Have your kids ever asked to host a tea party for all their imaginary fairy friends? But you didn’t have the proper sized cups? No worries, you can make your own miniature cups! All you need is a pressure cooker and some Styrofoam cups.
We had a pressure cooker donated to our home laboratory so we could prepare these little cups. It is a plug in style and came with no instruction booklet. I had to trust that all my years of experience operating industrial autoclaves would prepare me for operating this household appliance. Phew, no problems to report and all went as planned. But make sure you know how to safety operate your pressure cooker before attempting to shrink cups (or make dinner).
You can shrink just a plain Styrofoam cup, or you can add to the fun by getting the kids to decorate them first with permanent markers (we used Sharpies). Craft time AND science, how fun is that?
There is a 20 minute waiting period, so after setting it up send the kids elsewhere to play. (Because watching a pressure cooker work, even our fancy digital one, is not that exciting, and you don’t want to hear “Is it done yet?” a million times).
Get the kids to return when the time is up and you are going to release the pressure. I think the big release of steam was one of the highlights. Screams of “That’s cool!” were plenty. But be sure to tell them to stay clear for safety reasons.
When you can finally take the lid off, the giggles may start! (Although, in our case I’m pretty sure it was the grown-ups who thought it was just adorably funny!). The cups will be very delicate, and will crumble if squeezed too tightly. You might want to warn your kids of this too, helps prevent potential tears in case their cup breaks.
- Pressure cooker
- Styrofoam cups
- Wire grate
- Small aluminum pie plate
- Permanent Markers (not the washable kind, we used Sharpies)
- Paper and Pencil
- Preparation: 5-10 minutes (not including cup colouring time)
- Experiment: 20 minutes
- Observations and Results: 10+ minutes
- Optional: Colour the cups with permanent markers. You can colour the outside, inside and bottom.
- Optional: Measure the height of the Styrofoam cup with a ruler and write this number down (and any other measurements you would like to make beforehand).
- Get the kids to make some other observations before the shrinking event. What does the cup look like? Feel like? Get them to write these down too!
- Make sure you know how to safety operate your pressure cooker before proceeding.
- Add water to completely cover the bottom of your pressure cooker. A pressure cooker needs water to produce steam or it won’t work. You may even damage it without adding the water.
- Put a small aluminum pie plate on the wire grate that’s part of the pressure cooker. If your pressure cooker doesn’t have a wire grate just make sure you use a pressure cooker safe dish that sits stably on the bottom of the cooker (i.e. you don’t want it floating around on the water). Whatever you use, make sure the cup stands upright and out of the water.
- Check that the Styrofoam cup will not block the pressure release valve in any way. Blocking this valve could result in the pressure cooker exploding (any extra pressure produced must be able to escape). Note: There will be some expansion of the cup prior to its shrinking. So imagine the cup is slightly larger and double check that this valve is clear.
- Properly close and lock your pressure cooker.
- Set the time for about 20 minutes. And start it up!
- After 20 minutes, you must release the inside pressure BEFORE you attempt to open the pressure cooker! Be sure everyone is clear of the pressure release valve during this step.
- When you open the lid you should see a miniature Styrofoam cup.
- Optional: Measure the height of the miniature cup with a ruler and compare this to its height before shrinking.
- Optional: Calculate the change in height. In our case the original height was about 9 cm and afterwards it shrunk down to about 4.5 cm. Let’s do some math…
- Make any observations about how the cup changed after shrinking. And remember to warn the kids that these cups are delicate.
Tips and Tricks
We found that the addition of the pie plate really helped the cup to maintain its shape. Also, we attempted 1, 2 and 3 cups at a time in the pressure cooker. Doing one cup at a time always worked best. We also played around with the time, and 20 minutes work well for us, but your pressure cooker may be different, you may need to experiment a bit first. And try to remove the cups after you release the pressure, cups that we “forgot” about and were left to sit in the hot cooker past the 20 minutes never held their shape very well. I think they were starting to melt.
Oh, and did I mention make your kids do something else during the 20 minutes of “cooking”? Seriously, this is important to maintain peace in your household.
And warn the kids that the mini cups are delicate. Compare them to Grandma’s fancy china. I find if you warn the kids that things could break, the risk of a complete meltdown is lessened…. sometimes.
Science Secrets Revealed
When you are boiling a pot of water on the stove with no lid, say it’s spaghetti night, the water will boil at 100°C. It doesn’t matter how much heat you add to the pot, the temperature of the boiling water and steam will never go above 100°C. Any extra heat you give the pot after it reaches its boiling temperature will only make the change from liquid to gas (steam!) happen faster (the water molecules are doing some wild fast dancing), but there will be no change in their temperature.
The story is different in a closed, and tightly sealed, container like our pressure cooker. When the water inside this sealed pot boils, steam is produced but this steam (unlike in our open spaghetti pot) has no place to go, it’s trapped inside the pot. This causes a build-up of pressure. This pressure pushes on the liquid water making it harder and harder for the little water molecules to break free (turn into steam). So now the water will boil (change from a liquid to a gas) at a temperature GREATER than 100°C. Crazy! That’s why you can cook so much faster in a pressure cooker, because you can achieve a higher temperature.
So how does this make our Styrofoam cups shrink? It’s all about the pressure! When you start up the pressure cooker, the water that we added will start to change from its liquid to its gaseous state (our steam) and the pressure will rise (remember the steam is trapped). This increased pressure will act on our Styrofoam cup. These cups are made out of polystyrene and are about 95% air. Did you notice how light these cups are? Imagine there are TONS of little air bubbles between all the polystyrene molecules. When the pressure builds, it will SQUEEZE the polystyrene molecules closer, and closer together, and POP the air out! As that air is pushed out, the cup becomes compressed. Take a look at the miniature cup. How does the surface look after some of these air bubbles are removed? These little cups are compact versions of the original cups.