Measuring Mass and Volume

Your school bag, your water bottle, a handful of marbles — they all take up space and they all contain matter. But how would you measure the space a marble takes up, or tell exactly how much matter is in a stone? You cannot wrap a ruler around a lumpy rock. Scientists solved this problem long ago, and once you see it, you will never forget it. Let us find out how.

Parents: each section has a "predict first" step — please let your child make their guess before tapping the reveal. The interactive experiment in the Volume section is the heart of this note; doing it together takes about ten minutes and really locks in the keyword.

By the end you will be able to explain how to measure the volume of any shape of object, find the mass of anything using a balance, and write the correct units every time. The four ideas to master are: volume, water displacement, mass, and the correct units for each.

Volume — the space something takes up

Put your hand flat on your desk. Now make a fist. Your fist takes up more space than your flat hand. That extra space your fist fills is volume. Bigger objects take up more space — they have more volume.

For a neat shape like a rectangular block, you can measure its length, width, and height with a ruler, then multiply them together. But what about a stone? Or a key? Or a crumpled ball of foil? You cannot get a ruler to measure all their bumpy edges. There is a clever trick for that — and it uses water.

Water displacement — measuring irregular shapes

Here is the experiment your science teacher would set up on the lab bench. Fill a measuring cylinder with water to a known level — say, 50 ml. Now carefully lower an irregular object into the water. The water level rises. The amount it rises equals the volume of the object, because the object pushed aside (displaced) exactly its own volume of water.

This method is called water displacement. It works for any solid object that does not dissolve in water and is small enough to fit in the cylinder. A stone, a key, a coin — all easy to measure this way.

Try it yourself below. Pick a different object each time and read off the rise.

🔬

Water displacement — measure any irregular object

🤔 Predict first: How can you measure the volume of a stone that has no neat shape?

Wrap it in a rulerWater displacementWeigh it on a balance

Mass — the amount of matter inside

Pick up an empty crisp packet. Now pick up a full packet. The full one feels heavier — it contains more matter. The amount of matter in an object is its mass. A bigger, denser object has more mass than a small, light one.

Mass is not the same as size. A small piece of lead has far more mass than a large piece of foam, because lead has more matter packed into a smaller space. Two objects can look the same size but have very different masses.

We measure mass with a balance. A balance has two pans. You place the object on one pan and add known standard masses (sometimes called weights) to the other pan until both sides are level. The total of the standard masses equals the mass of the object.

Units — getting the numbers right

Science answers without units are incomplete. The marker needs to see the right unit next to your number.

What you are measuring	Unit	Symbol
Volume (liquid or solid)	millilitre or centimetre cubed	ml or cm³
Mass (small object)	gram	g
Mass (heavy object)	kilogram	kg

One useful fact to remember: 1 ml of water has a mass of 1 g. That is why the water displacement method works so neatly — 20 cm³ of water displaced means 20 ml of water pushed aside.

Watch out — these are easily mixed up

Quick recap

🎯 Mastery check

Answer all 8 — your progress is saved on this device.

1. A student places a stone in a measuring cylinder. The water level rises from 40 ml to 55 ml. What is the volume of the stone?

   40 cm³55 cm³15 cm³

2. Which instrument is used to measure the mass of an object?

   Measuring cylinderRulerBalance

3. A student writes that a marble has a volume of "8". Why is this answer incomplete?

   The number is wrongThere is no unit — it should be 8 cm³ or 8 mlVolume cannot be measured for a sphere

4. You want to find the volume of a metal key. You cannot measure it with a ruler because of its irregular shape. What is the best method?

   Wrap the key in clay and mould it into a cubeUse water displacement with a measuring cylinderPlace the key on a balance

5. A large sponge and a small iron nail are placed on a balance. The nail tips the pan. Which object has more mass?

   The sponge, because it is biggerThe nail, because it has more matter in a smaller spaceThey have the same mass

6. The correct unit for the volume of an irregular solid measured by water displacement is…

   kgcm³ or mlcm

7. A student places an apple on one pan of a balance and adds 150 g and 50 g standard masses to the other pan to make it level. What is the mass of the apple?

   100 g150 g200 g

8. Why can water displacement NOT be used to find the volume of a sugar cube?

   Sugar is too light to sinkSugar dissolves in water, so the reading would be wrongSugar is a liquid

See my score