Science required practicals
How do you do the density required practical?
Measure the mass on a balance, find the volume, then use density = mass ÷ volume. For a regular block, work out the volume from its length, width and height. For an irregular object, lower it into a full eureka can and collect the water it pushes out in a measuring cylinder. For a liquid, measure the mass of a known volume. Give answers in g/cm³ or kg/m³ (1 g/cm³ = 1000 kg/m³).
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the density practical investigate?
It measures the density of solids and liquids: how much mass is packed into each unit of volume. Mass is the easy part, because a balance gives it to you. The real work is finding the volume, which needs a different method for a regular shape, an irregular one and a liquid.
It is a required practical on AQA GCSE Physics and on AQA GCSE Combined Science: Trilogy, and it sits in the particle model of matter topic.
What equipment do you need?
You need a balance for the mass, and different tools for the volume depending on what you are measuring.
- A top-pan balance
- A ruler, plus Vernier callipers or a micrometer for small objects
- Regular solids, such as metal or wooden blocks
- Irregular objects that sink in water, such as a stone or a lump of modelling clay
- A eureka can (also called a displacement can) and a measuring cylinder
- Thread, for lowering objects into the water
- The liquids you are testing
What is the method?
Measure the mass first, while the object is still dry. Then find the volume in the way that suits the object.
- Regular solid: measure its mass on the balance. For a cuboid, measure the length, width and height, and multiply them together to get the volume.
- Irregular solid: measure its mass first. Fill the eureka can with water until it runs out of the spout, and wait until it stops dripping.
- Put an empty measuring cylinder under the spout. Tie the object to a thread and lower it gently into the can until it is completely under the water.
- When the dripping stops, read the volume of water in the measuring cylinder. That is the volume of the object.
- Liquid: stand an empty measuring cylinder on the balance and set it to zero. Pour in 10 cm³ of the liquid and record the mass, then keep adding 10 cm³ at a time, recording the total mass each time.
- Repeat your measurements and work out a mean density for each object and liquid.
What are the variables?
This practical measures a property rather than testing a relationship, so there is less to change. You pick the object or liquid, then measure its mass and volume to work out its density.
| Variable | In this practical |
|---|---|
| Independent variable | The material: the object or liquid you are testing |
| Dependent variable | Its density, worked out from the mass and volume you measure |
| Control variable | The water in the eureka can, filled to the spout and left to stop dripping before each object |
| Control variable | The balance: the same one each time, set to zero before every reading |
| Control variable | The temperature of the liquid, since a liquid's density changes slightly as it warms |
How do you work out and present the results?
Use density = mass ÷ volume, or ρ = m ÷ V. A mass in grams and a volume in cm³ give g/cm³. A mass in kilograms and a volume in m³ give kg/m³. To go from g/cm³ to kg/m³, multiply by 1000, because 1 g/cm³ = 1000 kg/m³.
Worked example: a metal block measures 5.0 cm × 4.0 cm × 2.0 cm, so its volume is 40 cm³. Its mass is 316 g, so ρ = 316 ÷ 40 = 7.9 g/cm³, which is 7900 kg/m³. A stone of mass 54 g that pushes 20 cm³ of water out of the eureka can has ρ = 54 ÷ 20 = 2.7 g/cm³, or 2700 kg/m³.
For the liquid, plot mass (y-axis) against volume (x-axis). You should get a straight line through the origin, and its gradient is the density. If 50 cm³ of cooking oil has a mass of 46 g, its density is 46 ÷ 50 = 0.92 g/cm³. That is less than water (1.0 g/cm³), which is why oil floats on water.
How do you make it accurate and safe?
Read the measuring cylinder at eye level, from the bottom of the curved water surface (the meniscus). Use the smallest measuring cylinder that holds the water, because its scale is finer. With the eureka can, wait for the dripping to stop before and after you lower the object in, and lower it gently so no water splashes over the side. For blocks, measure each side in two or three places and use the mean.
On safety: wipe up spilt water straight away so nobody slips. Lower heavy objects on a thread rather than dropping them in.
What do exam questions ask about it?
Expect to calculate a density, often with a unit conversion, or to rearrange ρ = m ÷ V to find a mass or a volume. Method questions ask how to find the volume of an irregular object, or why each step of the eureka can method matters.
Here is one we wrote in the exam style: "A student uses a eureka can to find the volume of a key. Explain why she waits for the can to stop dripping before she puts the measuring cylinder under the spout. (2 marks)" A good answer says that water already dripping would be collected too, so the volume measured would be bigger than the volume of the key.
Common mistakes
- Weighing the object after it has been in the water. Water clinging to it adds mass, so weigh it first.
- Reading the top of the meniscus. It makes the volume look bigger than it is.
- Mixing units, such as a mass in grams with a volume in m³. Keep grams with cm³, and kilograms with m³.
- Converting the wrong way. 2.7 g/cm³ is 2700 kg/m³, not 0.0027 kg/m³.
- Leaving part of the object above the water. It has to be completely covered, or its volume comes out too small.