Explain why the same molecule can have the same density at different temperatures

Question

The density of water at 98 degrees Celsius is 0.95979 g/cm^3.  The density if water at -8 degrees Celsius is 0.99865 g/cm^3.  Explain why the same molecule can have the same density at different temperatures?

Answer 1

Density is the amount of mass in a specific volume of a substance.

 

Usually substances expand when they are heated and contract when they are cooled.  Because of that, their density will usually increase as the temperature decreases -- because the same amount of mass takes up a smaller volume.

 

However, some substances have properties that affect this relationship in other ways.  Water, for example, follows this same relationship as it cools, but when the temperature reaches 4° C, the bonds between molecules cause the density to rise as it cools further.  Once the water freezes and becomes ice, the normal relationship returns and the density of the ice decreases as the temperature decreases further.

 

So sequence of fall, rise and fall of the density (as the temperature falls) means that there will be different temperatures at which water/ice have the same density.