NSES Content Standard B 
Physical Science: Properties and changes of properties in matter
Grades 5-8, page 154

A substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the sample. A mixture of substances often can be separated into the original substances by using one or more of the characteristic properties.

Benchmark 4D The Physical Setting: Structure of Matter
Grades 6-8, page 78
Equal volumes of different substances usually have different weights.