Frozen Saltwater and Negative Numbers

Much emphasis is now being placed on representing mathematics practically and visually (and quite rightly). For obvious reasons, it’s harder to do this with negative numbers. Here’s a classic activity that I came across on my first Primary Science Teaching Trust conference for showing children negative numbers in context. It can also be used to answer the question ‘Why do we put salt on the roads when it’s icy?’

Salt

Have a container filled with icy water and add quite a lot of salt. By putting the temperature probe into and out of the icy solution, the children will be able to see how the temperature changes (and how numbers change from positive to negative and vice versa) as the temperature goes above and below 0 degrees. Most dataloggers come with software that will allow you to graph this pattern as well as displaying the temperature.

By adding the salt, the freezing point of the solution becomes lower. At the maximum level of saturation for salt (according to Google), the freezing point for a saline solution is -21 degrees Celsius. This demonstrates that the salt doesn’t make the water hotter, as I’ve heard children suggest, but that it changes the freezing point. It’s also worth noting that if the temperature were to fall below -21 degrees it would be pointless to grit the roads.

This context could be used simply as a demo of negative numbers, or it could lend itself to a more extended scientific enquiry. Let’s just hope that thawing ice on the roads isn’t a subject that is too topical for too long!

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