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Measuring the density of regular solid shapes

Apparatus and materials

For each student group

Ruler, 15 or 30 cm

Micrometer

Vernier callipers

Chemical balance (0-1 kg)

Access to cuboidal samples of different metals and non metals, e.g. materials kit

Eureka can (OPTIONAL)

Aluminium cooking foil, sheet of (OPTIONAL)

Paper, sheet of (OPTIONAL)

‘Unknown’ object (a covered or painted regular object made of the same material as one of those used above OPTIONAL)

Procedure

a Measure the three dimensions of each regular object, repeating each measurement at two or more places. Depending on the size of the measurement, use the ruler, vernier callipers or a micrometer.

b Calculate the volume, V, of the object.

c Measure the mass, m, of the object, using the balance.

d Calculate the density, D, of each sample, using D = m/V.

e Compare your results with the accepted values for each material.

Teaching notes

1 Students will need to have studied density previously and be familiar with the density equation. Examples may have used cm3 as the unit of volume and g/cm3 as the unit of density, or m3 and kg/m3. Either sets of units are generally acceptable, but all length measurements must use the same unit.

2 The displacement method with a eureka can be used as a second method, particularly if the sample shape is not rectangular. If both methods are used for the same sample, students could compare their two values and decide which one is more accurate.

3 How Science Works Extension: In this experiment choose materials whose density is known to quite a high degree of accuracy, so that it is possible for students to perform a numerical comparison between the 'true' and measured values. They could be expected to find this data themselves or be provided with it. A good comparison will express the difference between the measured value and the true value as a percentage of the true value.

As an extension activity, you could ask students whether or not they can be confident that two different objects are made of the same material. Ask them to find the density of a sheet of aluminium cooking foil and compare it to that of the solid aluminium block. This same approach could be taken with a block of wood and a piece of paper. You might say "isn't paper made from wood?" as you pass the student the materials. Alternatively, you could give the students a sample of an unknown material and then samples of known materials, perhaps as a forensic test. The unknown object may need to be painted or coated in some way so that its surface does not provide other hints.

A critical point to make here is that, whilst they may be able to state with a great deal of confidence that two materials have the same density (or not), this does not mean that they are made from the same material. Further (chemical) testing would be needed to make an absolute statement of that kind.

This experiment was safety-checked in December 2006