## Measuring density

Classroom Activity for 11-14

What the Activity is for

This activity allows pupils to make some measurements to measure the density of a material for themselves.

What to Prepare

- A simple balance or scale calibrated in grams or kilograms to find the mass
- A ruler
- A measuring jug or cylinder

What Happens During this Activity

Pupils find the mass of an object and then its volume. By dividing mass by the volume they can calculate the density of the material from which the object is made. The maths is simple – just a division of two numbers.

The volume can be measured using a ruler if the object is a regular shape such as a cube (length × width × height). For irregular shapes such as a lump of plasticine use a measuring jug or a scientific measuring cylinder. By immersing the object in water, the volume can be seen as equal to the extra volume which the water appears to reach on the scale.

A note on units:

The units of density are the units of mass divided by the units of volume. If you measure the mass in grams and the volume in cubic centimetres then the density can be stated in units of gram / cubic centimetre (gram centimetre^{-3}
). For example the density of plasticine might be 2.5 gcm^{-3}.

For larger volumes, such as water in a swimming pool, the mass might be measured in kilogram and the volume in cubic metre. The density might then be quoted as 1000 kilogram / cubic metre. (Incidentally a swimming pool full of plasticine would have a very large mass. The density would be calculated from measurements in kilogram and cubic metre and would be quoted as 2500 kilogram / cubic metre. However it is still plasticine. The density is the same, no matter how much of it you are measuring).

It is important to note that the density of a material is unchanged should its value be quoted in different units. It is still the same stuff. So water might be quoted in a book as having a density of 1 kilogram / cubic centimetre but also 1000 kilogram / cubic metre. They amount to the same thing. Similarly, a density of 2.5 gram / cubic centimetre is the same density as 2500 kilogram / cubic metre.