Conservation of Mass

This lesson covers the law of conservation of mass as required by the AQA GCSE Chemistry specification (4.3.1). Conservation of mass is a fundamental principle that underpins all quantitative chemistry. You need to understand why mass is conserved in chemical reactions, be able to explain apparent changes in mass during reactions involving gases, and balance symbol equations to reflect this law.

The Law of Conservation of Mass

The law of conservation of mass states that no atoms are lost or made during a chemical reaction. This means the total mass of the products is always equal to the total mass of the reactants. This law was first established by the French chemist Antoine Lavoisier in the 18th century.

In any chemical reaction, atoms are simply rearranged into new substances. The same atoms that were present at the start are still present at the end — they are just bonded together in different ways. Because no atoms are created or destroyed, the total mass cannot change.

Principle	Explanation
Atoms are conserved	The same number and type of atoms exist before and after a reaction
Mass is conserved	The total mass of reactants equals the total mass of products
Bonds break and form	Existing bonds between atoms break, and new bonds form to create products
No atoms are created or destroyed	Atoms are rearranged, not made or lost

Exam Tip: When explaining conservation of mass, always use the phrase "no atoms are lost or made" — this is the exact wording AQA mark schemes use. Saying "no atoms are created or destroyed" is also acceptable.

Balanced Symbol Equations

A balanced symbol equation shows that the number of atoms of each element on the reactant side equals the number of atoms of the same element on the product side. This is a direct consequence of conservation of mass.

How to Balance an Equation

Write the unbalanced equation with correct formulae.
Count the atoms of each element on both sides.
Add coefficients (large numbers in front of formulae) to balance each element.
Check all elements are balanced.
Never change the chemical formulae themselves.

Worked Example: Balancing an Equation

Balance the equation for the reaction of magnesium with oxygen:

Unbalanced: Mg + O2 --> MgO

Step 1: Count atoms on each side:

Left: 1 Mg, 2 O
Right: 1 Mg, 1 O

Step 2: Balance oxygen by placing a 2 in front of MgO:

Mg + O2 --> 2MgO
Left: 1 Mg, 2 O | Right: 2 Mg, 2 O

Step 3: Balance magnesium by placing a 2 in front of Mg:

2Mg + O2 --> 2MgO
Left: 2 Mg, 2 O | Right: 2 Mg, 2 O

Balanced equation: 2Mg + O2 --> 2MgO

Exam Tip: When balancing equations, work through one element at a time. Start with the element that appears in the fewest formulae. Never alter a chemical formula (e.g. changing H2O to H2O2) — only change the large number in front.

Conservation of Mass in Practice

When you carry out a reaction in a sealed container (closed system), the mass reading on a balance will not change. This is because no substances can enter or leave.

However, if a reaction takes place in an open container (open system), the mass reading on the balance may appear to change:

Observation	Explanation
Mass appears to decrease	A gaseous product escapes into the atmosphere (e.g. carbon dioxide produced when a carbonate reacts with acid)
Mass appears to increase	A gas from the atmosphere is incorporated into the product (e.g. when magnesium burns in air, oxygen from the air combines with the magnesium)

In both cases, mass is still conserved overall — the apparent change is because a gas has either escaped from or entered the reaction vessel.

Reactions Where Mass Appears to Decrease

When a metal carbonate reacts with an acid, carbon dioxide gas is produced. If the reaction takes place in an open flask on a balance, the mass reading will decrease because the CO2 escapes into the air.

Example: Calcium carbonate + hydrochloric acid --> calcium chloride + water + carbon dioxide

CaCO3 + 2HCl --> CaCl2 + H2O + CO2

The CO2 gas escapes, so the mass on the balance goes down. If you could capture and weigh the CO2, the total mass would be unchanged.

Other Examples

Thermal decomposition of carbonates (e.g. heating copper carbonate releases CO2).
Reactions between metals and acids that produce hydrogen gas.
Combustion reactions where gaseous products (CO2 and H2O vapour) escape.

Exam Tip: If a question asks "explain why the mass decreases when calcium carbonate reacts with hydrochloric acid in an open beaker", you MUST mention that carbon dioxide gas is produced AND that it escapes into the atmosphere. Both points are needed for full marks.

Reactions Where Mass Appears to Increase

When magnesium burns in air, oxygen from the atmosphere combines with the magnesium to form magnesium oxide. If you weigh the magnesium before and after burning in an open crucible, the mass increases because oxygen atoms have been added.

Example: 2Mg + O2 --> 2MgO

The magnesium oxide has a greater mass than the original magnesium because it now contains oxygen atoms from the air.

Visualising Conservation of Mass

The following diagram illustrates how atoms are rearranged but conserved during a chemical reaction:

graph LR
    A["Reactants"] -->|"Bonds break"| B["Individual Atoms"]
    B -->|"New bonds form"| C["Products"]
    D["Total mass of reactants"] --- E["= Total mass of products"]

    style A fill:#3498db,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#9b59b6,color:#fff
    style E fill:#9b59b6,color:#fff

Closed vs Open Systems

A closed system is one where no substances can enter or leave. An open system allows substances (particularly gases) to escape or enter.

System Type	Mass on Balance	Reason
Closed (sealed flask)	Stays the same	No substances can enter or leave
Open (gas escapes)	Appears to decrease	Gaseous product leaves the container
Open (gas absorbed)	Appears to increase	Gas from atmosphere is incorporated into the product

Practical Application

In a required practical, you may measure the mass of a reaction in an open conical flask. You would observe the mass decreasing over time if a gas is being released. Plotting a graph of mass against time would show a curve that levels off when the reaction is complete.

Balancing More Complex Equations

Worked Example: Combustion of Ethane

Balance: C2H6 + O2 --> CO2 + H2O

Step 1: Count carbons: 2 on left, 1 on right --> place 2 in front of CO2:

C2H6 + O2 --> 2CO2 + H2O

Step 2: Count hydrogens: 6 on left, 2 on right --> place 3 in front of H2O:

C2H6 + O2 --> 2CO2 + 3H2O

Step 3: Count oxygens: (2 x 2) + (3 x 1) = 7 on right, 2 on left. Need 7/2 = 3.5 O2. Multiply everything by 2:

2C2H6 + 7O2 --> 4CO2 + 6H2O

Balanced equation: 2C2H6 + 7O2 --> 4CO2 + 6H2O

Exam Tip: If you end up with a fraction when balancing, multiply all coefficients by the denominator to get whole numbers. AQA GCSE equations must always have whole-number coefficients.

Summary

The law of conservation of mass states that no atoms are lost or made during a chemical reaction.
The total mass of the reactants always equals the total mass of the products.
Balanced symbol equations have the same number of atoms of each element on both sides.
In open systems, mass may appear to change because gases escape or are absorbed from the surroundings.
Mass appears to decrease when a gaseous product (e.g. CO2) escapes.
Mass appears to increase when a gas from the air (e.g. O2) is incorporated into the product.
In a closed system, the measured mass stays the same.

Exam Tip: Conservation of mass is one of the most frequently tested topics in quantitative chemistry. Be prepared to explain it in terms of atoms being rearranged, and always link apparent mass changes to gases entering or leaving an open system.