Final answer:
The density of helium inside a balloon decreases when its volume doubles, because density is inversely proportional to volume if the mass remains constant.
Step-by-step explanation:
When a helium balloon rises in the air and its volume doubles, the density of the helium inside the balloon decreases. Density, which is mass per unit volume, decreases proportionally if the volume increases given that the mass of helium does not change significantly. Since balloons filled with helium float because helium's density is less than the density of air, the balloon's volume expansion while rising is a direct result of the decrease in density of the helium gas due to reduced pressure and possibly changes in temperature.