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The main gas used in a stink bomb is hydrogen sulphide.

Chemistry - Balancing Equations (AQA)

Balancing equations is about fairness in atoms. Practise checking both sides match so your chemical reactions obey conservation of mass and make scientific sense on paper.

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Fascinating Fact:

Equations must obey conservation of mass, the number of each atom is the same on both sides.

In GCSE Science (Chemistry), you balance symbol equations so that every atom is accounted for on both sides of the arrow. This shows conservation of mass and makes equations match real reactions.

Key Terms

Chemical equation: A written way of showing which substances react and which substances are formed in a reaction.
Coefficient: A big number in front of a formula that tells you how many units of that substance are involved.
Conservation of mass: The rule that the total mass, and therefore the total number of each atom, stays the same in a closed reaction.

Frequently Asked Questions (Click to see answers)

What does conservation of mass mean in GCSE Chemistry?

Conservation of mass means no atoms are lost or gained during a chemical reaction. The number of each type of atom is the same before and after the reaction.

How do I balance a chemical equation step by step?

First count the atoms on each side, then add or change coefficients to match them, checking one element at a time until every atom balances on both sides.

Why do chemical equations have to be balanced?

Equations must be balanced so they obey conservation of mass and accurately show the real amounts of substances reacting and being produced in the experiment.

Try These Related Quizzes

1 .

How many atoms of hydrogen, sulfur and oxygen are there in sulfuric acid, formula H₂SO₄?

1, 1, 1

2, 2, 4

1, 1, 3

2,1,4

A small number after the symbol for an atom in the formula tells you how many of that atom are present. Seeing the symbol just on its own means that you only have one of that sort of atom

2 .

What non-metals are found in sodium silicate?

Sodium and silicon

Silicon and oxygen

Sulfur and hydrogen

Silicon and carbon

The name of a chemical will often indicate what elements are present. Sodium is a metal so, for this question, can be ignored. The non-metal part is the 'silicate'. The 'silic' indicates the element silicon and the 'ate' is a code that indicates that it is combined with oxygen. This rule works with most chemicals that you will meet at GCSE so something like phosphate indicates you have phosphorous combined with oxygen. Other useful parts of this chemical code are 'ite', which also indicates oxygen is in there, and 'ide' which indicates that the non-metal is there on its own

3 .

What do their names tell you about sodium sulfide and sodium sulfate?

Sodium sulfide is soluble in water, sodium sulfate is not

Sodium sulfate is highly flammable, sodium sulfide is not

Sodium sulfide is a solid, sodium sulfate is a gas

Sodium sulfate has oxygen atoms joined to the sulfur atoms but sodium sulfide hasn't

Chemical names do not tell you any of the physical or chemical properties of a compound so you can immediately dismiss the first 3 options

4 .

The main gas used in a stink bomb is the highly toxic hydrogen sulfide. What elements does this compound contain?

Only hydrogen and sulfur

Only hydrogen and sulfide

Hydrogen, oxygen and sulfur

Water and sulfur

Easy if you were paying attention to the useful comment of question 2! You should have been able to dismiss answers 2 and 4 because there is no such element as sulfide and water is a compound, not an element

5 .

How many atoms of oxygen in this thermal decomposition - CaCO₃ ⇌ CaO +CO₂

1 on the left, 2 on the right

1 on the left, 3 on the right

3 on the left, 3 on the right

3 on the left, 2 on the right

So the equation is balanced

6 .

You might find it easier to have a pen and paper handy for this and the rest of the questions. A student is balancing an equation that involves Na₂SO₄. She needs to have 6 atoms of sodium, how would she write this in her answer?

3Na₂SO₄

3(Na₂SO₄)

Na₆SO₄

None of the above

Writing a large number in front of a formula multiplies everything in the formula by that number. Na₂ tells her that there were 2 sodium atoms there to begin with so multiplying the formula by 3 would give her the 6 she needed. That would also increase the number of sulfur and oxygen atoms

7 .

Bill needs to balance an equation that involves aluminium oxide, formula Al₂O₃. He requires a total of 12 oxygen atoms to balance his equation. What should he write?

12Al₂O₃

Al₂O₁₂

Al₆O₁₂

4Al₂O₃

OK, getting the gist of things now? You should have immediately dismissed answers 2 and 3 as they change the formula

8 .

Balance the equation Na + H₂O → NaOH + H₂.

2Na + 2H₂O → 2NaOH + H₂

Na + 2H₂O → NaOH + H₂

2Na + H₂O → 2NaOH + H₂

2Na + 2H₂O → 2NaOH + 2H₂

There are 2 Na atoms, 4 H atoms and 2 O atoms on each side of the equation. All the other answers have different numbers of the atoms on both sides of the equation

9 .

Balance the equation for the reaction CuO + Al → Cu + Al₂O₃

2CuO + Al → 2Cu + Al₂O₃

3CuO + 2Al → 3Cu + Al₂O₃

3CuO + 3Al → 3Cu + Al₂O₃

CuO₃ + Al₂ → Cu + Al₂O₃

We hope that you didn't choose the fourth one - remember, do not change formulae to balance equations. In the exam, any formulae you are given for balancing equations will be correct. If you alter them, they will then be wrong

10 .

In a blast furnace, there are several chemical reactions. The main one for smelting the iron from the iron ore is Fe₂O₃ + CO → Fe + CO₂.
Which numbers should be put in front of each formula in order to balance the equation?

no number, 2, no number, 2

3, 3, 2, 3

2, 2, 3, no number

no number, 3,2,3

Remember, start off by counting individual atoms on each side then see what you need to get the same on both sides of the equation

You can find more about this topic by visiting BBC Bitesize - Calculations in chemistry - AQA

Author: Kev Woodward (PGCE, Science & Chemistry Teacher, Quiz Writer)