The molecules are farther apart in a gas.
When heated, gases typically expand and become less dense. So, a gas would be least dense when heated.
'Least' is a comparison word. Compared to iron and lead yes cotton is least dense. Compares to oxygen gas and hydrogen gas, no it is not least dense.
Gas
Saturn is the least dense planet in our solar system.
Saturn is the least dense of the gas giants, even out of all of the planets in our solar system. Its average density is around 0.7 g/cc (less than water).
Saturn is the least dense of the gas giants, even out of all of the planets in our solar system. Its average density is around 0.7 g/cc (less than water).
Hydrogen gas is considered the least dense element. It is less dense than air, which is why it is often used in balloons.
Helium has the least density among the substances listed. It is a very light gas and is less dense than air, oxygen, ammonia, and chloride.
solid, liquid then gas
Hydrogen (gas)Oxygen (gas)PumiceWater (liquid)GraniteSteelLeadGold
The density of solid substances is generally higher than that of liquids, which in turn are higher than gases. This is because the particles in a solid are tightly packed together, while in a liquid they are more spread out, and in a gas, they are far apart.
Oil will sink in water, so is the least dense.