WHAT is used instead of soap because it will not create an insoluble s c u m when mixed with hard water.

Hard water contains salts of calcium and magnesium principally as bicarbonates, chlorides, and sulfates. Ferrous ion may also be present;

Explanation:

Hard water contains salts of calcium and magnesium principally as bicarbonates, chlorides, and sulfates. Ferrous ion may also be present; oxidized to the ferric form. It appears as a reddish brown stain on washed fabrics and enameled surfaces. Water hardness that is caused by calcium bicarbonates is known as temporary, because boiling converts the bicarbonates to the insoluble carbonates; hardness from other salts is called permanent. Calcium and magnesium ions in hard water react with the higher fatty acids off soap to form an insoluble gelatinous curd, thereby causing a waste of the soap.    

D. most models represent a real world object in a simpler way

The density would be 1.622