Back on November 29th we started an experiment on our front porch, with four plastic jars filled with water. At the start of the experiment, the jars looked like this:
Here's what the experiment looked like five days ago, before the temperature hit the freezing mark.
Now, here’s what the experiment looked like on Friday, December 4th, after the air temperature went below freezing, hitting 24 degrees Fahrenheit overnight:
Here's what the water jars looked like after a hard freeze. Three of the five jars had frozen overnight: A, B, and D. Also, we had added a fifth jar: In Jar E, the water that had been BOILED first, then cooled, put in the jar, and sealed with a lid that had NO holes in it. (Thanks to J of Science Museum of Minnesota for suggesting this addition to the experiment.)
Three of the jars had frozen water in them, including the jar with SALTY water. No one predicted that! The two jars that did not freeze were both sealed tight — there were no holes in their lids.
Here’s a closer look at Jar A, which started out as cold, fresh tap water, and was covered by a plastic lid with holes drilled in it:
A closer look at Jar A shows intersecting sheets of ice, more densely frozen closer to the top of the jar.
Jar B showed a similar pattern, although the sheets of ice were more horizontal than vertical:
Jar B also had sheets of ice, more densely frozen towards the top of the jar -- but the ice sheets were more horizontal than vertical.
Jar D was most surprising. I was expecting the salty water wouldn’t freeze, but the top part of the jar was frozen fairly solid. However, the bottom of the jar was not frozen, but had a bunch of bubbles sticking to the side of the jar:
In Jar D, notice the clear division between frozen water above and unfrozen water below.
So, that leaves us with two mysteries to solve:
- Why did most of the water in the salty jar freeze?
- Why did the water in the sealed jars not freeze?
I have some ideas, but I won’t tell you about them yet. However, I will give you one hint about the second mystery. Here’s what the lower part of salty Jar D looked like on December 2, 2009 — three days after the experiment started, but before the temperature dropped below freezing:
Here's what Jar D looked like three days after I had mixed in several handfuls of sidewalk salt, but before the first hard freeze. How can this help us understand why the top of the jar froze, but the bottom did not?
Feel free to tell us your ideas in the comments section (below) or on Facebook.