Explaining the concept:
To explain this, we first need to know what density actually is:
Density is the mass of a given substance per unit volume, which means, that is the amount of mass present in unit volume (1 cm³ , 1m³)
Impure water is water, but with impurities added to it
Why is the density of Impure water is greater:
When we compare the mass of equal volumes of Water and Impure water, we will find that the mass of Impure water will be more, that is because the extra mass is of the impurities
Since impure water has impurities uniformly spread in it, we will find more matter in unit volume of Impure water than in Pure water
That is why the density of Impure water is greater
