Jeremy, a musterer on Mt Rosa Station, Central Otago, asks :-

Is there a container that wont expand at all under pressure inside it?

John Campbell, a physicst at the University of Canterbury, responded.

No. Even if we made one that didn't expand with pressure it too would eventually explode.

We can work out the maximum pressure that a vessel could sustain inside it by considering the strongest force we would need to pull two atoms apart. This is the electrostatic force between two charged ions, eg the sodium and chlorine ions in table salt (NaCl.) One electron from the sodium atom is transferred to the chlorine atom. Someone doing 6th form physics should be able to use Coulomb's law to calculate the force of attraction between opposite electrical charges which are about the diameter of an atom apart (about one tenthousandth of a millionth of a metre). That force is actually very small but it does act over a very tiny area, ie about the cross-sectional area of an atom, so the pressure (the force per unit area) can be very high.

This pressure needed to pull atoms apart works out to be about a million million Newtons per square metre. To put this in perpective, this pressure is about ten million times the pressure of the air around us. Rupture pressures approaching this are measured in specially grown very fine needles of single crystals of pure metals.

In practice, materials have an ultimate internal strength of only about ten thousand times atmospheric pressure. High pressure cylinders used by welders and aqualung divers are filled to only 200 times atmospheric pressure to be on the safe side. The problem is that in pure metals whole planes of atoms can easily slide over each other without rupturing. We can make metals harder, and therefore more brittle, by breaking up the planes, for example by adding zinc to copper to produce hard brass. That is why a soft metal such as copper is used for water pipes. When water freezes to ice it expands. The copper can flow to expand with it whereas the old steel pipes couldn't flow so would crack during hard frosts, causing a flood when the ice thawed back to water.

We can have much higher pressures in compression because we cannot force one atom inside another. At the bottom of the deepest ocean the pressure is 1000 times atmospheric pressure. The highest pressure we can produce in a laboratory (for example, between two small diamonds) is 100,000 atmospheres, and at the center of the Earth it is a million atmospheres. In large planets and stars the pressure can be a lot higher.