Question

how does the density of warm water compared with the density of cold water

Answer 1

As you chill water, the molecules start to move closer together.
Thus, as you chill water, the density of the water increases (because there are more molecules per square meter)

As you heat water, the molecules start to move around more.
Thus, as you heat water, the density of the water decreases (because there are less molecules per square meter)

Answer 2

Just to re-enforce a bit: That's absolutely correct.

Between the temperatures of 100 C and 0 C (the boiling and freezing points of water, respectively), the density of water increases as temperature gets lower, and for the same reason stated. The molecules get closer to one another, so there is more mass in a given volume.

A small extra, and rather cool (in my opinion) note about water: When it freezes, it actually expands! The density of ice is less than the density of water! This is why ice floats on water. There aren't a lot of substances (at least common ones, if at all) that exhibit this behavior.

Answer 3

Oh, one small correction. It'll actually start to expand just before freezing.

In most cases, the above is true. But from 4 C - 0 C, the density starts increasing a little bit.

However, between what we would call warm water and cold water, the density of cold will be higher.

Answer 4

Bleh, small correction to my correction: between 4 and 0, the density starts decreasing as water is cooled.

Answer 5

Here is a cool video showing the power of water expanding! https://www.youtube.com/watch?v=4i5r65QGUpw

This is why we have to be careful in the winter when freezing could happen in the pipes of our homes.