Choose a substance you're familiar with. what are its physical and chemical properties? how would you measure its density? what are some physical and chemical changes it can undergo?

you can look at magnesium, it can react with oxygen to form oxides. (chemical) it is malleable and a solid at room temperature. (physical)

to measure its density, the mass and volume can be worked out and from this density too. look up the equation, it is quite easy :) 
physical changes -- it can be melted, and oxidized 

the chemical changes of oxidation magnesium looses electrons to form oxides, this is a chemical reaction- chemical change..--- use to get the density use (rho) or density D = M/V