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Weigh everything before, weigh everything after — in a closed reaction the number on the balance never changes. Matter is not created or destroyed, only rearranged.
- In a chemical reaction, matter can be neither created nor destroyed .
- The total mass of reactants = total mass of products .
- Proposed by Antoine Lavoisier in 1789 .
- In Set-up 1 the flask was open to the air.
- The carbon dioxide gas escaped into the surroundings.
- So the final reading was less — the mass was not really lost, the escaped gas just was not weighed.
- Known as the Father of Modern Chemistry .
- Proposed the Law of Conservation of Mass , which applies to every chemical reaction.
- He stated that in every operation an equal quantity of matter exists both before and after it.
In this Activity, we will verify the Law of Conservation of Mass using the reaction between sodium sulfate and barium chloride.
- Place two clean, dry 100 mL conical flasks on a balance and mark them A and B; set to zero.
- Pour ~10 mL of 1% sodium sulfate solution into Flask A.
- Pour ~10 mL of 1% barium chloride solution into Flask B.
- Record the total mass of both solutions before mixing (Fig. 9.4a).
- Mix the two solutions, then weigh again (Fig. 9.4b) and compare.
- Keep both the conical flasks on the digital weighing balance. It prevents the error caused by a small amount of solution that may remain stuck to the walls of the conical flask during transfer.
- This reaction has been carried out in the open system, since no gas is formed.
- Given: Zinc + Hydrochloric acid (dilute) → Zinc chloride + Hydrogen.
- Hydrogen gas is formed, so it can escape into the air.
- Design a closed set-up that traps the hydrogen.
- Weigh the whole set-up before the reaction.
- Weigh it again after the reaction is over.
- Equal masses confirm the hypothesis that mass is conserved.
4.0 g calcium carbonate + 2.92 g hydrochloric acid → 1.76 g carbon dioxide + 0.72 g water + 4.44 g calcium chloride (closed container).
Total reactants = 4.0 + 2.92 = 6.92 g . Total products = 1.76 + 0.72 + 4.44 = 6.92 g .
Reactants = products, so the Law of Conservation of Mass is obeyed .
12 g carbon + 32 g oxygen → 44 g carbon dioxide. If 2.4 g of carbon reacts fully, how much CO₂ forms?
1 g carbon gives 44 12 g CO₂, so 2.4 g gives 44 12 × 2.4 = 8.8 g of carbon dioxide.
- 1. Burning 10 g ethanol in an open beaker with no residue does not violate the law — the products (carbon dioxide and water vapour) escaped as gases, so their mass was not weighed.
- 2. 20 g hydrogen + 160 g oxygen → 20+160 = 180 g of water (mass is conserved).
- Any measurement has an uncertainty of about ± 1 in the last digit.
- So small variations in a digital balance reading are normal.
- They lie within experimental error.
- The weight can therefore be taken as constant.
NCERT Question 11 — 5.3 g of sodium carbonate
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Who proposed the Law of Conservation of Mass, and in which year?
Show Answer
Antoine Lavoisier, in 1789. -
State the Law of Conservation of Mass.
Show Answer
Matter can neither be created nor destroyed in a chemical reaction. -
Why did the mass appear to drop in Experimental set-up 1?
Show Answer
The carbon dioxide gas produced escaped from the open flask. -
Is mass conserved in a physical change too?
Show Answer
Yes. Dissolving salt in water shows no change in total mass.
- Law of Conservation of Mass — matter can be neither created nor destroyed in a chemical reaction.
- Physical change — a change such as dissolving in which no new substance forms and the mass stays the same.
- Chemical change — a change in which new substances form; mass is conserved in a closed system.
