Measuring the mass

To find a density you need two measurements: mass and volume. Start with mass.

Mass is the amount of matter in an object (Grade 6). It's measured with a balance, like the ones you see in shops.

Measure it (Activity 9.3). Switch on a digital balance. It should read zero; if not, press the tare (reset) button.

Put a clean, dry watch glass or a piece of butter paper on the pan. The reading changes. Now press tare again, so it reads zero with the watch glass on it. This is the key step: the balance now ignores the watch glass and counts only what you add.

Gently place the object, say a stone, on the watch glass and read the display. That's the stone's mass, for example 16.400 g. Any balance in your school will do.

To weigh a liquid, use a beaker instead of a watch glass: put it on, tare, then pour in the liquid.

A word of caution. In daily talk mass and weight mean the same, but not in science:

Most balances (except two-pan ones) actually measure weight, but their scales are marked in mass units, so they show grams or kilograms.