Question

Cotton is a solid but it floats on water. Why?

Answer 1

The parameter which decides whether a thing will float or sink is density. So try thinking from a density point of view why cotton being a solid is still able to float on water.

Complete answer:
Let us understand density in simpler words, Density is the measure of the mass of an object per unit volume. It is denoted as $'\rho '$.
$\rho =\dfrac{M}{V}$
Where $\rho$ is the density, $M$ is the mass in kgs and $V$ is the volume in ${{m}^{3}}$. Therefore, the SI unit of density is $kg/{{m}^{3}}$.
Density describes how closely packed the particles are in a solid, liquid or gas. As we know, In solids the particles are tightly packed and cannot move freely whereas in liquids particles are loosely packed and can move freely. So it means the density of solid is greater than the density of liquids.
If an object is more dense than water, it will sink when placed in water and if it is less dense than water it will float.
So as per above logic, solids are denser than water so they should sink when placed in water but why does cotton float then?
It is because of its porous structure in which air is trapped as a result its volume increases(as air has occupied the pores) and due to this density decreases but once the pores get filled with water it will sink in the water.

Note:
One important feature about density is that it is an intensive property, which means that it is a property that is the same no matter how much of a substance is present.
So, a litre of water has the same density as that of a single drop of water.