Conservation of Mass

This lesson covers the law of conservation of mass as required by the AQA GCSE Combined Science Trilogy specification (8464). You will understand why mass is conserved in chemical reactions, how to explain apparent mass changes in open systems, and how to perform calculations using this law.

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The Law of Conservation of Mass

No atoms are created or destroyed in a chemical reaction. The total mass of the products is always equal to the total mass of the reactants.

This law was established by Antoine Lavoisier in the 18th century. It is one of the fundamental principles of chemistry.

Why Is Mass Conserved?

In a chemical reaction:

• Bonds between atoms break and new bonds form.
• The atoms are rearranged into new substances.
• But the number and type of atoms stays exactly the same.

Since no atoms are gained or lost, the total mass cannot change.

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Demonstrating Conservation of Mass

Experiment in a Sealed Container

If a reaction takes place in a sealed flask on a balance:

• The mass reading stays exactly the same before and after the reaction.
• This works for precipitation reactions, neutralisation reactions, or any reaction in a sealed system.

Apparent Mass Changes in Open Containers

In open containers, the mass reading may appear to change. This is because gases can enter or leave the container.

flowchart LR
    subgraph OPEN["Open Container"]
        direction TB
        R["Reaction occurs"]
        G["Gas produced or\ngas absorbed"]
    end
    G -->|"Gas escapes"| D["Mass APPEARS\nto decrease"]
    G -->|"Gas absorbed\nfrom air"| I["Mass APPEARS\nto increase"]
    style D fill:#ef4444,color:#fff,stroke:#dc2626
    style I fill:#22c55e,color:#fff,stroke:#16a34a
    style R fill:#3b82f6,color:#fff,stroke:#2563eb

Observation	Explanation	Example
Mass decreases	A gas is produced and escapes into the atmosphere	Acid + carbonate → CO₂ escapes
Mass increases	A gas from the air combines with the reactant	Magnesium + oxygen from air → MgO
Mass stays the same	No gas enters or leaves (sealed system or no gas involved)	Precipitation in a sealed flask

Exam Tip (AQA 8464): The total mass is always conserved — it is only the measured mass that appears to change because the balance cannot weigh gases that have escaped or entered.

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Worked Examples

Example 1 — Mass Decrease (Gas Escapes)

Question: 10.0 g of calcium carbonate is heated strongly in an open crucible. After heating, 5.6 g of calcium oxide remains. What mass of carbon dioxide was released?

$\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$CaCO3​(s)→CaO(s)+CO2​(g)

Using conservation of mass:

$\text{mass of CO}_2 = \text{mass of CaCO}_3 - \text{mass of CaO}$mass of CO2​=mass of CaCO3​−mass of CaO

$\text{mass of CO}_2 = 10.0 - 5.6 = 4.4 \text{ g}$mass of CO2​=10.0−5.6=4.4 g

Example 2 — Mass Increase (Gas Absorbed)

Question: 4.8 g of magnesium is burned in air to form magnesium oxide. The magnesium oxide has a mass of 8.0 g. What mass of oxygen reacted?

$2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$2Mg(s)+O2​(g)→2MgO(s)

$\text{mass of O}_2 = \text{mass of MgO} - \text{mass of Mg}$mass of O2​=mass of MgO−mass of Mg

$\text{mass of O}_2 = 8.0 - 4.8 = 3.2 \text{ g}$mass of O2​=8.0−4.8=3.2 g

Example 3 — No Change (Sealed System)

Question: A student mixes solutions of lead nitrate and potassium iodide in a sealed flask on a balance. The total mass before mixing is 250.0 g. What is the total mass after the yellow precipitate of lead iodide has formed?

$\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \rightarrow \text{PbI}_2(s) + 2\text{KNO}_3(aq)$Pb(NO3​)2​(aq)+2KI(aq)→PbI2​(s)+2KNO3​(aq)

Answer: 250.0 g — the mass does not change because no substance enters or leaves the sealed flask.

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Explaining Mass Changes Using Particle Ideas

When a question asks you to explain a mass change, use these key phrases:

For mass decrease:

• "A gas (name it, e.g. CO₂) is produced during the reaction."
• "The gas escapes into the atmosphere."
• "The atoms in the gas are no longer on the balance, so the measured mass decreases."
• "Mass is still conserved overall — the 'missing' mass is the gas that escaped."

For mass increase:

• "The metal reacts with oxygen from the air."
• "Oxygen atoms from the air combine with the metal atoms."
• "The oxygen atoms add mass to the product."
• "Mass is still conserved — the increase equals the mass of oxygen that combined."

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Conservation of Mass and Balanced Equations

A balanced equation is a direct representation of conservation of mass. The same number of each type of atom appears on both sides.

For example: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO

	Left side	Right side
Mg atoms	2	2
O atoms	2	2

Total $M_r$Mr​ on left: $(2 \times 24) + (2 \times 16) = 48 + 32 = 80$(2×24)+(2×16)=48+32=80

Total $M_r$Mr​ on right: $2 \times (24 + 16) = 2 \times 40 = 80$2×(24+16)=2×40=80

Both sides equal 80 — mass is conserved.

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Common Mistakes to Avoid

Mistake	Correction
"Atoms are destroyed so mass is lost"	Atoms are never created or destroyed — mass changes are due to gases entering or leaving
"The reaction creates new mass"	Mass cannot be created — apparent increases are from gases (e.g. O₂) joining the reaction
Forgetting to name the gas	Always state which gas escapes or is absorbed
Confusing sealed vs open systems	In sealed systems, mass always stays the same; in open systems, it can appear to change

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Summary

• The law of conservation of mass states that no atoms are created or destroyed in a chemical reaction.
• Total mass of reactants = total mass of products (always).
• In open containers, mass may appear to change because gases escape or are absorbed.
• In sealed containers, the measured mass stays the same.
• Balanced equations demonstrate conservation of mass — equal atoms on both sides.
• Always name the gas and explain whether it escapes or is absorbed from the air.

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Extended Worked Examples — Conservation of Mass Calculations

Conservation of mass questions reward students who write the equation, calculate $M_r$Mr​, and track every substance. Below are further worked examples that combine conservation of mass with relative formula mass ($M_r$Mr​) and mole reasoning.

Worked Example 4 — Finding the Missing Mass

Question: 5.6 g of calcium oxide reacts completely with carbon dioxide to form calcium carbonate.

$\text{CaO}(s) + \text{CO}_2(g) \rightarrow \text{CaCO}_3(s)$CaO(s)+CO2​(g)→CaCO3​(s)

If 10.0 g of calcium carbonate is produced, what mass of carbon dioxide reacted?