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Grade 9 / Middle 3 (age 14-15)

Law of Conservation of Mass

Law of Conservation of Mass

In a chemical reaction atoms only rearrange; they are not created or destroyed, so total mass before equals total mass after. It is like rebuilding Lego: the house becomes a car, but the number of bricks stays the same. In an open system mass can seem to drop when gas escapes, yet counting that gas still conserves mass. On this page, compare the balance before and after a reaction to see how mass is kept.

Mass in Chemical Reactions?
⚖️ Intuition — Rebuilding Lego Blocks
①Build a Lego house, then rearrange into a car
②The number of blocks does not change
③Same with chemical reactions: atoms rearrange, none appear or disappear
Check With a Balance
Why is Mass Conserved?
🔬 Atoms Are Conserved
①Reaction = rearrangement of atoms
②No atoms appear or vanish
③Same kinds and counts of atoms before and after
④Therefore the total mass is the same
Summary
Conservation of Mass
Total mass before = Total mass after
Lavoisier proved it with precise experiments (1774)
🎯 Exam Points
①Conservation of mass: total mass unchanged in a reaction
②Reason: atom types and counts are preserved
③In an open system, escaping gas seems to lose mass; include the gas and it matches
④Holds for precipitation and combustion reactions
⑤Lavoisier confirmed it with sealed vessel experiments
Examples and Unit Test
Example 1
Inside a sealed container, 56 g of steel wool reacts completely with 16 g of oxygen to form iron oxide. What is the mass of the iron oxide produced?
1
By the law of conservation of mass, total mass before = total mass after.
2
Total mass before = 56 g + 16 g = 72 g, so the iron oxide produced is also 72 g.
▸ 72 g (56 + 16 = 72, mass conserved)
Atoms only rearrange and none appear or disappear, so the total mass stays the same.
Example 2
When wood is burned in an open dish, the ash seems lighter than the original wood. Does this break the law of conservation of mass?
1
In burning, the wood reacts with oxygen and makes gases such as carbon dioxide and water vapor that escape into the air.
2
If you collect the escaped gases too, the mass of (wood + oxygen) = the mass of (ash + gases), so it is conserved.
▸ It is not broken: including the escaped gases, the mass before and after is equal
In an open system, the mass only seems to change because gases enter and leave.
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).
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Thermal Energy
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Definite Proportions
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