(a)[3]
Explain the differences in density in solids, liquids and gases by referring to how far apart the molecules are.
(b)[3]
A hydrogen nucleus (proton) can be treated as a sphere with radius $1 \times 10^{-15}\,\text{m}$. Calculate the density of a hydrogen nucleus.
(c)[1]
The density of hydrogen gas in a pressurised cylinder is $4\,\text{kg m}^{-3}$. Suggest a reason why this value is far smaller than your answer in (b).