Applies to: AQA, OCR Gateway, Edexcel (combined and separate), Edexcel International GCSE, and Cambridge IGCSE's listed derived quantities.

Drop a stone and an apple into a bowl of water. The stone sinks straight to the bottom. The apple floats. Both of them have mass, so why does one sink and the other doesn't? The answer is density, and it's one of the most useful required practicals because it turns a bit of maths into something you can actually watch.

What it tests: The mass per unit volume of regular solids, irregular solids, and liquids.

What you need: Kitchen scales, a measuring jug, a ruler, and a few household objects, ideally a mix of things that sink and things that float.

How to do it: For a regular shaped object, like a bar of soap or a wooden block, measure its length, width and height with a ruler and multiply them together to get the volume. Weigh it on the scales to get the mass. Density equals mass divided by volume. For an irregular object, like a stone or a small toy, fill a measuring jug partway with water, note the level, drop the object in, and note how much the water level rises. That rise in millilitres is the object's volume, since one millilitre of water displaced equals one cubic centimetre. For a liquid, weigh an empty container, add a measured volume of the liquid, weigh it again, and subtract to find the mass of the liquid alone.

Safety note: Standard kitchen care, nothing hazardous, just be mindful with anything breakable near water.

Why it matters for the exam: Density questions usually give you a mass and a volume and ask you to calculate the density, or give you two densities and ask you to explain why one object floats and another sinks. Having actually measured and calculated it yourself, rather than just plugging numbers into a formula on a worksheet, tends to make the logic stick.

On motivation: If density has always felt like "just maths" rather than real science, that's a fair reaction to how it's often taught. Watching the stone sink and the apple float, then working out the numbers that explain why, turns it back into something you can actually picture. You're not bad at density if a formula alone didn't make sense. Formulas rarely do, on their own.