Chemistry · Unit 5

Chemical Reactions

Reactions and how to describe them

A reaction rearranges atoms into new combinations. Nothing is created or destroyed, which is why equations balance and why balancing them is not busywork.

The unit covers reactants and products, exothermic and endothermic reactions, and an introduction to acids and bases as the most common reaction type met in practice.

This unit breaks down into 16 short steps and 95 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 16
Questions: 95
Difficulty: 1-3

What this unit covers

Endothermic Reaction
Reactant
Product
Exothermic Reaction
Acids and Bases

Where this fits

Needs Chemical Bonding. Moles and Calculations turns this qualitative picture into arithmetic.

Where people slip

Exothermic means energy leaves the system, so the surroundings warm up. The reaction losing energy and the beaker feeling hot are the same event described from two sides.

How the unit is structured

Chemical Reactions runs as 16 short steps that unlock in order. 12 are practice rounds and 4 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 16 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 95 in the unit altogether.

Acids and Bases

Fill the blankLevel 1
1. A solution with a pH lower than 7 is called an ____.
- Acidcorrect
- Base
- Salt
- Alkali
Solutions below pH 7 are acidic.
Multiple choiceLevel 1
2. A neutral solution, like pure water, has a pH of what?
- 7correct
- 0
- 1
- 14
A pH of 7 is neutral, neither acidic nor basic.
Build the sentenceLevel 2
3. Build the general word equation for a reactive metal reacting with an acid.
Answer: acid + metal -> salt + hydrogen
A reactive metal reacts with an acid to make a salt plus hydrogen gas.
Choose all that applyLevel 2
4. Which of these substances are basic (pH above 7)? Select all that apply.
- Soapcorrect
- Baking sodacorrect
- Lemon juice
- Vinegar
Soap and baking soda are bases; lemon juice and vinegar are acids.
Guess the numberLevel 2
5. On the pH scale, what value is exactly neutral?
Answer: 7 pH
A pH of 7 is neutral - the midpoint of the 0 to 14 scale.
Sequence recallLevel 2
6. Recall the neutralisation word equation in order: acid + alkali makes salt + water.
Answer: acid -> alkali -> salt -> water
In neutralisation, acid + alkali -> salt + water.
Sort into groupsLevel 2
7. Sort each substance as an acid or a base.
Answer: Lemon juice = Acid; Vinegar = Acid; Soap = Base; Baking soda = Base
Lemon juice and vinegar are acids; soap and baking soda are bases.

Endothermic Reaction

Multiple choiceLevel 1
8. What does an endothermic reaction do with energy?
- Absorbs it from the surroundingscorrect
- Releases it as heat
- Destroys it completely
- Turns it into new mass
Endothermic reactions take in energy, often making the surroundings feel colder.
True or falseLevel 2
9. Photosynthesis is an endothermic process because it absorbs energy from sunlight.
Answer: True
Plants take in light energy to make glucose, so photosynthesis absorbs energy.
Type the answerLevel 2
10. Thermal decomposition only keeps going while you heat it, because it takes in energy. What is the one-word name for a reaction that takes in energy?
Answer: endothermic
A reaction that takes in energy from its surroundings is described as endothermic.
True or falseLevel 2
11. Heating a metal carbonate so that it breaks down into a metal oxide and carbon dioxide is called thermal decomposition.
Answer: True
Thermal decomposition uses heat to break a compound down into simpler substances.
True or falseLevel 2
12. The thermal decomposition of a metal carbonate is endothermic because it must be heated continuously to keep going.
Answer: True
Thermal decomposition takes in energy to break the compound apart, so it is an endothermic reaction.

Exothermic Reaction

Fact or fibLevel 1
13. An exothermic reaction releases heat to its surroundings.
Answer: True
Exothermic reactions give out energy, usually as heat, so the surroundings warm up.
Picture questionLevel 1
14. 🎆 This firework is burning brightly in the sky. What type of chemical reaction is happening?
- Combustioncorrect
- Freezing
- Dissolving
- Melting
A firework burning is combustion, a reaction with oxygen that releases heat and light.
True or falseLevel 1
15. Burning a fuel such as wood or gas is an exothermic reaction.
Answer: True
Combustion releases heat and light, so it is exothermic.
True or falseLevel 1
16. In a hand warmer, an exothermic reaction is used to warm your hands.
Answer: True
A hand warmer uses an exothermic reaction that releases heat to its surroundings.
Sort into groupsLevel 2
17. Sort each change as exothermic (gives out heat) or endothermic (takes in heat).
Answer: Burning natural gas on a stove = Exothermic; A hand warmer heating up = Exothermic; Heating limestone to break it down = Endothermic; A cold pack getting cold when squeezed = Endothermic
Exothermic changes release heat and warm up; endothermic changes take in heat and cool down.
True or falseLevel 2
18. Neutralising a strong acid with a strong alkali releases heat, so it is an exothermic reaction.
Answer: True
Neutralisation gives out energy as the temperature of the mixture rises, making it exothermic.

Product

Fact or fibLevel 1
19. When iron rusts it slowly reacts with oxygen, so rusting is a type of oxidation.
Answer: True
Rusting is the slow oxidation of iron as it reacts with oxygen and water.
Fill the blankLevel 1
20. In the equation NaCl(s), the state symbol (s) tells you the sodium chloride is a ____.
- solidcorrect
- liquid
- gas
- solution
The state symbol (s) means the substance is a solid.
Guess the numberLevel 1
21. In the balanced equation 2H2 + O2 -> 2H2O, how many water molecules are produced?
Answer: 2 molecules
The big number 2 in front of H2O shows that 2 water molecules are produced.
Choose all that applyLevel 2
22. Which of these can be signs that a chemical reaction has happened? Select all that apply.
- A colour changecorrect
- Bubbles of gas formingcorrect
- A solid (precipitate) appearingcorrect
- An object changing shape when you squeeze it
A colour change, gas being produced, and a new precipitate all suggest a chemical reaction; changing shape is only a physical change.
Match the pairsLevel 2
23. Match each type of reaction to its description.
Answer: Combustion = Burning a fuel in oxygen; Neutralisation = An acid reacts with a base; Thermal decomposition = A compound breaks down when heated; Precipitation = Two solutions form an insoluble solid
Combustion, neutralisation, thermal decomposition and precipitation are common reaction types with distinct descriptions.
Multiple choiceLevel 2
24. In the reaction 2H2 + O2 -> 2H2O, what is the product?
- Watercorrect
- Hydrogen
- Oxygen
- Salt
Hydrogen and oxygen are the reactants; water is the product formed.

Reactant

Build the sentenceLevel 1
25. Build the word equation for magnesium burning in oxygen.
Answer: magnesium + oxygen -> magnesium oxide
Magnesium reacts with oxygen to form the product magnesium oxide.
Fact or fibLevel 2
26. Adding a catalyst makes a reaction go faster but does not change what products are made.
Answer: True
A catalyst only speeds up the reaction; the same products are formed.
Fill the blankLevel 2
27. A reaction where a more reactive metal pushes a less reactive metal out of its compound is called a ____ reaction.
- displacementcorrect
- combustion
- decomposition
- neutralisation
In a displacement reaction a more reactive metal takes the place of a less reactive metal in a compound.
Guess the numberLevel 2
28. To balance the equation N2 + __ H2 -> 2NH3, how many H2 molecules are needed?
Answer: 3 H2 molecules
2NH3 contains 6 hydrogen atoms, so 3 H2 molecules are needed to balance the equation.
Match the pairsLevel 2
29. Match each state symbol used in equations to what it means.
Answer: (s) = solid; (l) = liquid; (g) = gas; (aq) = dissolved in water
State symbols show the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Choose all that applyLevel 3
30. Using the reactivity series, which of these displacement reactions will actually occur? Select all that apply.
- Mg + CuSO4 -> MgSO4 + Cucorrect
- Zn + FeSO4 -> ZnSO4 + Fecorrect
- Cu + FeSO4 -> CuSO4 + Fe
- Ag + CuSO4 -> Ag2SO4 + Cu
Magnesium and zinc are more reactive than the metals in their salts, so those reactions happen; copper and silver are too unreactive to displace iron or copper.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Chemical Reactions

95 questions across 16 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Chemical Reactions

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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