Expert: George Maxwell - 1/12/2008

Question
Hello, thanks for youre time. One day I urinated in a plastic bottle. I meant to properly discard the contents soon after in the toilet and flush the bottle before discarding, but I forgot. The next day when I remembered, I found the thin plastic bottle had sucked right in, just like an old science project we did at school where the air had been removed from a canister creating a vaccum. I wondered why this happened.. And where did the air go? The bottle cap was screwed on tight.  Thanks, Simon.

Answer
Hi, and thanks for your question.

This effect will work with any warm liquid,and relies on the fact that warm air takes up more space than cold air.

It only happens when the lid of the vessel is tight fitting - as the top to your bottle was.

Urine is quite warm, often even slightly above the standard body temperature of 37 degrees centigrade because it's stored deep inside you.

When you add this to the bottle, it immediately heats up the air inside, so that when you put the cap on, you have two litres (or however big the bottle was) of warm air.

Leaving it overnight gradually allowed the air inside to cool and contract. This lowered the pressure inside the bottle, allowing atmospheric pressure to crush the sides inwards: your bottle was crushed by tonnes of atmospheric pressure.

To reiterate,the air inside didn't go anywhere, it just cooled down and so packed together more tightly, taking up less room in the bottle.

You can experiment with this idea; when teaching, I get a two-litre bottle and cover the bottom with very hot water. If you leave it to cool down, it gradually collapses,and if you throw cold water at it, it crumples almost immediately.

The pressure from the atmosphere is so great that this even works with metal vessels: the following website describes an experiment to crush a can by his method, and has pictures of the same effect accidentally crushing a steel rail wagon that had been cleaned with steam.
http://www.delta.edu/slime/cancrush.html

This will only work if the vessel is sealed soon after it has been heated. If not, as the air cools, more air from outside will be drawn in to replace the contracting air, and will equalise the pressure.

The effect of hot air taking up more space than cold air is due to the fact molecules move around more when they're hot, and hit the sides of their container more often, exerting more pressure. For a classic demonstration, see the heating up of a thin fabric envelope of air with burning propane gas - the hot air takes up so much space that the fabric envelope inflates, and you have a hot air balloon.

Hope this helps.