Freeze Water...Backward
This is a pretty simple experiment, but it’s one of the coolest things to watch! The principle we are learning today is that water can exist in a liquid state colder than 32 degrees Fahrenheit. In fact you may have seen this happen if you leave an unopened water bottle in your car on a cold night. It looks unfrozen, but when you pick it up or open it, the whole thing freezes.
What you need:
Unopened water bottle (purified water)
Bowl of ice
What you do:
Put an unopened bottle of purified water into the freezer. It will take about 2 or 2.5 hours to supercool. All those words sound fancy, but this is the water you can buy a case of at the local grocery store.
PRO TIP: put a cup of tap water in at the same time as the bottle. When that has frozen but the bottle of water has not, the water in the bottle is supercooled.
At that point, take the bottle out of the freezer and do the experiment!
There are a few different ways to do this. My favorite way is with the bowl of ice. Here’s a video demonstration of me doing this version of the experiment. Carefully open the water bottle without disturbing it too much. Then slowly pour your water over the ice in one spot (don’t try to spread it around). Watch how the water starts to freeze as it hits the ice, then before it hits the ice, then all the way back up to your bottle! You can experiment with how fast to pour the water and how far away the bottle can be from the ice and still make this work. I suggest starting just a couple inches away from the ice to make sure it will work the first time.
Instead of using the bowl of ice, you can slam the unopened bottle down on a hard surface (that sounds more aggressive than it needs to be, just forcefully put it down on a hard, flat surface and watch the water freeze). This will also work to freeze the water.
What is happening:
Purified water has different molecular structure than tap water. There are other elements in tap water, but purified water is just the hydrogen and oxygen. Those other elements can help start the nucleation process where water molecules join together to create a solid structure of ice. Purified water doesn’t get that same jump start from the other elements, so it can exist as a liquid at a temperature lower than 32 degrees while waiting for the nucleation process to take over.
When you shake the bottle or slam it down on the table, you change the way the molecules are arranged in the bottle. They are colder than freezing, so when the start bumping into each other, they join together into the structure of ice. Pouring the water over ice is a little more complicated, though it starts with the same principle. The molecules are colder than freezing and you are disturbing their structure. The air directly above the ice is slightly cooled by the ice, so as you pour the supercooled water in your bottle toward the ice, it is additionally cooled and freezes the moment it encounters the ice.
This is also how some clouds form! Water starts to collect on tiny particles in the air and form a cloud of liquid and solid water.