[neilep (OP)]

I understand that liquids are very difficult to pressurise or compress.

So, Taking ' water ' for example.........how much can it be squashed ?

Fire extiguishers have water under pressure !

What liquids can be squashed more or less ?

[daveshorts]

Water can be pressurised as much as you like, the issue is that they don't compress so when you release the pressure they don't expand - doing damage, squirting out of tings etc.

A fire extinguisher works like an aerosol or a soda syphon, there is a compressed gas above the liquid which can expand and therefore pushes the liquid out.

[neilep (OP)]

Water can be pressurised as much as you like, the issue is that they don't compress so when you release the pressure they don't expand - doing damage, squirting out of tings etc.

A fire extinguisher works like an aerosol or a soda syphon, there is a compressed gas above the liquid which can expand and therefore pushes the liquid out.

This is facinating..THANKS DAVE...you have clarified the true meaning of what I meant to ask...I was particularly interetsed in compression..(NOT Pressure...sorry about that)

Does it not compress at all ?...not even a slight bit ?...what about other liquids ?

[lightarrow]

I understand that liquids are very difficult to pressurise or compress.

So, Taking ' water ' for example.........how much can it be squashed ?

http://en.wikipedia.org/wiki/Bulk_modulus

Writing the bulk modulus formula in another way: ΔV = -V•ΔP/K

K for water is 2.2•10⁹, so, if you compress it of 100 atm, then:

ΔV ≈ -0.5•10^-2V . It means that 1 litre of water becoms 1 litre minus 5 millilitres.

Certainly K depends on temperature and pressure; I assume it could be considered as constant in a practical range.

I don't know about other liquids.

[daveshorts]

I would have thought compressibility would increase with temperature, if the water expands thermally there will be more space in between the molecules... Of course, it being water, strange things probably happen below 4°C where it expands with reducing temperature...

[lightarrow]

I have corrected the value of K in my previos post: it's multiplied by 10⁹ not by 10^-9.
There are different values for K (or its inverse, compressibility) according if we calculate it isothermally, adiabatically ecc.
I don't know how K varies with the temperature, but what you say seems resonable, Dave.

[eric l]

One thing seems left out in this discussion :  if you compress a liquid, it has to be contained in something.  And the pressure on the liquid will be transferred to that container.  This will make the container stretch (rheologists will call it strain).  The expansion of the container will often be more important than the actual reduction in volume of the liquid.
I also remember some emulsions behaving in a rather strange way under pressure - can this be due to differences in compressability ?

[lightarrow]

I also remember some emulsions behaving in a rather strange way under pressure - can this be due to differences in compressibility ?

If you explain precisely which is this behaviour, we can try to answer to your question, otherwise it would be difficult.

[eric l]

I remember the case of an oil-in-water emulsion, that had a milky aspect at ambient pressure (high turbidity mainly), that seemed to become more like a one phase system (lower turbidity) when subjected to increasing pressures.  After lowering the pressure, the turbidity was high again.  In the case of a latex (PVAc if my memory does not fail me) we had a similar decrease in turbidity at high pressure, but after the pressure was released, we ended up with two separate phases instead of one emulsion.
(At that time we were looking for a simple and quick method to evaluate the stability of emulsions.  Behaviour under pressure was quickly dropped as a possible method, in favour of diluting and observing the phase separation after 15 minutes rest.)