A favourite pupil misconception is that gas has no weight. This is because gas particles are widely spread out, unlike solids or liquids, which are condensed with particles touching.
To demonstrate that air has weight, take a two-litre plastic bottle and insert a rubber bung with a short glass tube and rubber tube with screw clip, which fits a cycle pump with a non-return valve. Place the bottle on a top-pan balance and set to zero.
Now pump air in - about 40 strokes. Tighten the clip and weigh the bottle again. It should now be about 1g. (Rehearse the number of pumps it takes to get 1g of air into the bottle).
Unscrew the clip a little to let the air out, quick enough to hear the hissing but slow enough to see the balance return gently to zero. Place 1g of modelling clay on to the bottle, which is still on the balance, to return it to its air-filled weight. This demonstrates there is the same amount of matter in that tiny ball of modelling clay as in 40 pump-fulls of air
Resources at Science Issues and the National Curriculum
Keith Ross teaches science education at Gloucestershire university