What is the difference between a mixture and compound

A compound is made up of a single type of molecule. A mixture, on the other hand, is made up of different substances that can be distinguished from one another.
As an example:
Water (pure water) is a compound since it's only made up of water molecules
Seawater is a mixture since it is made up of water molecules and soluble salts

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged