Representing Chemical Change — Science Sage Gr10

Word Equations

A word equation describes a chemical reaction using the names of the reactants and products. Reactants (starting substances) are written on the left, products (new substances formed) on the right, joined by an arrow showing the direction of change.

hydrogen + oxygen → water

Word equations are useful for describing what happens in a reaction, but they do not show quantities — for that we need a balanced symbol (chemical) equation.

Balanced Symbol Equations

A symbol equation uses chemical formulae instead of names, and includes coefficients (numbers placed in front of formulae) to show the exact ratio in which substances react and form.

2H₂ + O₂ → 2H₂O

This equation reads: "2 molecules (or moles) of hydrogen react with 1 molecule (or mole) of oxygen to form 2 molecules (or moles) of water."

Reactants → Products | Coefficient = number of formula units; Subscript = number of atoms within one formula unit

The Law of Conservation of Mass

The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction — atoms are only rearranged. This means the total mass of the reactants must equal the total mass of the products, and the same number of each type of atom must appear on both sides of a correctly balanced equation.

Why this matters: Because atoms are neither created nor destroyed, we must "balance" an equation by adjusting coefficients until the number of atoms of every element is equal on both sides. This is not optional — an unbalanced equation misrepresents reality.

Rules for Balancing Equations

Balancing a chemical equation means adjusting the coefficients (the numbers placed in front of each formula) until the atoms of every element match on both sides. There are strict rules:

You may only change the coefficients (the numbers in front of a formula) — never the subscripts within a formula.
Changing a subscript changes the identity of the substance itself (e.g. changing H₂O to H₂O₂ turns water into hydrogen peroxide — a completely different compound). This is not allowed.
Balance one element at a time, usually leaving oxygen and hydrogen until last, since they often appear in the most formulae.
Once you think the equation is balanced, count every atom on both sides again to check.
Coefficients should be the smallest possible whole numbers (a ratio like 2:4:2 should be simplified to 1:2:1).

Worked example — Balancing: Fe + O₂ → Fe₂O₃
Step 1: Count atoms. Left: 1 Fe, 2 O. Right: 2 Fe, 3 O. Not balanced.
Step 2: Balance Fe first: 2Fe + O₂ → Fe₂O₃. Now Fe is balanced (2 = 2).
Step 3: Balance O: right has 3 O, left has 2 O (in O₂). Use a common multiple — multiply O₂ by 3 and Fe₂O₃ by 2: 4Fe + 3O₂ → 2Fe₂O₃.
Step 4: Recheck: Left: 4 Fe, 6 O. Right: 4 Fe, 6 O. Balanced!
Final balanced equation: 4Fe + 3O₂ → 2Fe₂O₃

State Symbols

State symbols are written in brackets immediately after a formula to show the physical state of each substance in the equation:

Symbol	Meaning	Example
(s)	solid	Fe(s)
(l)	liquid	H₂O(l)
(g)	gas	O₂(g), CO₂(g)
(aq)	aqueous — dissolved in water	NaCl(aq)

Example with state symbols: 2Mg(s) + O₂(g) → 2MgO(s). This tells us solid magnesium reacts with oxygen gas to form solid magnesium oxide.

The Five Basic Reaction Types

Most chemical reactions can be grouped into five broad categories:

Type	Pattern	Example
Synthesis (combination)	A + B → AB	2Mg(s) + O₂(g) → 2MgO(s)
Decomposition	AB → A + B	CaCO₃(s) → CaO(s) + CO₂(g)
Single displacement	A + BC → AC + B	Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Double displacement	AB + CD → AD + CB	AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Combustion	fuel + O₂ → CO₂ + H₂O	CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Synthesis = "join together" | Decomposition = "break apart" | Displacement = "swap partners" | Combustion = "burn in oxygen"

IEB Extension — Ionic Equations & Net Ionic Notation

Full equation vs ionic equation: When reactions happen in aqueous solution, many compounds actually exist as separate ions rather than whole formula units. An ionic equation shows only the species that actually take part in the reaction, leaving out "spectator ions" (ions that appear unchanged on both sides).

Worked example: The reaction AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) can be rewritten showing all soluble ionic compounds as free ions:
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
Na⁺(aq) and NO₃⁻(aq) appear unchanged on both sides — they are spectator ions and can be removed. This gives the net ionic equation:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
This shows the true chemistry of the reaction: silver ions and chloride ions combine to form insoluble silver chloride precipitate.

Checking net ionic equations: A correctly balanced net ionic equation must balance not only atoms but also charge on both sides. In the example above: left side charge = (+1) + (−1) = 0; right side charge (AgCl is neutral) = 0. Charge is balanced, confirming the equation is correct.

Equation Balancer

Not yet balanced

Equation

How to use

Use the + / − steppers under each formula to change its coefficient. Watch the atom-count table below — matching rows turn green. Get every row green to balance the equation. Click "New Equation" above to try another.

Question 1 (1 mark)

Which of the following correctly balances the equation: __N₂ + __H₂ → __NH₃?

N₂ + 3H₂ → 2NH₃ is correct. Check: N atoms — left 2, right 2×1=2. H atoms — left 3×2=6, right 2×3=6. Both sides balance.

Question 2 (1 mark)

Which rule of balancing chemical equations is correct?

You may only adjust coefficients (the numbers placed in front of a formula). Changing a subscript would change the identity of the substance itself, which is not allowed — this would misrepresent the actual chemistry.

Question 3 (1 mark)

What does the state symbol (aq) mean in a chemical equation?

(aq) stands for "aqueous", meaning the substance is dissolved in water. The other state symbols are (s) for solid, (l) for liquid, and (g) for gas.

Question 4 (1 mark)

CaCO₃(s) → CaO(s) + CO₂(g) is an example of which reaction type?

This is decomposition: one compound (CaCO₃) breaks down into two simpler substances (CaO and CO₂). The general pattern is AB → A + B.

Question 5 (2 marks)

The Law of Conservation of Mass states that in a chemical reaction:

The Law of Conservation of Mass means atoms are simply rearranged, never created or destroyed. This is why a correctly balanced equation must show equal numbers of each type of atom on both the reactant and product sides.

Question 6 (1 mark)

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) is best classified as:

This is single displacement: zinc (a more reactive metal) displaces copper from copper sulfate solution, following the pattern A + BC → AC + B.

Question 7 (Analysis)

Magnesium reacts completely with oxygen gas to form magnesium oxide: 2Mg + O₂ → 2MgO. 12.0 g of magnesium reacts completely with 8.0 g of oxygen gas, with no leftover reactant of either kind. What mass of magnesium oxide forms, and what type of reaction is this?

This combines two ideas. First, the Law of Conservation of Mass: since no reactant is left over, the total mass of products must equal the total mass of reactants — 12.0 g Mg + 8.0 g O₂ = 20.0 g MgO. Second, reaction classification: two elements (Mg and O₂) combine to form a single compound (MgO), which is the definition of a synthesis (combination) reaction — not decomposition (one → many), single displacement (element swaps into a compound), or combustion (though burning magnesium does release light, "combustion" specifically refers to reaction with oxygen releasing heat/light and is usually reserved for fuels containing carbon and hydrogen).

IEB Question 1 IEB (2 marks)

Write the net ionic equation for: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), after removing spectator ions.

Na⁺(aq) and NO₃⁻(aq) appear unchanged on both sides of the full ionic equation, so they are spectator ions and are removed. The net ionic equation showing only the species that actually react is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

IEB Question 2 IEB (1 mark)

In addition to balancing the number of atoms, what else must a correctly written net ionic equation balance?

A correct net ionic equation must balance both atoms AND overall charge. For example, in Ag⁺(aq) + Cl⁻(aq) → AgCl(s), the left side charge is (+1) + (−1) = 0, matching the neutral AgCl on the right.

Show all balancing working. Include state symbols where requested.

Question 1

Write balanced symbol equations (with coefficients only — no subscripts may be changed) for the following word equations:
(a) magnesium + oxygen → magnesium oxide
(b) hydrogen + nitrogen → ammonia
(c) sodium + water → sodium hydroxide + hydrogen
(d) potassium chlorate → potassium chloride + oxygen

Question 2

Balance the following equations by inserting the correct coefficients:
(a) __Al + __O₂ → __Al₂O₃
(b) __C₃H₈ + __O₂ → __CO₂ + __H₂O
(c) __Ca(OH)₂ + __HCl → __CaCl₂ + __H₂O
(d) __Fe₂O₃ + __CO → __Fe + __CO₂

Question 3

Explain the Law of Conservation of Mass in your own words. Use the balanced equation 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s) to show that the number of iron atoms and the number of oxygen atoms are equal on both sides. (4)

Question 4

For each equation below, (i) add appropriate state symbols, and (ii) identify the reaction type (synthesis, decomposition, single displacement, double displacement, or combustion):
(a) 2Na + Cl₂ → 2NaCl
(b) AgNO₃ + NaCl → AgCl + NaNO₃ (solutions reacting, silver chloride is an insoluble solid)
(c) CH₄ + 2O₂ → CO₂ + 2H₂O
(d) Zn + 2HCl → ZnCl₂ + H₂ (zinc metal reacting with hydrochloric acid solution)

Question 5

A learner writes the following "balanced" equation: Fe + O₂ → FeO₂. Explain why this is INCORRECT, referring specifically to the rules of balancing equations, and provide the correct balanced equation. (3)

Question 6 — Testing Conservation of Mass Experimentally

A learner reacts marble chips (calcium carbonate) with hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The reaction is carried out twice on an electronic balance — once in an open flask, and once in a flask sealed with a balloon over the mouth to trap any gas produced:

Setup	Initial mass (g)	Final mass (g)
Open flask	65.20	63.85
Sealed flask (with balloon)	65.20	65.20

(a) Calculate the apparent mass change for the open flask.
(b) Explain, in terms of the reaction equation, why the open flask appears to lose mass.
(c) Explain why the sealed flask shows no change in mass, and state which setup correctly demonstrates the Law of Conservation of Mass.
(d) Based on your answer to (a), calculate the mass of CO₂ gas produced in the reaction.