I place an ice cube inside a snack size baggie for each student. I ask each student to identify if it a solid, a liquid or a gas. Then I ask if they can change this solid to a liquid or gas. They always say liquid first.
We have a race to see who can melt their cube the fastest. I tell them the bag must stay closed and they may not put it in their mouths. They usually get the idea to go outside in the sun as we live in Texas and it is very hot outside this time of year. (We do matter at the beginning of the school year too.) So I have the rest of our supplies ready to go outdoors.
I tell the students to come show me when their solid has become a liquid. After they have water in their baggie I tell them to open it up and I drop in a half tablet of generic Alka Seltzer and then "zip" the baggie closed for them. I tell them to shake it gently and watch what happens.
It will start to expand from the gas. I ask them what is happening? They quickly realize it is gas. They can see that gas really does take up space and they love the balloon effect.
I have never had a baggie burst. Some students do not seem to get as much gas and are disappointed if their baggie does not get large, so I give them another half tablet.
They love this experiment!