# Measuring the density of air 1

##### Class practical

This is a very quick experiment that provides evidence that air has mass. This still surprises some students!

#### Apparatus and materials

For each student group

Party balloon, large

Bucket

Measuring jug

Electronic balance

#### Health & Safety and Technical notes

The bucket should be large enough to fully immerse an inflated balloon. A kitchen bin works well.

The electronic balance should be sensitive to 0.1 g or less.

#### Procedure

a Inflate the balloon.

b Immerse the balloon in the bucket of water and note the level the water rises to.

c Take the balloon out and leave it to dry.

d Pour water from a measuring jug into the bucket until the water level matches the level with the immersed balloon. Record how much water you added. This is the volume of the balloon.

d Check that the balloon is completely dry.

e Measure the mass of the balloon on the electronic scales.

f Burst the balloon near the neck. Measure the mass of the empty balloon.

Analysis

Calculate mass of air in the inflated balloon in kg.

b Calculate the volume of the inflated balloon in m3

c Calculate the density of air in kg m-3

d Compare your answer with others in the class.

#### Teaching notes

1 This simple experiment ignores the buoyancy of air displaced by the balloon and so is not accurate but it does yield surprisingly consistent results. Individual results can be compared and a measure of uncertainty agreed.

2 If air is particulate, it would be expected to possess mass. That it does so is shown in this experiment. However, it would also have mass if it is an infinitely extendable fluid!

This experiment was safety-checked in September 2004

Thanks to Doug Fraser for pointing out an error on this page, now corrected. Editor