There are many states of matter. Previously we have covered solids (here) and liquids (here and here). This time we will focus on gases. Gas is a state of matter in which the molecules have so much energy to move that forces holding the molecules together is small or zero. This causes the distance between molecules to increase. Gases have no definite shape and will occupy the entire container it is in. And, like in transforming solid to liquid, warmth is required. Liquid becomes a gas if enough heat is added. Also, a gas will turn into a liquid if enough heat is removed. I have demonstrated this conversion to my daughter every time she watches me boil water. I have told her about how the liquid water is becoming gas (water vapour) due to the heat. I’ve also told her how this vapour is essentially the same stuff that clouds and fog are. So the basic principle is fairly well established in that conversion in our house. However, we decided to experiment a bit further. We have done some air experiments previously, which can be found here.
Bottle in freezer-experiment: This time we experimented with the volume of air in different temperatures. We took two clear plastic bottles. Basically, the weaker the plastic the better as changes in this experiment are easier to see. We put one of them in the freezer (temperature -18 celsius) and left one at room temperature. Then we waited for a while and then took the cold bottle out. The sides had caved in (bottle on the left in the image). I explained that cold air takes up less space and then we watched as room temperature heated the air inside the bottle and the caved sides filled out. We repeated this a few times.
Then we tried to do the egg in the bottle experiment also playing with the volume of air at different temperatures. This experiment has given me no end of grief. The idea is simple as is the science behind it. You have heated air in a container. In the usual version of this experiment the air is heated by a lighted match stick dropped into the bottle. You then place an egg on top of the container and, as the air cools, the air inside the container takes up less space. The egg has allowed the heated air leave the bottle, but does not let more air in. Instead the egg is sucked inside the bottle finally allowing for air pressure to stabilise. This is the idea. However, finding the right kind of a bottle with the right size opening has proven to be rather difficult. This experiment was coined when (and where) they brought milk outside doors in glass bottles. This kind of glass bottle is just the right kind and the opening is just the right size for an egg to be sucked in. We don’t have those. So I first tried this with Avent bottle designed for expressed mothers milk. The first time we did this, I succeeded. The soft-boiled egg was sucked in rather quickly. Daughter then wanted to repeat this experiment and I repeatedly failed. First of all the egg broke in the first successful attempt.
The mistakes I made and attempts to correct them were as follows:
- As the soft-boiled egg broke in the first successful attempt, I hard-boiled the second one. That didn’t change much.
- Then I noticed that I had placed the egg so that air could seep through to the bottle.
- After several failed attempts, I did some research online and found I could use water balloons instead. So I took a different bottle, a small water balloon and matches and tried again.
- I got tired of the match stick and decided to use warm water instead.
- That worked a lot better and it sucked the water balloon most of the way in, but not completely.
- Moving the balloon a bit from side to side (not pushing it in as that is unnecessary) finally allowed the greater air pressure outside the bottle to push the balloon into the bottle which had a smaller air pressure. Why? Water “wants” to go down (of course it has nothing to do with wanting and everything to do with gravity, but anyway…) and left to its own devices will go as down as it can. This creates pressure between the balloon and the bottle making it difficult for the balloon to move. Moving the water inside the balloon lessens the pressure and helps the balloon to move where air pressure pulls (from inside the bottle) and pushes it (from outside the bottle). So kind of worked, but needed help.
Basically, you can use a water balloon. I suggest introducing the experiment with seeing the balloon get sucked part way in the bottle. It does go in sufficiently by itself that it is quite difficult to remove. Asking the child to help removing it works well. This helps to introduce the concept of the volume of air and the effect heat has on it (warm air has a greater volume as a similar amount of cold air). I also suggest using warm water instead of fire as it is easier to control and less chance of burnt fingers and kids playing with matches.
We moved onto blowing candles out with CO2. I wanted to do this experiment to show that there are more than one kind of air/gas. The idea behind this experiment is that CO2 (carbon dioxide) does not react with burning materials and it replaces the oxygen needed for the fire to continue. So it puts out the candle or match (which I ended up using). This is another experiment we had trouble with. For the original experiment you need baking powder, vinegar, two glasses, and a candle. You first put the candle in a glass and light it. Second, you combine the vinegar and baking powder in a second glass. They start to bubble and create CO2. Do not breathe this in as it can be very dangerous. As it can replace oxygen around the fire, it can also do this in ones lungs. This is why I use only a small amount of vinegar and baking powder, so the amounts of CO2 are small too. And I ask the children to stay back. Thirdly, you pour the CO2 out of the glass with the vinegar and baking powder into the glass with the candle inside and watch it go out. The reason this works is that carbon dioxide (CO2) is a heavier gas than the air around us. This is why it stays in the glass and it is possible to pour out of it. It does not float around.
My problem with this experiment is that it is quite difficult to get the flow of CO2 right so that it does put out the candle, or then you would need more vinegar and baking powder I am happy using. So I remade the experiment so that I had a candle and matches and I combined vinegar and baking powder in a glass. I then lighted the match and lowered it to the glass with the vinegar and baking powder bubbling away making CO2. It went out clearly before touching the liquid. I then repeated the experiment three times and the match always went out. The experiment was much easier this way and repeating it was no problem.
Our final air-experiment at this time was building a wind whirligig (see image below). We used cardboard that was a bit thicker than just paper. Then we used some small beads to put in between the corners. Other things you need are scissors, a ruler or some other way fo drawing straight lines, a pen, a nail, and a wooden stick. First take a square piece of paper or cardboard. Second draw lines from corner to corner and cut these 2/3 of the way (approximately). Thirdly start construction by alternating between a bead and the top of a corner and putting them through the nail. Finally attach to a stick. Then just go out and watch it whirl. This experiment worked as a continuation with the previous air experiments proving that air is a substance. We feel it as it moves, as the little molecules that make up that air bombard us. We can trap it into bags and play with them. They are otherwise known as balloons. And as the molecules of air flow past, they will make the whirligig whirl.
If you try these experiments at home and your child asks questions, please let me know what types of questions they ask in the comment section so I can further develop these instructions. If you have questions about these experiments or instructions, leave me a comment and I will answer and also improve these instructions. Also, please remember like and share if you find this useful.
Copyright text and images: Satu Korhonen. You are free to try these experiments out, use them in your teaching. But instead of copying the text or images, link back to this page.