Final answer:
The mass of a mixture of substances can be determined by adding the masses of each substance. The mass can be used as evidence of the conservation of matter if it remains constant before and after a chemical reaction.
Step-by-step explanation:
The mass of a mixture of substances a and b can be determined by adding the masses of each substance. For example, if the mass of substance a is 10 grams and the mass of substance b is 5 grams, then the total mass of the mixture would be 15 grams.
This mass can be used as evidence of the conservation of matter because according to the law of conservation of mass, the total mass of substances in a chemical reaction remains constant. So, if the mass of the mixture before the reaction is 15 grams, then the mass of the mixture after the reaction should also be 15 grams.
If there is a difference in mass before and after the reaction, it indicates that there might have been a loss or gain of matter, which would suggest a violation of the conservation of mass.