in the law of conservation of matter what happens to atoms when they combine to form a new substance?

Question

I am a mom trying to help my daughter and I have no clue and this information is not in her book she brought home

Answer 1

The resultant mass is always equal to the starting mass. The mass will be in the resulting new substance plus any energy added to form the new substance, or minus any energy that was released by forming the new substance. The energy is measured by E=mc^2

Answer 2

using a magnifying glass, I set a piece of paper on fire (I use a glass so I don't add anything to the paper)

I start with a piece of paper.

I end with; heat, light, smoke, water vapor, ash.

I did not create or destroy anything...I just changed its form