Why do solids expand when heated?

breadmonkey

So, on a molecular level what's going on? Electrons are all whizzing around really fast and.....?

Also, while I'm here, are liquids truly incompressible or could you compress them to a tiny degree if you applied a really massive stress?

humbert

yup they(atoms/molecules) are whizzing around and bumping into stuff more, that's pressure.

liquids are compressible but not very.

breadmonkey

humbert wrote:

yup they(atoms/molecules) are whizzing around and bumping into stuff more, that's pressure.

But why does that make the solid expand?

themole

breadmonkey wrote:

But why does that make the solid expand?

Because as the molecules get more energy and bump off each other they push eachother further apart. So, the space between the molecules increases and hence the overall volume of the solid increases.

Professor_Fink

Hi Breadmonkey,

The reason most solids expand when heated has very little to do with electrons, and is mostly about lattice vibrations. By lattice vibrations, I mean vibrations of the individual atoms and molecules which make up the solid.

You can think of the potential in a lattice as a hill, high near the atom and lower as you get further away. If another atom is placed close to the first, then it is pushed away, since positions closer to the atom have higher potential. In solids we have both attractive an repulsive forces, which means that our hill goes down, below the zero line, and then back up to zero as you move away to infinity. Here zero potential means that the atom is unaffected by the lattice. The exact form of this potential depends on the type bonding within the solid.

The lowest point is called the equilibrium seperation, and is the distance between atoms at zero temperature. As we add temperature, the atoms can vibrate about this low point. The more energy we add (increasing the temperature) the further atoms can get from the equilibrium position. This leads to an expansion of the solid.

Hope this helps.

slipss

so why does water contract when heated? when most things expand, whats special about water, if anything?

SOL

Water only exhibits that behaviour at certain temperatures i.e. 0-4C this is because as the water molecules lose heat they lose speed and start to organise themselves, but their organised state actually takes up more space then when they are free flowing (below 4C), of course once we go above 4 normal order is restored and the molecules gaining more energy means they push eachother farther apart and hence the sample expands... in short. The reasons for this is to do with the unusually strong intermolecular forces in water, as well as the shape of the molecule, and probably more stuff that I have forgotten or omitted... what you'll want to know next is why hot water freezes faster than cold water...

tak

First - the expansion question.

What Prof Fink said is true as far as it goes.
But it's worth saying too that the interatomic forces that hem the vibrating atom in as it oscillates on either side of its equilibrium position are not symmetrical.
That is, the repulsive force felt as the atoms approach one another by one unit distance from the absolute zero separation is greater than the attractive force as they move one unit distance apart from it.
Thus, as the atoms oscillate due to their kinetic energy (heat) they spend slightly more time beyond their absolute zero equilibrium separation than they do within it. This means that the time-average separation of atoms that vibrate is slightly more than those of atoms with no vibrational energy, i.e. the absolute zero separation. The length/breadth/height of an iron bar is a multiple of the time-average separation between the adjacent atoms.
Hence the expansion.
As the kinetic energy increases this imbalance between attractive and repulsive forces becomes more marked and the material continues to expand.

Of course the degree of expansion depends on the particular values of the force and potential functions for that material.
So, one can have say an alloy of Fe and Ni (~36% Ni) called Invar whose rate of thermal expansion at normal temperature ranges is so small that it hard to measure. It used be the material of choice for clock's pendulums so their frequency did not increase unduly in hot weather.

Water in the liquid state expands on heating. Be clear on that.
When it freezes it does not contract like most (but not all) materials - it expands.
Grey cast iron does the same thing. (This has broken many a badly designed mould!)
Other materials do the same.
The change of phase going from the liquid to the solid on cooling, from one configuration of atoms to another, each with their own potential energy need not always involve a contraction in volume.

Tak.

JoeB-

I was reading something about Einstein Bose concentrates and it mentioned that they exhibited properties like a fluid which was a superfluid and incompressible...
Superfluid means no viscosity but are all fluids not incompressible in theory?

Spyral

There are usually exceptions to every rule but some fluids aren't easily compressable such as hydrolic fluid in a hydrolic ram.

Professor_Fink

Spyral wrote:

There are usually exceptions to every rule but some fluids aren't easily compressable such as hydrolic fluid in a hydrolic ram.

All conventional fluids are compressible to some degree, but the change in volume is very small compared to the force necessary to achieve it. In Bose Einstein condensates the atoms occupy a degenerate ground state. If you compress them then you are adding energy to the system, raising them out of this ground state. There is an energy gap present above the ground state, however, so small pressures are not enough to change the state, rendering them incompressible for small pressures.

Tar.Aldarion

Don't some solids contract when heated?

Professor_Fink

Tar.Aldarion wrote:

Don't some solids contract when heated?

Yes, elestic bands do, for example.

There is a somewhat more complicated reason for this. The solids fo these substances tend to contain very long molecules, which are aligned at low temperatures, but when heated rotate, causing the bang to contract.

Tar.Aldarion

Yeah, thanks. I was just at odds with the opening post. It should be, why do soldis change size when heated.

I remember hearing about some metal that contracted when heated, can't remember much though.

tak

http://www.sciencedaily.com/releases/2004/11/041119015323.htm

ZrW2O8 being the phase concerned.

Tar.Aldarion

Oh, thanks for that.