Want this question answered?
water
The answer depends on what the numbers measure. If they are the masses of equal volumes of substances, then the substance with mass 0.8 is denser. On the other hand, if the numbers refer to the volumes of equal masses of two substances, then the substance with volume 0.7 is denser.
substances
No, not unless they are made of the same substance. Different substances have different densities, which means that the same volumes will have different masses.
The question isn't worded very well, so I'm not sure this is the answer to what you were trying to ask. The bulk mass of a substance has no impact whatsoever on the melting point of that substance. Covalently bonded molecules with higher molecular masses tend to have higher melting points, if the substances are roughly the "same kind of substance", which is an ill-defined term so you shouldn't put too much faith in this until you've studied chemistry enough to have developed a sort of intuition about what it means.
yes
substances
substances
isotopes
Yes
This question cannot be answered. ppm is a ratio: so many parts of one substance per million parts of another. That can only be converted into mcg if the relative masses of the two substances are known.
The total mass of products is unchanged from the total mass of the reactants, but the masses of particular substances among the reactants or products change.