This grade 8 science lesson on balancing equations notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on balancing equations and check yourself with the quiz.
Quick answer: A chemical equation shows reactants turning into products. Atoms are only rearranged, never created or destroyed, so a balanced equation has the same number of each kind of atom on both sides. To balance it, change coefficients in front of formulas, never the small subscripts.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2, MS-PS1-5
Balancing equations notes: the key ideas
What happens to atoms in a chemical reaction?
When wood burns, iron rusts or a tablet fizzes in water, new substances form. A chemical reaction turns the starting substances, called reactants, into different substances, called products. The bonds in the reactants break, the atoms move around, and new bonds form in the products.
A chemical equation is a short way of showing a reaction, with the reactants on the left, the products on the right and an arrow meaning “produces” between them.
Atoms are never created or destroyed in a chemical reaction; they are only rearranged. So every element has the same number of atoms on both sides of a correct equation, and the total mass does not change.
How do you write a chemical equation?
Start with a word equation, such as hydrogen + oxygen → water. Then replace each name by its formula: H₂ + O₂ → H₂O. Hydrogen and oxygen are written as molecules with two atoms (H₂ and O₂). Often extra letters in brackets, called state symbols, show the physical state.
| State symbol | Meaning | Example |
|---|---|---|
| (s) | solid | Mg(s) |
| (l) | liquid | H₂O(l) |
| (g) | gas | H₂(g) |
| (aq) | dissolved in water | HCl(aq) |
How do you balance an equation?
H₂ + O₂ → H₂O is not yet correct, because there are two oxygen atoms on the left but only one on the right. We fix it using coefficients, the big numbers in front of formulas. The balanced version is 2 H₂ + O₂ → 2 H₂O. The picture shows how the atoms are simply regrouped.
Balance: Mg + O₂ → MgO (burning magnesium ribbon).
Count atoms. Left: Mg 1, O 2. Right: Mg 1, O 1.
Put 2 before MgO to make 2 oxygen atoms: Mg 1 on the left but 2 on the right. Put 2 before Mg to match.
Recount: left Mg 2, O 2; right Mg 2, O 2. 2 Mg + O₂ → 2 MgO.
Do not change a subscript to make the numbers match. Writing H₂O₂ instead of H₂O would make a different substance (hydrogen peroxide). Only the coefficient in front of a formula may be changed.
| Atom | Left side: N₂ + 3 H₂ | Right side: 2 NH₃ | Equal? |
|---|---|---|---|
| N | 2 | 2 | yes |
| H | 3 × 2 = 6 | 2 × 3 = 6 | yes |
Balance: N₂ + H₂ → NH₃ (making ammonia for fertiliser).
Nitrogen: 2 on the left, 1 on the right, so put 2 before NH₃. Now the right has 2 N and 6 H.
Hydrogen: 6 needed on the left, and each H₂ has 2, so put 3 before H₂.
N₂ + 3 H₂ → 2 NH₃.
How is mass conserved in a reaction?
Because every atom survives, the total mass of the reactants equals the total mass of the products. If a gas escapes from an open container, the balance reading may drop, but the missing mass is still there in the gas.
In a lime kiln in Rajasthan, limestone breaks down: CaCO₃ → CaO + CO₂. If 100 g of CaCO₃ gives 56 g of CaO, what mass of CO₂ forms?
Mass of products = mass of reactants, so CO₂ = 100 − 56 = 44 g.
Balance the elements that appear in only one substance on each side first, and leave elements such as oxygen or hydrogen, which often appear in several substances, until last.
- Reactants turn into products; bonds break and new bonds form.
- Atoms are conserved, so each element has equal numbers of atoms on both sides.
- Balance by changing coefficients only, never subscripts.
- Example: N₂ + 3 H₂ → 2 NH₃ has 2 N and 6 H on each side.
- Total mass of reactants = total mass of products in a closed system.
Balancing equations and conserving atoms: frequently asked questions
What is a balanced chemical equation?
It is an equation with the same number of atoms of every element on both sides. This reflects the fact that atoms are not created or destroyed in a reaction, only rearranged.
Why can you not change the subscripts when balancing?
The subscripts are part of the identity of the substance. Changing H₂O to H₂O₂ makes a different compound, so you would no longer be describing the same reaction. Only coefficients can change.
What are reactants and products?
Reactants are the substances that react and are written on the left of the arrow. Products are the new substances formed and are written on the right.
How do state symbols help?
They show whether each substance is a solid (s), liquid (l), gas (g) or dissolved in water (aq). They tell you, for example, whether a gas is released.
Does mass change in a chemical reaction?
The total mass of all reactants equals the total mass of all products. A balance may show a loss if a gas escapes from an open container, but the mass is still accounted for in that gas.
