Chemical Equations: Quick Revision

A brief note on Chemical Equations. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• In a chemical change, new substances form. Signs: a gas, a change of colour, a precipitate, a change of temperature or of state.
• A chemical equation has the reactants on the left, the products on the right, and an arrow that means gives.
• State symbols: $(s)$ solid, $(l)$ liquid, $(g)$ gas, $(aq)$ dissolved in water. Conditions such as heat or light go over the arrow.
• Balance with the numbers in front only. Never change a formula.

Counting atoms

A worked example

Balance $\text{Fe}_2\text{O}_3 + \text{C} \rightarrow \text{Fe} + \text{CO}$.

1. Iron. $2$ on the left, so $2\text{Fe}$.
2. Oxygen. $3$ on the left, so $3\text{CO}$.
3. Carbon. Now $3$ on the right, so $3\text{C}$.
4. Count. $\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}$: Fe $2 = 2$, O $3 = 3$, C $3 = 3$.

Practice with new numbers

A program to try

The program counts the atoms in three formulas, with brackets and with numbers in front.

Mistakes to avoid

• Changing a formula to balance, such as writing $\text{H}_2\text{O}_2$ for water.
• Writing oxygen as O instead of $\text{O}_2$.
• Forgetting the bracket. The number after it multiplies everything inside.
• Taking melting for a reaction. A change of state alone makes nothing new.

Check yourself

1. What do the state symbols $(l)$ and $(aq)$ mean?

2. Balance $\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2$.

3. How many oxygen atoms are there in $4\,\text{HNO}_3$?

4. Give two signs of a chemical reaction that you see when magnesium ribbon burns.