Question

Which of the following is true of chemical reactions?

Chemical reactions occur as a result of a change in mass.
Chemical reactions occur when gas is heated.
Chemical reactions occur when matter changes state.
Chemical reactions occur as a result of different substances combining with each other.

Answer 1

I should probably clarify and let you answer this, but, a \(\sf \color{red}{chemical~reaction}\) is when the molecule changes because two of them react. They \(\ must\) change. Bonds must be broken and form a new one.
For example, when you leave some metal outside, made of say, Iron, over tie you will see that the iron metal begins to turn brownish. This is a chemical change, because the Iron, Fe, bonds with oxygen air, forming iron oxide.

You have Fe + O\(_2\) \(\rightarrow\) Fe\(_2\)O\(_3\)

Answer 2

Now, can you answer this?