Plastic is a good insulator which means that it does not conduct heat very well. This makes it safer to use plastic coated tongs to pick up hot things. If you used pure metal ones and held a hot beaker, the tongs would get hot quite quickly because heat travels very rapidly through metal (a good conductor).
Here’s an interesting factoid for you: the French mathematician Joseph Fourier was obsessed by heat. He worked out the maths of hot heat travels along a hot poker and in doing so invented a technique that is known as the Fourier Transform (FT).
This proved to be a very useful development (although the other French mathematician were very snooty about it a the time – probably because they didn’t understand it properly!). The maths of the FT is now used to compress music into MP3 files for an iPod. We also use FT maths in our X-ray experiments to work out protein structures!
I don’t know (I don’t think they did have plastic in my day). May be it is to insulate the tongs so you don’t burn your hand?
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