How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Balance the equation: Al + O₂ → Al₂O₃.
Before any working — hunt first
Given
Numbers and units already in the sentence: no numbers yet — only a name, a diagram, or a worded unknown. Copy each one into a column. A name without a number is not given.
Missing
After you copy the given, the letter that still has no number is missing. The paper is pointing at: Balance the equation: Al + O₂ → Al₂O₃. Circle that word — that is the missing variable.
Find
We are required to find: Balance the equation: Al + O₂ → Al₂O₃. Write that as a symbol on the left, with a question mark, before you touch a formula.
Calculate
Write Al₂O₃ has 2 Al and 3 O with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives Balance the equation: Al + O₂ → Al₂O₃.
In other words — as if you have never seen this before
If you have never seen this before: Count atoms of each element. Oxygen in the air is O₂, so oxygen changes in twos. Do not change the subscripts. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
Unbalanced: aluminium and oxygen form aluminium oxide.
3. Which method — and why this one?
Count atoms of each element. Oxygen in the air is O₂, so oxygen changes in twos. Do not change the subscripts.
4. Step-by-step working
01 / 05
Count the outcomes
Right side: 2 Al and 3 O. Left side starts as 1 Al and 2 O.
Al₂O₃ has 2 Al and 3 OWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Multiply
Two formula units of Al₂O₃ give 4 Al and 6 O. That needs 3 O₂ and 4 Al.
4 Al+3 O₂→2 Al₂O₃Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Sense-check
Al: 4=4. O: 6=6.
balancedWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 05
Step F — Box the answer with its unit
4 Al + 3 O₂ → 2 Al₂O₃
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
4 Al + 3 O₂ → 2 Al₂O₃
Copy onto your answer sheet — in this order
Formula: Al₂O₃ has 2 Al and 3 O
Answer: 4 Al + 3 O₂ → 2 Al₂O₃
6. Check it like a marker
Writing Al₂ + O₃ → Al₂O₃ changes the substances. Only the big numbers in front may change.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”