what is the density
Density is the mass per unit volume of a substance. On Earth, you can assume mass is the same as weight, if that makes it easier. If you're not still in school, then you probably forgot you ever even heard it. The definition of density, makes a lot more sense with a little bit of explanation. As long as an object is made up of molecules, and thus has size, it has a density. Density is just the weight for a chosen amount (volume) of the material. Water's density varies with temperature