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.
Reactants and products
- Reactants are the substances you start with.
- Products are the new substances made.
- The arrow means "react to make" or "produces".
Word equations
A word equation names the reactants and products.
Conservation of mass
Atoms are not created or destroyed in a reaction. They are only rearranged. So:
Formula equations and state symbols
A formula equation uses chemical formulas instead of names.
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 ) 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.
- Physical: no new substance
- Chemical: new substance
- Gas, colour, heat
- Light, precipitate
- Reactants → products
- Word equations
- Formula equations
- State symbols s, l, g, aq
- Atoms rearranged
- Mass in = mass out
- H₂: squeaky pop
- O₂: relights splint
- CO₂: limewater milky
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
Sign in to take the quiz
13 questions with instant marking. It is free, and your scores are saved to your account so you can carry on from any device.
Turn on JavaScript to sign in and take the quiz.
Try 3 questions now
Which is a chemical change?
- A Ice melting
- B Sugar dissolving in tea
- C Toast burning
- D Water boiling
Show answer
C: Toast burning
Burning makes new substances.
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
Show answer
D: Water changes from liquid to gas when heated
Boiling is a physical change. Steam is still water.
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
Show answer
A: Magnesium and oxygen
Reactants are on the left of the arrow.