# Chemical Reactions and Equations

> Physical and chemical changes, signs of a reaction, conservation of mass, and writing word and formula equations.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/chemical-reactions/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The sealed bottle
Over 200 years ago in France, Antoine Lavoisier wanted to settle an argument. When wood burns, the ash weighs less than the wood. When iron rusts, it gets heavier. Was mass being made and destroyed?

Lavoisier did his reactions inside **sealed** glass containers and weighed everything before and after. Every time, the total mass stayed exactly the same. The ash only seemed lighter because gases escaped into the air. The rust only seemed heavier because it took oxygen from the air.

His conclusion is one of the most important rules in chemistry: **in a chemical reaction, mass is never created or destroyed**. Atoms just rearrange.

**In short:** A chemical reaction turns reactants into new substances called products, shown by signs like a gas, a colour change, a temperature change or a precipitate. Mass is conserved: the total mass of the products equals the total mass of the reactants.

## Physical and chemical changes

| | Physical change | Chemical change |
|---|---|---|
| New substance? | **No** | **Yes** |
| Easy to reverse? | Usually | Usually not |
| Examples | Melting ice, dissolving sugar, boiling water, cutting paper | Burning, rusting, cooking an egg, baking a cake |

A **chemical reaction** is a chemical change. Bonds between atoms break and new bonds form, making new substances.

## Signs of a chemical reaction

Look for evidence that something new has formed:

- **Gas produced**: bubbles or fizzing.
- **Colour change**.
- **Temperature change**: gets hot or cold.
- **Light** or sound given off.
- **Precipitate**: a solid forms when two solutions are mixed, making the liquid cloudy.
- **Smell** changes.

### Watch out: Bubbles in boiling water are not a reaction
Boiling water makes bubbles of steam, but steam is still water. No new substance forms, so it is a physical change. Always ask: is there a **new substance**?

## Reactants and products

- **Reactants** are the substances you start with.
- **Products** are the new substances made.
- The arrow means "react to make" or "produces".

$$\text{reactants} \rightarrow \text{products}$$

## Word equations

A **word equation** names the reactants and products.

$$\text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide}$$

$$\text{hydrochloric acid} + \text{sodium hydroxide} \rightarrow \text{sodium chloride} + \text{water}$$

## Conservation of mass

Atoms are not created or destroyed in a reaction. They are only **rearranged**. So:

$$\text{total mass of reactants} = \text{total mass of products}$$

### Worked example: Using conservation of mass
12 g of magnesium burns completely in oxygen to make 20 g of magnesium oxide. What mass of oxygen reacted?

$$12 + m_{\text{oxygen}} = 20 \quad\Rightarrow\quad m_{\text{oxygen}} = 8 \text{ g}$$

### Exam tip: Open containers
If a reaction in an **open** container seems to **lose** mass, a gas has escaped. If it seems to **gain** mass, a gas from the air (usually oxygen) has been added. In a **closed** system the mass stays the same.

## Formula equations and state symbols

A **formula equation** uses chemical formulas instead of names.

$$\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}$$

**State symbols** show the state of each substance:

| Symbol | Meaning |
|---|---|
| (s) | solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous: dissolved in water |

The big numbers in front (like the 2 in $\mathrm{2MgO}$) are **coefficients**. They show how many units of each substance react. You will learn how to choose them in the next chapter.

## Common types of reactions

| Type | Pattern | Example |
|---|---|---|
| **Synthesis** (combination) | A + B → AB | Iron + sulfur → iron sulfide |
| **Decomposition** | AB → A + B | Calcium carbonate → calcium oxide + carbon dioxide |
| **Combustion** | Fuel + oxygen → oxides | Methane + oxygen → carbon dioxide + water |
| **Acid + metal** | Acid + metal → salt + hydrogen | Zinc + sulfuric acid → zinc sulfate + hydrogen |
| **Neutralisation** | Acid + base → salt + water | Hydrochloric acid + sodium hydroxide → sodium chloride + water |
| **Precipitation** | Two solutions → insoluble solid | Silver nitrate + sodium chloride → silver chloride (white solid) + sodium nitrate |

The chapter Types of Chemical Reactions explains each type in detail, including how to predict the products.

### Key idea: Tests for common gases
- **Hydrogen**: a lit splint gives a squeaky **pop**.
- **Oxygen**: a glowing splint **relights**.
- **Carbon dioxide**: turns **limewater milky** (cloudy).

**Mind map**

```text
Chemical reactions
  Changes
    Physical: no new substance
    Chemical: new substance
  Signs
    Gas, colour, heat
    Light, precipitate
  Equations
    Reactants → products
    Word equations
    Formula equations
    State symbols s, l, g, aq
  Conservation of mass
    Atoms rearranged
    Mass in = mass out
  Gas tests
    H₂: squeaky pop
    O₂: relights splint
    CO₂: limewater milky
```

### Year 11: Energy in reactions
Reactions also involve energy changes.

- **Exothermic** reactions **release** energy to the surroundings, which get hotter. Examples: combustion, neutralisation, respiration.
- **Endothermic** reactions **absorb** energy, so the surroundings get colder. Examples: photosynthesis, dissolving ammonium nitrate (used in instant cold packs), thermal decomposition.

Breaking bonds **takes in** energy. Making bonds **gives out** energy. If more energy is released making new bonds than was used breaking the old ones, the reaction is exothermic overall.

### Year 12: Ionic and net ionic equations
In solution, ionic compounds split into their ions. In the precipitation of silver chloride:

$$\mathrm{Ag^+(aq) + NO_3^-(aq) + Na^+(aq) + Cl^-(aq) \rightarrow AgCl(s) + Na^+(aq) + NO_3^-(aq)}$$

Ions that appear unchanged on both sides are **spectator ions** ($\mathrm{Na^+}$ and $\mathrm{NO_3^-}$). Cancel them to get the **net ionic equation**:

$$\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}$$

Every neutralisation between a strong acid and a strong base has the same net ionic equation: $\mathrm{H^+(aq)} + \mathrm{OH^-(aq)} \rightarrow \mathrm{H_2O(l)}$.

## Quick summary

- A **chemical change** makes a new substance. A **physical change** does not.
- Signs of a reaction: gas, colour change, temperature change, light, precipitate, smell.
- Reactants → products. Write reactions as word equations or formula equations.
- **Conservation of mass**: total mass of reactants = total mass of products.
- State symbols: (s), (l), (g), (aq).
- Gas tests: hydrogen pops, oxygen relights a glowing splint, carbon dioxide turns limewater milky.

## Quiz

Try these questions from the chapter quiz.

1. Which is a chemical change?
   - A. Ice melting
   - B. Sugar dissolving in tea
   - C. Toast burning
   - D. Water boiling
   - Answer: C. Toast burning. Burning makes new substances.

2. Which observation is NOT evidence of a chemical reaction?
   - A. A precipitate forms
   - B. The mixture gets hot
   - C. A gas is given off when two solids react
   - D. Water changes from liquid to gas when heated
   - Answer: D. Water changes from liquid to gas when heated. Boiling is a physical change. Steam is still water.

3. In the reaction magnesium + oxygen → magnesium oxide, the reactants are:
   - A. Magnesium and oxygen
   - B. Magnesium oxide only
   - C. Oxygen only
   - D. Magnesium oxide and oxygen
   - Answer: A. Magnesium and oxygen. Reactants are on the left of the arrow.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/chemical-reactions/#quiz