Kid Science : Frozen Ocean

A couple of weeks ago, I was in the car with my kids - Emma, 7, and Jack, 4 - when the subject of water freezing came up. I think it was the first day of the season when we had sub-freezing temperatures and I was trying to impress upon them the importance of making sure they had their hats and gloves. During this conversation, one of them (I think it was Jack) asked if the ocean ever froze. I told them that it did on very cold places like the Arctic and Antarctica, but that around here it did not generally freeze because the salt in the water prevented it (As much as may have wanted to, I didn't go into the physics of molecular motion and ice crystal formation - I'll save that for when they're a few months older!).

A light bulb went off in Emma's head - it was an amazing thing to witness. "I know what we can do! Let's put a container of water without salt and another container of water with salt outside and see if they freeze!" I can't explain how proud I was at that moment. Notice that Emma did not just want to put a container of salt water outside and see what would happen. She wanted to put both salt water and fresh water out - she had just designed a simple controlled experiment. I was smiling all the way to work that day.

The weather warmed up for a bit after that conversation, so we had to wait until yesterday to carry out Emma's experiment. After dinner, we pulled out a couple of bowls, a measuring cup, a kitchen scale, and some table salt.

I was trying to keep everything as accurate as I could so we added 500mL of tap water to each bowl and added approximately 17.5g of salt to one of them (this gives a salinity of 3.5% or 35 ppt, the average salinity of ocean surface waters, and only slightly higher than the salinity we would expect for the ocean off of New England). I asked the kids what else we needed to do and Emma realized that we needed some way to identify each bowl. She had some trouble coming up with a good system. Jack, of course, figured it out - "Emma, we just need to write 'salt' and 'no salt' on a piece of paper and tape it to the bowls."


The kids put the bowls outside on the back deck and went off to bed. I watched the temperature outside drop through the evening: 30^oF, 28^oF, 25^oF, 23^oF, 20^oF. Uh oh. I hadn't anticipated such a cold night. Salt water doesn't freeze at 32^oF, but it will freeze - especially sitting there in a little bowl without currents or waves - in fact, water with a salinity of 35ppt will freeze at 28.8^oF. The reason the ocean as a whole doesn't freeze even when it's sub-28.8^oF outside is that it has such a huge thermal mass that the water itself doesn't often get to 28.8^oF. You'll see some freezing in shallow areas with little wave activity, like along the edges of tidal marshes. I was starting to fear that our little controlled scientific experiment was going to backfire and we would wake up to two frozen bowls of water and the kids would logically conclude that the salt did nothing to prevent freezing. D'oh!

When I went to bed sometime around midnight, the temperature outside was 18.8^oF. Should I bring the bowl of salt water inside and then wake up before the kids, run downstairs, and put it back outside? No. That's just too deceitful and is completely counter to the lesson I was trying to teach about scientific thinking. I'll just hope for the best - perhaps the salt water wouldn't freeze as much as the fresh water. I went to bed hoping for a watery slush.

When we brought the bowls in this morning, sure enough the salt water had froze. But thankfully, like I had hoped, it was different from the fresh water. The fresh water was frozen solid, smooth like a hockey rink. The salt water was slushy, not firm at all. The kids marvelled at the difference. They banged their knuckles on the hard fresh water ice and stuck their fingers through the salt water slush. "Wow, the salt water isn't really frozen!". They played with, touched, and tasted the ices for a few minutes then headed off to get dressed for the day. Whew. To paraphrase a cogent comic - Science. It works.