Can someone answer this question please. Is this a physical change or chemical change?

Balance this equation co2(g) + h2o (g) show that the balanced equation obeys the law if conversation of mass

Plz choose one of the compounds from the table and explain how you know the numbers of atoms in your formula. is it possible for two different compounds to be made from the exact same two elements? why or why not? with a limited number of elements (less than 120 are known), does this mean we also have a small number of compounds or do we have a large number of compounds in this world?

What happens when an acid dissolves?