a large piece of polystyrene floats on water, while a small stone sinks, even though the stone is smaller.
Density is mass per unit volume. Polystyrene has a low density, meaning that a given volume of polystyrene has a small mass, smaller than the mass of the same volume of water. Because it is less dense than water, it floats. The stone has a high density - a given volume has a large mass, larger than the mass of the same volume of water. Because it is more dense than water, it sinks.
A student uses a measuring cylinder to find the volume of 10 identical metal spheres. The initial volume of water in the cylinder is 50 cm³. After adding the 10 spheres, the water level rises to 65 cm³. The mass of the 10 spheres is 150g. Calculate the density of the metal used to make the spheres. Give your answer in g/cm³.
1. Calculate the volume of the spheres:
Volume of spheres = Final volume - Initial volume = 65 cm³ - 50 cm³ = 15 cm³
2. Calculate the volume of one sphere:
Volume of one sphere = Total volume / Number of spheres = 15 cm³ / 10 = 1.5 cm³
3. Calculate the mass of one sphere:
Mass of one sphere = Total mass / Number of spheres = 150 g / 10 = 15 g
4. Calculate the density:
Density = Mass / Volume = 15 g / 1.5 cm³ = 10 g/cm³
*Density of the metal is 10 g/cm³*
Describe how you would accurately determine the density of an irregularly shaped stone that sinks in water using laboratory equipment. Explain each step and the measurements you would take.
1. Measure the mass: Use a balance to find the mass (m) of the stone in grams (g).
2. Measure the volume (using displacement):
* Partially fill a measuring cylinder with a known volume (V1) of water and record this volume.
* Carefully lower the stone into the measuring cylinder ensuring it is fully submerged. Make sure no water splashes out.
* Record the new volume (V2) of the water level.
* The volume of the stone (V) is the difference between the two readings: V = V2 - V1. The volume will be in cm³.
3. Calculate the density: Use the formula Density = Mass / Volume. Substitute the values of mass (m) and volume (V) to calculate the density of the stone in g/cm³.
*This method uses the principle of displacement to determine the volume of an irregularly shaped object.*
A block of wood has a volume of 0.005 m³ and a mass of 4.0 kg. The density of water is 1000 kg/m³. Calculate the density of the wood. Will the wood float in water? Show your working.
Density = Mass / Volume
Density of wood = 4.0 kg / 0.005 m³ = 800 kg/m³
Yes, the wood will float. The density of the wood (800 kg/m³) is less than the density of water (1000 kg/m³). Objects float if their density is less than the density of the fluid they are placed in.
A plastic ball has a density of 800 kg/m³, while a glass marble has a density of 2500 kg/m³. Both are placed in a container of water, which has a density of 1000 kg/m³. Explain why the plastic ball floats but the glass marble sinks.
The plastic ball floats because its density (800 kg/m³) is less than the density of water (1000 kg/m³). An object will float if it is less dense than the fluid it is in.
The glass marble sinks because its density (2500 kg/m³) is greater than the density of water (1000 kg/m³). An object will sink if it is more dense than the fluid it is in.
Liquid A has a density of 0.8 g/cm³ and Liquid B has a density of 1.2 g/cm³. If these two liquids do not mix, which liquid will float on the other? Calculate the difference in mass between 50 cm³ of Liquid A and 50 cm³ of Liquid B.
1. Identify which liquid floats:
Liquid A will float on Liquid B because Liquid A has a lower density (0.8 g/cm³) than Liquid B (1.2 g/cm³). Less dense objects float on more dense objects.
2. Calculate mass of Liquid A:
Density = Mass / Volume
Mass = Density x Volume
Mass of A = 0.8 g/cm³ x 50 cm³ = 40 g
3. Calculate mass of Liquid B:
Mass of B = Density x Volume
Mass of B = 1.2 g/cm³ x 50 cm³ = 60 g
4. Calculate the difference in mass:
Difference = Mass of B - Mass of A
Difference = 60 g - 40 g = 20 g
Answer: Liquid A will float on Liquid B. The mass difference is 20 g.
You have two liquids, X and Y, which do not mix. Liquid X has a density of 1.1 g/cm³ and Liquid Y has a density of 0.9 g/cm³. Describe what will happen when Liquid Y is poured into a container already partially filled with Liquid X.
Liquid Y will float on top of Liquid X. This is because Liquid Y has a lower density (0.9 g/cm³) than Liquid X (1.1 g/cm³). Less dense liquids float on more dense liquids. The liquids will form two separate layers, with Liquid Y on top and Liquid X at the bottom.
Tips to avoid common mistakes in Density
- ● To find added-substance density, divide the added substance's mass INCREASE by its volume INCREASE (ρ = Δm/ΔV).
- ● Density is mass/volume, so for an *added* substance, calculate density using the *increase* in mass divided by the *increase* in volume.
- ● Remember the density formula, ρ = m/V, applies to *changes* in m and V if you're finding the density of *added* material.
- ● Always convert to standard SI units (kilograms, meters, seconds) first, especially before using any formula.
- ● Fix the density formula in your mind: density = mass / volume (with density usually measured in kg/m^3 or g/cm^3).
More topics in Unit 1 — Motion, forces and energy
Density sits alongside these Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
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Cambridge syllabus keywords to use in your answers
These are the official Cambridge 0625 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
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