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    Density required practical: method, results and exam tips

    By Engr. Mehmood Kakar · Updated

    Quick answer

    To find density, measure mass with a balance and volume by the right method, then use ρ = m ÷ V. For a regular solid, measure its sides and multiply. For an irregular solid, measure the water it displaces in a eureka can or measuring cylinder. For a liquid, measure a volume in a measuring cylinder and weigh it on a zeroed balance.

    What you are finding

    The density practical asks you to find the density of three kinds of thing: a regular solid, an irregular solid and a liquid. Mass is always measured the same way, on a top-pan balance. What changes is how you find the volume. Once you have both, use ρ = m ÷ V.

    The numbers in the worked examples below are sample values to show the calculation, not real measurements.

    Regular solids

    1. Measure the mass of the object on a balance.
    2. Measure its length, width and height with a ruler, or vernier callipers for small objects.
    3. Multiply the three to get the volume of a cuboid.
    4. Divide mass by volume.

    A cube has sides of 2.0 cm and a mass of 21.6 g. Calculate its density.

    1. V = 2.0 × 2.0 × 2.0 = 8.0 cm³
    2. ρ = m ÷ V = 21.6 ÷ 8.0
    3. ρ = 2.7 g/cm³ (2700 kg/m³)

    Irregular solids

    A stone or a lump of modelling clay has no sides to measure, so you find its volume by displacement.

    1. Measure the mass of the object.
    2. Fill a eureka can with water until it drips from the spout, and wait for it to stop.
    3. Put an empty measuring cylinder under the spout.
    4. Lower the object gently into the can on a thread. Collect all the water that overflows.
    5. Read the volume of water in the cylinder. This equals the volume of the object.

    If the object fits in a measuring cylinder, you can instead note the water level before and after lowering it in. The rise in level is the object's volume.

    A stone has a mass of 39 g and displaces 15 cm³ of water. Calculate its density.

    1. ρ = m ÷ V = 39 ÷ 15
    2. ρ = 2.6 g/cm³ (2600 kg/m³)

    Liquids

    1. Put an empty measuring cylinder on the balance and zero it (press tare).
    2. Pour in some liquid, for example 50 cm³, reading the bottom of the meniscus at eye level.
    3. Read the mass shown on the balance. Because you zeroed it, this is the mass of the liquid only.
    4. Divide mass by volume.

    50 cm³ of a liquid has a mass of 40 g. Calculate its density.

    1. ρ = m ÷ V = 40 ÷ 50
    2. ρ = 0.80 g/cm³ (800 kg/m³)

    g/cm³ and kg/m³

    In the lab you measure grams and cm³, so g/cm³ comes out naturally. Exam questions often want kg/m³. To convert, multiply by 1000: 2.7 g/cm³ = 2700 kg/m³.

    If you convert before dividing, remember that 1 m³ is 1,000,000 cm³, not 100 or 1000. The units guide explains why.

    Errors and accuracy

    • Parallax error. Read the measuring cylinder with your eye level with the bottom of the meniscus.
    • Splashing. Dropping the object into the eureka can splashes water out without it going into the cylinder, so the volume reads too low. Lower it slowly on a thread.
    • Air bubbles. Bubbles stuck to an irregular object add to the displaced volume. Tap the object gently to release them.
    • Resolution. A small measuring cylinder has finer scale divisions, so use the smallest one that holds the water.
    • Repeats. Repeat each measurement and calculate a mean to reduce the effect of random errors.

    Exam questions on this practical

    Questions usually ask you to describe a method, name a piece of equipment, explain an error or calculate a density from given results. A 6-mark "describe a method" answer should cover how you find mass, how you find volume, the calculation and at least one way to improve accuracy. Write it as numbered steps; it is easier for you to check and for the examiner to mark.

    Practice

    1. A block of wood measures 5.0 cm × 4.0 cm × 2.0 cm and has a mass of 32 g. Calculate its density in g/cm³.

    V = 5.0 × 4.0 × 2.0 = 40 cm³ → ρ = 32 ÷ 40 → ρ = 0.80 g/cm³

    2. A key has a mass of 62.4 g. When lowered into a measuring cylinder, the water level rises from 30.0 cm³ to 38.0 cm³. Calculate its density.

    V = 38.0 − 30.0 = 8.0 cm³ → ρ = 62.4 ÷ 8.0 → ρ = 7.8 g/cm³ (7800 kg/m³)

    3. Explain why the volume of an irregular object might be measured as too small when using a eureka can.

    Some water may splash out of the can without being collected, or water may be left in the spout. Either way, less water reaches the measuring cylinder than the object displaced.

    Frequently asked questions

    What equipment do you need for the density practical?

    A top-pan balance, a ruler (or vernier callipers) for regular objects, a eureka can and measuring cylinder for irregular objects, and a measuring cylinder for liquids.

    How do you find the volume of an irregular object?

    Fill a eureka can to the spout, lower the object in gently, and collect the water that overflows in a measuring cylinder. The volume of water collected equals the volume of the object.

    Why do you read the bottom of the meniscus?

    Water curves up at the sides of a measuring cylinder. Reading the bottom of the curve, with your eye level with it, gives the correct volume and avoids parallax error.

    How do you improve the accuracy of the density practical?

    Use a balance with a higher resolution, vernier callipers for small dimensions, the smallest measuring cylinder that holds the volume, and repeat each measurement to calculate a mean.

    Engr. Mehmood Kakar

    Written by Engr. Mehmood Kakar

    Physics educator and engineer (MSc Physics, B.Ed, M.Ed) with 8 years of hands-on teaching, from college students to teacher training. About the author · LinkedIn (opens in a new tab)

    Checked against the AQA GCSE Physics (8463) specification on 14 November 2026.