Mass is conserved in chemical reactions, but not in nuclear ones. All compounds have a definite composition by mass. When elements combine to form compounds, energy is released. Decomposition reactions are always endothermic. Some acids are dangerous, but others are not.
Food spoils more slowly in the refrigerator.
A spark is needed to light your butane lighter. A carburetor is needed to make a gasoline engine work. Iron rusts, but gold and aluminum do not. Thousands of different chemicals can be made from crude oil. In a chemical reaction many changes can take place.