Why Can't We Compress Water but Can Compress Gas?

[Impulse2]

Excuse my ignorence here but as I understand you cannot compress water. Why not, is there not some space between any molecules, particularly following heating, that would allow compression? Assuming there isn`t then how is it we can compress a pure gas. Is the situation not the same?

 

[Vanadium 50]

Gasses are not liquids. Indeed, it's the space between molecules that make the properties of one different from the other.

There is a lot of space between gas molecules compared to liquids. That's why gasses are much more compressible than liquids.

 

[Impulse2]

Yes, but what is between the gas molecules and for that matter the liquid ones? Are you saying that there is a vacuum and they only partially pack down?

 

[Doug Huffman]

Liquid water is compressible, just not a lot. See bulk modulus of elasticity.

As pressure decreases and/or temperature increases then there is more distance between the molecules.

In a hard vacuum like interstellar space there may only be one molecule per cubic centimeter.

Avogadro's Number of water molecules is ~18 grams, the molar volume.

 

[gmax137]

Excuse my ignorence here but as I understand you cannot compress water.

As Doug Huffman just said, your understanding is incorrect. Liquids are not truly incompressible, but they are nearly so, and assuming that they follow the idealized behavior makes solving many problems simpler without significantly affecting the results. So, they are "approximately" incompressible, but the approximation is a very good one.

Of course, there are likely situations in which the approximation breaks down. The trick is to recognize those situations, and avoid using the approximation there.