Law of Conservation of Mass

The statement. Mass can neither be created nor destroyed in a chemical reaction — the total mass of the reactants equals the total mass of the products, provided nothing escapes or enters the reaction vessel.

Why "closed system" matters. A reaction like burning magnesium in air, or mixing two solutions, involves solids, liquids, and sometimes gases. If a gas produced during the reaction is allowed to escape into the open air before the final weighing, the measured mass of products will look smaller than the mass of reactants — not because mass was destroyed, but because some of it left the balance pan unmeasured. Carrying out the reaction in a sealed vessel, and weighing the whole vessel with its contents, is what lets the law be verified honestly.

The mass statement. For a reaction of the general form

the law requires

where: $m_A$ and $m_B$ are the masses of the substances that react, and $m_C$ and $m_D$ are the masses of the substances formed. At Class 9 level this is used as a plain numerical equation — you are not required to balance a symbolic chemical equation with formulae; that is introduced in Class 10 once you know chemical formulae well.

Historical grounding. Antoine Lavoisier established this experimentally in the late 1700s by carrying out reactions in sealed vessels and weighing reactants and products on a sensitive balance. His careful, quantitative measurements — a sharp break from the qualitative chemistry before him — showed total mass stayed constant to a very good approximation. Together with the Law of Constant Proportions, this mass-conservation evidence was one of the two experimental pillars on which John Dalton built his atomic theory: if atoms are neither created nor destroyed during a reaction, only rearranged into new combinations, then the total mass measured before and after must match.

Problem: 5.3 g of sodium carbonate reacts completely with 6 g of ethanoic acid in a closed flask. The products are sodium ethanoate, water, and carbon dioxide. The mass of sodium ethanoate formed is 8.2 g and the mass of water formed is 0.9 g. Find the mass of carbon dioxide produced, assuming no gas escapes.

Step 1. Write the mass statement for this reaction: mass of reactants equals mass of products, so $m_{\text{sodium carbonate}} + m_{\text{ethanoic acid}} = m_{\text{sodium ethanoate}} + m_{\text{water}} + m_{\text{CO}_2}$.

Step 2. Insert the known masses: $5.3 + 6 = 8.2 + 0.9 + m_{\text{CO}_2}$, which simplifies to $11.3 = 9.1 + m_{\text{CO}_2}$.

Step 3. Solve for the unknown: $m_{\text{CO}_2} = 11.3 - 9.1 = 2.2\ \text{g}$.

Answer: 2.2 g of carbon dioxide is produced. This only works because the flask was closed — had the CO$_2$ gas escaped into the air before the final weighing, the products would have appeared to weigh less than the reactants, wrongly suggesting the law had been broken.

• Forgetting gases. Weighing only visible solids or liquids and concluding mass was "lost" is a common error — an escaping or unweighed gas like CO$_2$ or O$_2$ is still part of the mass balance, so the reaction must be sealed to verify the law correctly.
• Confusing this with balancing equations. At this level the law is about total mass, not about balancing the number of atoms of each element in a symbolic chemical equation — that formal skill belongs to Class 10.
• Assuming "no visible residue" means "no mass." A gas or vapour still has mass even though you cannot see it collecting; "disappearing" smoke or steam has moved, not vanished.
• Extending it to nuclear reactions. The law as taught here applies only to ordinary chemical reactions; nuclear reactions, where mass and energy interconvert, are outside this syllabus.

Ask the tutor: Want to try a mass-balance problem with a gas that partly escapes, or see how this law connects to Dalton's atomic theory — ask the AI tutor to walk through another example step by step.