Science Experiments

Boiling Yoghurt Pots

Sat, 22nd Oct 2005

Part of the show Social Insects, Biting Bugs and a Potted History of Honey

What you Need

A Yoghurt Pot

A yoghurt pot (empty)


A Saucepan of water


A couple of tablespoons of salt

What to do

Add 2-3 tablespoons of salt to the water

Beware this will make the water boil at a higher temperature than normal,  so be extra careful and make sure there is a responsible adult involved.

Boil the yoghurt pot for 3-4 minutes, take it out with a spoon.

Is it the same as when you put it in?

What may happen

You will find that the yoghurt pot has shrunk down much smaller, almost forming a flat piece of plastic. Also the plastic has got much stiffer and thicker.

Shrunken Pot

Why does it happen?

In order to understand what is going on you have to know how a yoghurt pot is made. It starts of a a sheet of Polyethylene about 1mm thick. Polyethylene is made of great big long molecules up to a few million atoms long. In the original sheet the molecules are all wiggley.

The plastic is heated up enough to allow the molecules to move past each other and the plastic softens. A former is then pushed into the plastic and the air sucked out around it (this is called vaccum forming). This stretches out the molecules in the sides of the pot to be almost straight.

 The plastic is then cooled down and the former removed.

The plastic

The pot being formed

The Pot

The plastic is made up of long polymer molecules arranged randomly.

The plastic is heated and a former pushed into it.

The result is then cooled and the former removed.

What happens when you boil the pot?

When you heat something up you give the molecules energy, this causes them to vibrate and writhe. When you heat the yogurt pot to 110°C or so the molecules are writhing enough to move past each other so the plastic becomes soft and rubbery.

As they write the molecules in the parts of the pot that were stretched out will tend to get more wigglely and therefore shorter, pulling the base of the pot upwards and making it flat again.

The Pot

Heated Pot

Shrunken Pot

The molecules in the side of the pot are streched out.

When it is heated the molecules start to vibrate.

this causes them to become more wiggley again shortening the plastic that was stretched.

 This will happen to any plastic that softens when heated and is why crisp packets shrink if you put them on a fire and why lemonade bottles distort if you fill them with tea.

Dave Ansell


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