Physical and chemical changes

Atoms in molecules are held together by bonds. In a chemical reaction, the bonds between the atoms of the reactants break, the atoms rearrange, and new bonds form to make the products. No atoms are made or lost: the same atoms are simply joined in a new way. Think of a Lego model: you take apart a car and build a plane from the same bricks. The bricks are the atoms. A good example is baking soda (sodium bicarbonate) with vinegar (dilute acetic acid). They react to make carbon dioxide gas, water and a salt called sodium acetate. The gas fills a balloon. We can write the reaction in words: baking soda + vinegar → carbon dioxide + water + sodium acetate. The reactants are on the left, the products on the right, and the arrow means “makes”. Many reactions are useful: cooking, digestion, batteries and fireworks all depend on them.