Density is the measurement of how solid something is…or in more geeky terms, the density of a material is equal to its mass divided by its volume. That is the mass of the atoms or molecules that makes up a material, divided by, the volume or amount of space the material takes up.
So two objects made of the same material have the same density even though they may have different masses and different volumes. That’s because the ratio of mass to volume is the same for both objects. Confused yet?
The density of a material depends on temperature. The general relationship is that density decreases with increasing temperature. This is called thermal expansion. This is true for most compounds, as the temperature of the liquid increases; the density decreases as the molecules spread out from each other. And as the temperature decreases, the density increases as molecules become more closely packed.
But this pattern does not hold true for ice as the exact opposite occurs. Which is more dense? Water or ice? Well we all know the answer to that question. But why does this happen?
Water is the only liquid that becomes less dense as a solid. The expansion of water when you cool it to a lower temperature is unusual, since most liquids contract when they’re cooled. Even though the mass remains constant, the space or volume that it takes up also changes with temperature. As temperature goes up, so does the vibration of the materials molecules, leading to a drop in density.
All substances, including water, become less dense when they are heated and more dense when they are cooled. So if water is cooled, it becomes more dense and forms ice. Water is one of the few substances whose solid state can float on its liquid state! Why? Water continues to become more dense until it reaches 4°C, or 39°F. After it reaches 4°C, it becomes less dense.
This is all due to something called hydrogen bonding, but I’ll spare you the details.