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When you think of crystals, you might think of something hard, like a gemstone or ice. You also might think of something that takes a long time to form, either underground, in your freezer, or in a crystal-growing science experiment. With the right mixture of chemicals, though, you can grow instant crystals. What’s even cooler is that as these crystals form, they give off heat! Today we’ll be working with something that’s sometimes called “hot ice,” even though it isn’t really ice.
Materials
- 4 cups of white vinegar
- 4 tablespoons of baking soda
- A big pot
- A big Pyrex measuring cup
- A stirring spoon
- A plate
- An adult
Process
Use your measuring cup to measure out 4 cups of vinegar into your pot.
Add 4 tablespoons of baking soda to the pot, one at a time. Between tablespoons, wait until the fizzing has died down. No baking soda and vinegar volcanoes today!
Stir the mixture until all of the baking soda has dissolved and all of the fizzing has completely stopped.
Have your adult boil the mixture on medium-low heat. While they’re doing that, wash, rinse, and dry your Pyrex measuring cup. We’re going to need it again.
Keep slowly boiling the mixture until about 75% of the water has boiled out. You want no more than about 1 cup of liquid left in the pot.
Pour that 1 cup back into the Pyrex measuring cup and put it in the fridge. You’ll want to leave it there for about 30-45 minutes. It has to be cool for the crystal formation to work.
You’ll notice some powder stuck to the inside of the pot once you pour out the mixture. Use your spoon to scrape a bit off and set it aside for later.
Once your mixture is nice and cool, take it out of the fridge. Put a pinch of the powder you gathered into the middle of your plate.
Now, very slowly, start to pour your liquid onto that pile of powder. Something incredibly cool should start to happen.
Once you’ve observed the crystal formation, you can touch it, too. What do you feel? Be careful, though. It might sting if you have a cut or scrape, and it’ll definitely sting if you get it in your eyes.
Summary
You just made hot ice! If everything went to plan, you watched crystals form rapidly right in front of your eyes. They weren’t tough, unbreakable crystals, as you probably noticed when you touched them. You could probably mash them up fairly easily as you were basking in their warmth. So what happened here?
It all starts with the baking soda and vinegar. You probably already knew that when you mix them, they fizz up. This is because the two together create a chemical reaction when two substances start to switch around the pieces they’re made of to make something new. One of the new things is carbon dioxide gas, which causes the fizzing. The other new thing is called sodium acetate. It’s what’s left over in the pot when all of the fizzing is done. Sodium acetate can be made to crystalize, but you have to remove some of the water that it’s mixed into. That’s why you boiled it.
When something is dissolved into water, it’s called a solution. Only so much can be dissolved into water. You can dissolve salt or sugar into water, but eventually, the water will have as much as it can handle and the salt or sugar will start settling on the bottom. Water that’s reached its limit is called saturated. Water can be made to hold more of a substance by heating it up, though. Sometimes when it cools back down, it will still have that same, larger amount of stuff dissolved in it and it hasn’t settled out yet. Water like this is called supersaturated. It doesn’t take much to disrupt a supersaturated solution and make the dissolved substance start to become solid again. That’s what the powder you used did.
Sodium acetate crystals (and almost all crystals, for that matter) need something to form around. They can’t just form on their own. The thing they form around is called a nucleus, and it causes the crystals to nucleate, or start growing. Sodium acetate crystals grow really fast!
But what about all that heat? Well, think of it this way. There’s always less heat in a solid than in that same substance when it’s a liquid. You need to cool water down to make ice. If you add extra heat, you can melt plastic or metal, but when they cool down, they become solid again. Usually, we think of the heat leaving before the liquid turns solid, but anytime a liquid makes a phase transition into a solid, the heat has to leave. When your sodium acetate, mixed into the liquid water and flowing all around, started to form solid crystals, it had to get rid of that heat, so the crystals were nice and warm until all of it was gone. If you let it sit until all of the heat is gone, you may even notice that it’s more solid!
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