Conservation of mass tells us total mass never changes. Yet in some experiments, the MEASURED mass appears to increase or decrease. This is NOT a violation of conservation of mass — it is because a GAS enters or leaves the reaction vessel.
WHEN MASS APPEARS TO DECREASE:
A GAS is PRODUCED and escapes into the atmosphere.
The gas molecules leave the container and are no longer weighed.
Example: Mg ribbon burning — ash (MgO) seems lighter than the ribbon, but this is because oxygen from AIR was added. Without accounting for the oxygen, mass appears lost.
WHEN MASS APPEARS TO INCREASE:
A GAS is ABSORBED or ADDED from the atmosphere.
Example: Magnesium burning in air:
2Mg(s) + O₂(g) → 2MgO(s)
Oxygen from the air is absorbed into the solid product → measured mass of solid INCREASES.
The oxygen molecules join the solid — weighed mass goes up.
Predicting Mass Changes
You can predict mass changes using the balanced equation and Mr values.
IF A GAS IS PRODUCED and escapes an open container:
Mass of container decreases by mass of gas produced.
IF A GAS IS ABSORBED from the air:
Mass of solid/container increases by mass of gas absorbed.
Example calculation:
2Mg + O₂ → 2MgO
Mr values: Mg = 24, O₂ = 32, MgO = 40
If 4.8 g of Mg burns completely in air:
Moles of Mg = 4.8 ÷ 24 = 0.2 mol
From equation: 2 mol Mg needs 1 mol O₂, so 0.2 mol Mg needs 0.1 mol O₂
Mass of O₂ absorbed = 0.1 × 32 = 3.2 g
Mass increase = 3.2 g (the oxygen from air added to the solid)
Expected mass of MgO = 4.8 + 3.2 = 8.0 g
Check: 0.2 mol MgO × 40 = 8.0 g ✓
Non-conservation Apparent Effects
Students sometimes observe apparent non-conservation — these all have explanations:
PRECIPITATION REACTIONS in closed containers:
No gas escapes or enters → mass stays exactly the same. ✓
HEATING A METAL IN AIR (open container):
Mass INCREASES — oxygen from air is absorbed.
If weighed in a closed container with air, mass stays constant (oxygen absorbed from sealed air pocket).
DECOMPOSITION reactions producing gas (e.g. CaCO₃ → CaO + CO₂):
In open container: mass DECREASES (CO₂ escapes).
In closed container: mass stays constant (CO₂ trapped).
BURNING HYDROCARBONS in open container:
CO₂ and H₂O vapour escape → apparent mass decreases.
KEY INSIGHT: If you cannot account for a mass change, look for a gas being produced or absorbed — conservation of mass is ALWAYS upheld in the universe, even if your experiment doesn't show it.
⚠️ Common Mistake
When a metal burns in air, the solid GAINS mass (oxygen is added from the air to form the oxide). Students often expect the solid to lose mass because 'burning destroys things' — but mass is always conserved. The apparent gain is real because you are adding oxygen from the atmosphere to the solid product.
📌 Key Note
Mass is always conserved. Apparent decrease: gas escapes (e.g. CO₂ from acid + carbonate). Apparent increase: gas absorbed from air (e.g. O₂ absorbed when Mg burns). Closed container: mass always stays the same. Look for gases when mass appears to change.
🎯 Matching Activity — Mass Increases, Decreases or Stays Same?
Predict what happens to the measured mass in each scenario. — drag the symbols on the right to match the component names on the left.
Mass decreases
Drop here
Mass increases
Drop here
Mass stays same
Drop here
Mass decreases
Drop here
Mass stays same
Drop here
Magnesium burning in air — oxygen from air absorbed into MgO solid product
Precipitation reaction in a sealed flask — no gas escapes or enters
CaCO₃ heated in a sealed tube — CO₂ trapped inside
CaCO₃ + HCl in an open flask — CO₂ gas escapes into the atmosphere
Zinc + sulfuric acid in an open tube — H₂ gas escapes
⚽ FIFA Worked Examples
Mass Change Prediction
4.8 g of magnesium burns completely in air: 2Mg + O₂ → 2MgO. Calculate the mass of MgO produced. Ar: Mg=24, O=16.
F
Use ratio from balanced equation: 2 × Mr(Mg) : 2 × Mr(MgO) = 48 : 80
I
Scale: 4.8 g of Mg. Scale factor = 4.8 ÷ 48 = 0.1
F
Mass of MgO = 80 × 0.1 = 8.0 g
A
8.0 g of MgO produced (mass increases by 3.2 g — the absorbed oxygen)
🎯 Test Yourself
Question 1 of 2
1. A student heats calcium carbonate in an open crucible. CaCO₃ → CaO + CO₂. What happens to the measured mass?
2. Why does the mass of iron increase when it rusts in air?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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🤖 Ask Mr Badmus AI
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I'll use FIFA for calculations and flag Higher/Triple content clearly.