IGCSE Physics: Density Practice Questions
Density is the mass per unit volume of a substance, calculated as density = mass divided by volume. An object floats in a liquid if its density is less than the density of that liquid.
Sub-topic 1.4 of Cambridge IGCSE Physics 0625 appears on both the theory and practical papers. Almost every mark lost in this topic comes from unit conversion between g/cm3 and kg/m3, or from a poorly described method. The questions below cover both.
What you need to know for Density
- Density equationDensity = mass divided by volume. Units are g/cm3 or kg/m3. To convert from g/cm3 to kg/m3, multiply by 1000.
- Regular solidMeasure the mass on a balance and calculate the volume from the measured dimensions, then divide.
- Irregular solidMeasure the mass on a balance. Find the volume by displacement, either the change in level in a measuring cylinder or the overflow from a displacement can.
- LiquidMeasure the mass of an empty measuring cylinder, add the liquid, measure the new mass and subtract. Read the volume at eye level from the bottom of the meniscus.
- Floating and sinkingAn object floats in a liquid if its average density is less than the density of the liquid, and sinks if it is greater. A steel ship floats because its overall shape encloses a large volume of air, lowering the average density.
- Density of water1.0 g/cm3, which is 1000 kg/m3. This is worth memorising, since many questions expect it without stating it.
IGCSE Physics Density questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A metal block has a mass of 486 g and measures 6.0 cm by 3.0 cm by 10.0 cm. Calculate its density in g/cm3 and in kg/m3.
Show the worked answer
- Volume = 6.0 x 3.0 x 10.0 = 180 cm3.
- Density = mass divided by volume = 486 divided by 180.
- Density = 2.7 g/cm3.
- To convert to kg/m3, multiply by 1000: 2.7 x 1000 = 2700 kg/m3. This value suggests the metal is aluminium.
Describe how you would measure the density of a small irregular stone accurately, naming the apparatus you would use.
Show the worked answer
- Measure the mass of the dry stone using a balance and record it in grams.
- Part fill a measuring cylinder with water and record the initial volume, reading at eye level from the bottom of the meniscus.
- Lower the stone gently into the water on a thread until it is fully submerged, avoiding splashing, and record the new volume.
- The volume of the stone is the difference between the two readings.
- Calculate density = mass divided by volume. Dry the stone before weighing, since water on the surface would increase the measured mass and give too high a density.
Explain why a steel ship floats on water even though the density of steel is about 7800 kg/m3, far greater than the density of water.
Show the worked answer
- An object floats when its average density is less than the density of the liquid it is placed in.
- A solid block of steel has a density of about 7800 kg/m3, far greater than the 1000 kg/m3 of water, so it sinks.
- A ship is not solid steel. Its hull is shaped to enclose a very large volume of air.
- The total mass of the ship divided by its total volume, including the air inside the hull, is less than 1000 kg/m3. The average density is therefore lower than that of water and the ship floats.
A liquid has a density of 0.80 g/cm3. Calculate the mass of 250 cm3 of this liquid, and state whether a plastic cube of density 1.2 g/cm3 would float or sink in it.
Show the worked answer
- Rearrange density = mass divided by volume to give mass = density x volume.
- Mass = 0.80 x 250 = 200 g.
- The plastic has a density of 1.2 g/cm3, which is greater than the 0.80 g/cm3 of the liquid.
- An object sinks when its density is greater than that of the liquid, so the cube sinks.
Common mistakes in this topic
- Converting between g/cm3 and kg/m3 in the wrong direction.
- Forgetting to subtract the initial water level when using displacement.
- Weighing a wet object, which raises the apparent mass.
- Reading a measuring cylinder from above rather than at eye level.
- Assuming an object floats or sinks from experience rather than from the densities given.
Exam tips
- Write the equation and rearrange it before substituting. Method marks come from the rearrangement.
- Memorise that water is 1.0 g/cm3 or 1000 kg/m3. Many questions rely on it without stating it.
- For any density practical, describe mass and volume measurements separately, then combine.
- Check the unit asked for in the question before converting anything.
- For a floating question, always finish by comparing the two density values explicitly.
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Density FAQs
How do I calculate density?
Divide the mass by the volume. If the mass is in grams and the volume in cubic centimetres, the density is in g/cm3. If the mass is in kilograms and the volume in cubic metres, the density is in kg/m3. To convert from g/cm3 to kg/m3, multiply by 1000.
How do I measure the density of an irregular object?
Measure the mass with a balance. Find the volume by displacement: record the water level in a measuring cylinder, lower the object in until fully submerged, record the new level, and subtract. Divide the mass by that volume. Dry the object before weighing it.
Why does a steel ship float?
Floating depends on average density, not on the density of the material alone. A ship's hull encloses a very large volume of air, so the total mass of the ship divided by its total volume is less than the density of water. The average density is therefore lower than 1000 kg/m3 and the ship floats.
What is the density of water?
1.0 g/cm3, which is the same as 1000 kg/m3. This value is often needed without being given in the question, so it is worth memorising. Any object with a density below this value will float in water, and any object above it will sink.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.