A teacher asked for a good experiment to show 8th graders that gas has mass.  “We have used balloons in the past,” she says, “but some of the kids still don’t make the connection.”

Paul Doherty replied:

I like to get a big weather balloon from a surplus store , inflate it until it is 1 meter in diameter or a little more and then a second balloon that is deflated.

Have a kid stand and throw the empty balloon at the back of their head…they feel almost no force.

Then throw the full one. It packs a noticeable punch due to the mass of moving air. the mass approaches a kilogram.

Of course you cannot weigh it using a scale due to buoyancy. You can only feel the mass by accelerating it or decelerating it.

And Eric Muller added:

Get some dry ice. It is solid Carbon Dioxide and it has noticeable mass. Lots of stores around the bay area sell dry ice. Many Safeways, Albertsons, bait shops, liquor stores, ice distributors and welding supply companies carry dry ice.

Weigh (or Mass) a chunk of dry ice. Put the chunk in a plastic bag and tie it off. It will sublimate and turn into a gas. The bag will expand noticeable. A solid, 44gram chunk of dry ice (that’s the size of a couple of fingers) will expand to around 22.4 liters of gas.

Gas has mass!