Balancing equations and state symbols

5 minAS Level2.3.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can balance a chemical equation using whole-number coefficients and add the correct state symbols: (s), (l), (g) and (aq).

Atoms don't vanish

In a chemical reaction, atoms are rearranged, never created or destroyed. So every atom on the left must turn up on the right. A balanced equation is chemistry's receipt: everything that goes in comes out.

The rule that matters most: change the big numbers in front (coefficients), never the small numbers inside a formula (subscripts). Changing a subscript changes the substance. $\text{H}_2\text{O}$ is water; $\text{H}_2\text{O}_2$ is hydrogen peroxide, which you'd rather not drink.

A method that works every time

Write the correct formula of every substance. Don't touch them again.
Balance the elements that appear in only one substance on each side first (usually C, then H in combustions).
Leave elements on their own (like $\text{O}_2$ , $\text{H}_2$ , Mg) until last: changing them doesn't mess anything else up.
If you're forced into a half, double every coefficient.
Count every element on both sides. Make sure the coefficients are the smallest whole numbers.

Worked example: Burning propane

Balance the combustion of propane: $\text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

Have a go first?

Worked example: Ethane, and the half trick

Balance $\text{C}_2\text{H}_6 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$ with whole numbers.

Have a go first?

Your turn

CalculateQuestion 1 · Apply

Answer the 3 questions above to continue.