Actually liquids CAN change volume - they just change less drastically than gases under the same conditions. An obvious example is the functioning of a thermometer. The liquid in the thermometer expands (changes volume) as it gets warmer. Usually the liquid will be in a large reservoir with a narrow tube attached because the volume change is not that big. The change in volume for a gas with the same change in temperature is much more noticeable. Liquids also change their volume with changes in pressure. Again, gases change a lot more than liquids with the same change in pressure - except near the critical point of the liquid. As an example: liquid water at 50 bar and 25 °C will have a density of about 1000 kg/m3. If the pressure is cut in half to 25 bar at 25 °C, the density drops to about 998 kg/m3 - a change of about 0.2%. If you do the same with a gas at 25 °C, the density will change by 50% (250 times the change in the liquid)!
The simple reason for this is that liquids are much more dense than gases. The molecules are much closer together so when you start to put pressure on them, they resist compression much more than gases will because they are already so close and repulsive forces between the molecules are much stronger. When pressure is dropped, liquids expand less because - again - the molecules are close together and the attractive forces are much more significant. The spacing of molecules in a liquid is a bit like the situation with Goldilocks - they don't want to be too close or too far apart
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