My son Jack and I took some time out from playing with the educational toys he received for Christmas and went to a hockey game over the holidays. Like most 7 year olds, his favorite part is when the Zamboni comes out to clean the ice. We had seen this a thousand times so I was stymied when Jack said, “Why is there smoke coming out of the back?” It’s steam I told Jack, they use hot water because it freezes faster. I could see the wheels turning now and knew I was getting in over my head. “But ice is cold, shouldn’t they use cold water, Dad”. Well, there was only one period to go and a thousand questions, most of which I didn’t have the answers to, so I told Jack that when we got home we would do our own science experiment to see whether hot or cold water froze faster.

To get the most out of this I thought we should treat it as a science project. I looked through some of the science toys Jack had and found a science kit that showed how to do some experiments. Jack and I made a plan and a list of all the things we’d need. We also searched the internet to see if there were any educational toys that might help us. We didn’t find any educational toys, but we did find some very interesting facts.

It turns out there are at least 3 physical processes that would explain why hot water freezes faster. First, hot water contains less dissolved air, this is why fish can die in very warm water, a lack of oxygen. Liquids conduct heat much better than gas so hot water would cool much more rapidly. Secondly, in a pool of warm water, the water at the surface would be hotter than the water below, hot water rises. This would create a larger temperature difference with the air and cool the water faster. Third, the warm water would initially melt the ice on the ground, forming a better bond that would cool the water faster. This also prevents layers of ice from chipping away.

So we filled up a jug with hot tap water and a second jug with cold water. We took 2 plastic cups and a short 2x4 outside. We filled each cup half full, the red one with hot water and the blue one with cold water and set them on the walk. Then we found a patch of ice on the driveway, laid the 2x4 in the center, and poured hot water on one side and cold water on the other side. It was 18 F so we went in to warm up.

We came back out an hour later. The 2 cups were mostly water. The red one had some ice above the water line and the blue one had some ice on the outer edge of the water surface.

On the driveway, both sides of the 2x4 had turned to ice. When we touched the ice we could see our fingerprint on the cold water side. The hot water side was frozen solid.

We weren’t sure what we just proved but we rushed back inside, wrote down our observations, drew a few pictures and had some hot chocolate before bedtime. I knew Jack was going to be excited when he explained his science experiment to his class on Monday.

So next time you see that steam rising behind the Zamboni you’ll know why. As for my backyard ice rink, if I can only wrap the garden hose around the barbeque …

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