Question

A solid is cooled to a very low temperature. Assuming the mass remains constant, how, (if at all), does this affect the density of the solid?

Answer 1

Most solid substances (all of them that I can think of)
contract (shrink) when they get cold.

         Density  =  (mass) / (volume)

If mass doesn't change but volume gets smaller,
then density increases.

Answer 2

Answer: When the solids reduce their temperature, usually happens a volume contraction, it means that the volume of the solid decreases.

Now, the density of something is defined as : D = $\frac{mass}{volume}$.

Then if the mass remains constant, and the volume gets smaller, the density of the solid gets bigger.