Video narration
In a sealed clear jar, two clear solutions meet and a white solid appears-where did it come from? Conservation of mass means matter isn't made or lost in a chemical reaction. In our closed jar, the ingredients can change form, but nothing leaves. Here, barium chloride solution and sodium sulfate solution react. The white solid is insoluble barium sulfate. It isn't extra matter: barium and sulfate particles have rearranged to form it, while sodium and chloride stay dissolved as sodium chloride. A visible change isn't a gain in matter. The balanced reaction is: BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq). Compare the sides: one barium, two chlorine, two sodium, one sulfur, and four oxygen atoms on each. Same atoms, regrouped. So the mass is the same, too. Back in the sealed jar, the contents look different, but nothing escaped. Where did the white solid come from? Nowhere new. Its atoms were already there.