Home > Outreach, physics > Knowing things you don’t know

Knowing things you don’t know

I give a schools talk called “What do Scientists do?”, often to S2 pupils before they make their Standard Grade subject choices. The ignorance over what types of jobs kids think are possible with science (and other STEM subjects) is quite worrying, considering the importance of the decisions they are about to make (but that’s a discussion for a different day). I usually start the talk by asking the kids if they can tell me ‘what science is?’. Often this is met with blank faces, but usually I we get to the idea that science is all about experiments. This I think is summarised quite nicely by a quote from ‘Zen and the Art of Motorcycle Maintenance’:

The real purpose of the scientific method is to make sure nature hasn’t misled you into thinking you know something you actually didn’t know

While this is maybe a bit ‘fancy’ a definition for second year school kids, I try and illustrate with an example, and ask ‘what happens when water gets to 0 degrees Celsius?’ The answer that we all ‘know’ is that the water will freeze. When asked if the students have ever done an experiment to check this the answer is usually ‘no’.

It then surprises the kids to know that you can cool water quite readily below zero degrees C without it freezing, a process called supercooling. This is because in addition to water being cold you also need something to freeze around, a so-called nucleation centre. Normally this will be an impurity in the water. But if you take purified or filtered water and then cool slowly you can get down to about -8 or -10 degrees Celsius without too much bother. And then the fun begins…

The water is clearly liquid (and it’s about -8 degrees Celsius) but freezes on impact with the bowl, which provides a surface to freeze on.

This behavior is mimiced in a lot of liquid aerosols in the atmosphere, which will crystallise on contact with bits of soot and other particles that are flying about. Ice aerosols are an important part of the weather/climate system, and can get down to about -40 (!) degrees Celsius).

Also supercooling occurs in many plants and animals, particularly fish that live in very cold waters. The fish obviously don’t want to freeze, but most of their cells are made of water. The fish then generate antifreeze proteins to stop the formation of any ice crystals, which if entering the body at the wrong point would lead to contact nucleation and death. The fact that fish have antifreeze producing proteins is something I find remarkable.

We have now unlearned something we thought we knew and have learned a bit more about cooling processes along the way. Hopefully the message that doing careful experiments under a range of conditions is really important has been shared, and that this is what science is all about.

Oh, and we haven’t even become to think about what happens if I do similar experiments under different pressure conditions.


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