Quick answer: Air is a mixture of gases, mainly 78 % nitrogen and 21 % oxygen, and it has mass: about 1.2 kg per cubic metre. This grade 7 science lab on atmospheric pressure gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can we weigh air? In this lab you add measured amounts of air to a football with a hand pump and watch the mass on a sensitive scale. The results let you calculate the mass of air in one stroke and the volume of air the pump moves.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-ESS2-5
Aim
To show that air has mass, to find the mass added by each stroke of a hand pump and to estimate the volume of air in one stroke.
Equipment
- A football or volleyball with a valve, deflated
- Hand pump and needle adaptor
- Digital scale reading to 0.1 g
- Notebook and pen for a results table
Safety
Do not over-inflate the ball, because it can burst and the valve can pop out. Keep your face away from the valve and the pump hose, and stop pumping if the ball feels very hard.
Method
- Let all the air out of the ball. Place it on the scale and record the mass.
- Connect the pump. Give exactly 10 full strokes and disconnect the pump immediately.
- Place the ball on the scale again and record the mass.
- Repeat steps 2 and 3 until you have done 40 strokes in total.
- Plot a graph of mass against the number of strokes.
Results
Sample measurements:
| Total pump strokes | 0 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| Mass of ball (g) | 392.4 | 398.5 | 404.2 | 410.4 | 416.3 |
Analysis questions
Question 1 Total gain
Calculate the total increase in mass after 40 strokes.Question 2 Mass per stroke
Calculate the mean mass added by one stroke.Question 3 The graph
Describe the graph of mass against strokes and state its gradient.Question 4 Volume of one stroke
The density of air is 1.2 g per litre. Calculate the volume of air added by one stroke.Question 5 Prediction
Predict the mass of the ball after 60 strokes.Question 6 Sources of error
Suggest two reasons why the mass gained may be smaller than expected.Question 7 Conclusion
State what the investigation shows about air.Question 8 Better method
Describe one change to the method that would make the mass gain of each stroke more precise.
