In a chemical reaction atoms only change partners; none are made and none are destroyed, so the masses of everything before the reaction add up to the same total as the masses of everything after it.
Take apart a Lego house and build a car from it, and you still have exactly the same bricks.
In an open dish a gas that forms can drift away and the mass seems to fall, but weigh that gas too and the total is unchanged.
The balance below holds the starting substances on the left pan and the products on the right, and the beam stays level.
What happens to mass in a reaction?
⚖️ Rebuild the bricks, keep the count
①Pull apart a Lego house and build a car, and the shape is completely different
②Every brick is still there, so the bricks weigh just as much as before
③In a chemical reaction the bricks are atoms, and the reaction only regroups them
Comparing the two pans of a balance
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An equal-arm balance with the reactants on the left pan and the products on the right pan, beam level
Why mass is conserved
🔬 Same kinds of atoms, same numbers
①A reaction makes new substances by letting atoms swap partners
②No atom appears or disappears, so the kinds and numbers of atoms are the same before and after
③Each atom has a fixed mass, so the total mass is the same too
④In 4Fe + 3O₂ → 2Fe₂O₃ both sides hold 4 iron atoms and 6 oxygen atoms
⑤In an open container gas can enter or leave and the mass seems to change; count that gas and the totals agree
Summary
Law of conservation of mass
Total mass before = Total mass after
Shown by Lavoisier, who weighed reactions carefully in sealed vessels (1774)
🎯 Points that often come up
①The total mass of the reactants equals the total mass of the products
②This is because the kinds and numbers of atoms do not change
③In a sealed container the total mass stays the same even when a gas forms
④In an open container escaping gas makes the mass drop, and oxygen taken from the air makes it rise
⑤It holds for precipitation reactions and for combustion alike
Worked examples and unit test
Example 1
In a sealed container, 56 g of steel wool reacts completely with 24 g of oxygen to form iron oxide (Fe₂O₃). What is the mass of the iron oxide?
1
Nothing enters or leaves a sealed container, so the total mass before the reaction equals the total mass after.
2
Before the reaction the total is 56 g + 24 g = 80 g. Afterwards only iron oxide is left, so it has a mass of 80 g.
▸ 80 g
This is the iron case on the balance above: 80 g on the left pan and 80 g on the right, and the beam is level.
Example 2
Wood burned in an open dish leaves ash that is much lighter than the wood. Has the law of conservation of mass been broken?
1
As wood burns it reacts with oxygen from the air, forming carbon dioxide and water vapour, and these gases escape from the open dish.
2
Collect and weigh all the escaped gas, and the mass of (wood + oxygen used) equals the mass of (ash + carbon dioxide + water vapour).
▸ No. Once the escaped gases are added in, the mass before and after is the same
Steel wool burned in the open does the opposite and gets heavier, because it takes oxygen from the air. Either way, counting the gas that moved keeps the mass the same.
Unit Test
Which statement about the law of conservation of mass is correct?
①When a chemical reaction happens, atoms newly appear or disappear
②In a reaction inside a sealed container, the total mass before and after is equal
③Mass is not conserved in a precipitation reaction
④If gas is produced in an open dish, the law of conservation of mass is broken
⑤The law of conservation of mass holds only for physical changes
▸ ② In a reaction inside a sealed container, the total mass before and after is equal
1
A chemical reaction is a rearrangement of atoms, so no atoms appear or disappear (① wrong), and mass is conserved in a precipitation reaction too (③ wrong).
2
Even if gas escapes an open dish, the mass is conserved once the gas is included (④ wrong), and conservation holds for chemical changes too (⑤ wrong); in a sealed-container reaction the total mass before and after is equal (② correct).