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KS3 Science · Year 7Chapter 9 · Chemical reactions and word equations
Year 7 · Chapter 9

Chemical reactions and word equations

By the end of this chapter you will be able to

  • Say how a chemical reaction differs from a physical change
  • Name the reactants and the products of a simple reaction
  • Write a word equation for a simple reaction, with the arrow and the plus sign
  • Give the signs that show a chemical reaction has happened
  • Say whether a reaction is exothermic or endothermic from the temperature change

You should already know

  • Physical changes and reversible changes of state (Chapter 4 of this book)
  • Elements and compounds, and symbols (Chapters 6 and 7 of this book)

Where this leads

  • Conservation of mass in chemical reactions (Chapter 10)

National Curriculum for England, Key Stage 3 science, Year 7. Content: C7 chemical reactions produce new substances; reactants and products, C7 word equations for simple reactions; signs of a chemical reaction (colour, gas, temperature, precipitate); energy changes (basic only).

Exercise P  Do you remember?

1
FIs melting a physical change or a chemical change? Write the word.
2
FWhat is the word for a substance made of two or more elements chemically bonded? Write the word.
3
FWhat is the symbol for hydrogen? Write the symbol.
4
FWhat is 25 minus 8? Write the number.
5
FWhat is the name of the gas in the air that we need for breathing? Write the word.
9.1

Chemical reactions make new substances

Year 7
C7: chemical reactions produce new substances; the starting substances are the reactants and the new substances are the products
Video lesson: Chemical reactions make new substancesComing soon
Story. Ruby sees a bright white light when a strip of magnesium is burnt by her teacher, and a white powder is left behind. “Is the powder just the magnesium, broken up?” she asks. Her teacher says, “No. A new substance has formed, from the magnesium and the oxygen in the air. That is a chemical reaction.”

A chemical reaction is a change in which one or more new substances are made. The starting substances are called the reactants. The new substances formed are the products.

A chemical change is usually difficult or impossible to reverse. A physical change makes no new substance, for example melting or dissolving.

magnesium+ oxygenreacts to formmagnesium oxidereactants (before the arrow) -> products (after the arrow)
Magnesium and oxygen (reactants) form magnesium oxide (product).

Key ideas

  • A chemical reaction makes one or more new substances.
  • Reactants are the starting substances; products are the new substances.
  • A physical change makes no new substance.
  • Chemical changes are usually hard or impossible to reverse.
Worked example 9.1.1
In the reaction of magnesium and oxygen, which substance is the product? Write the words.

Method. Find the new substance.

  1. Find the new substance. The magnesium and oxygen react to make a new white solid.
    new white solid
  2. Name it. The product is magnesium oxide.
    magnesium oxide
  3. Write the answer.
    magnesium oxide

Check. Magnesium and oxygen are the reactants. ✓

Worked example 9.1.2
Is melting ice a chemical change? Write yes or no.

Method. Check whether a new substance forms.

  1. Check. Ice and water are the same substance, water, in a different state.
    same substance
  2. Decide. No new substance forms, so it is a physical change, and the answer is no.
    no
  3. Write the answer.
    no

Check. Melting is physical, and can be reversed by freezing. ✓

Worked example 9.1.3
In the reaction of iron and oxygen, what are the reactants? Write the words.

Method. Find the starting substances.

  1. Find the starting substances. The reactants are the substances that go into the reaction.
    go in
  2. Name them. The reactants are iron and oxygen.
    iron and oxygen
  3. Write the answer.
    iron and oxygen

Check. Iron oxide is the product. ✓

Watch out

Watch out: a change that looks different is not always a chemical change. Melting, dissolving and boiling are physical changes.

Exam tip

Exam tip. To show a chemical reaction, say that a new substance is made, and name it.

Try it (with your teacher, or a grown-up at home). Look at a labelled picture of a burning candle, or a rusting nail. Name the reactants and the product, if you can. Explain why this is a chemical change, not a physical one. Do not burn anything yourself.

Exercise 9A  Chemical reactions make new substances

1
FWhat are the starting substances of a reaction called? Write the word.
2
FWhat are the new substances formed in a reaction called? Write the word.
3
FIs a chemical change usually easy to reverse? Write yes or no.
4
FIs dissolving sugar in water a chemical reaction? Write yes or no.
5
FIn a reaction, what kind of substance is magnesium oxide: a reactant or a product? Write the word.
6
FWhich of these is a physical change: burning, melting, or rusting? Write the word.
7
PSAli says the white powder from burning magnesium is just magnesium in smaller pieces. Is Ali right? Write yes or no.
8
PSRuby says that the magnesium and the oxygen are the products. Is Ruby right? Write yes or no.
9
PSWorking scientifically: a pupil says a chemical reaction has happened because the mixture looks different. Give one better piece of evidence. Accept any one: a new substance, a gas, or a temperature change.
10
RExplain why burning magnesium is a chemical reaction. Write one sentence.
11
RChallenge: explain why the reaction of iron and oxygen cannot be reversed by simply cooling the rust. Write one sentence.
9.2

Word equations

Year 7
C7: write word equations for simple reactions, with the reactants before the arrow and the products after it
Video lesson: Word equationsComing soon
Story. Sam is writing up the reaction of magnesium and oxygen. “How do I show what happened?” he asks. His teacher says, “Use a word equation. Put the reactants on the left, an arrow, and the products on the right.”

A word equation shows the reactants and the products in words. The reactants are written before the arrow, and the products are written after it. A plus sign (+) separates each substance from the next.

For example: magnesium + oxygen → magnesium oxide.

The table gives the word equations used in this chapter. Some are teacher demonstrations only.

ReactantsProductsNote
magnesium + oxygenmagnesium oxideteacher demonstration only
iron + oxygeniron oxidethe slow reaction that makes rust
calcium carbonate + hydrochloric acidcalcium chloride + water + carbon dioxideteacher demonstration only
sodium hydrogencarbonate + ethanoic acidsodium ethanoate + water + carbon dioxidethe fizzing of bicarbonate of soda and vinegar at home
magnesium + hydrochloric acidmagnesium chloride + hydrogenteacher demonstration only

Key ideas

  • Reactants go before the arrow; products go after it.
  • A plus sign separates the substances on each side.
  • Word equations use names, not symbols.
  • Count the products by counting the substances after the arrow.
Worked example 9.2.1
Write the word equation for iron reacting with oxygen to make iron oxide. Write the whole equation.

Method. Put the reactants on the left, and the product on the right.

  1. Left side. The reactants are iron and oxygen, separated by a plus sign.
    iron + oxygen
  2. Right side. The product is iron oxide, after the arrow.
    iron oxide
  3. Write the answer.
    iron + oxygen → iron oxide

Check. Two reactants and one product. ✓

Worked example 9.2.2
How many products are in the word equation for calcium carbonate reacting with hydrochloric acid? Write the number.

Method. Count the substances after the arrow.

  1. Read the products. The products are calcium chloride, water and carbon dioxide.
    three names
  2. Count them. Each product is separated by a plus sign, so there are three.
    3
  3. Write the answer.
    3

Check. There are two plus signs after the arrow, so three products. ✓

Worked example 9.2.3
In the word equation magnesium + oxygen → magnesium oxide, is magnesium a reactant or a product? Write the word.

Method. Recall what is on each side.

  1. Recall. The reactants are on the left of the arrow, and the products on the right.
    left
  2. Decide. Magnesium is on the left, so it is a reactant.
    reactant
  3. Write the answer.
    reactant

Check. Magnesium oxide, on the right, is the product. ✓

Watch out

Watch out: the arrow means “reacts to form”, not “equals”. Reactants go before it and products go after it.

Exam tip

Exam tip. Write a word equation with names, not symbols, and check every substance is on the correct side of the arrow.

Try it (with your teacher, or a grown-up at home). Write a word equation for the reaction of bicarbonate of soda and vinegar, using the name sodium hydrogencarbonate for bicarbonate of soda. Write the reactants before the arrow, and the products after. Check your equation with a partner.

Exercise 9B  Word equations

1
FWhat is written after the arrow in a word equation? Write the word.
2
FWhat does the plus sign mean in a word equation? Write the word: and, or, or equals.
3
FWhat word means the arrow in a word equation? Write the words.
4
FWrite the product of magnesium reacting with oxygen. Write the name.
5
FHow many reactants are there in the word equation for magnesium and oxygen? Write the number.
6
FHow many products are in the word equation for magnesium and oxygen? Write the number.
7
FIn the word equation iron + oxygen → iron oxide, which substance is a reactant? Write the name of one reactant.
8
FIn the word equation iron + oxygen → iron oxide, what is the product? Write the name.
9
PSWrite the reactants of the word equation for calcium carbonate and hydrochloric acid. Write the two names.
10
PSWrite the number of products in the word equation for sodium hydrogencarbonate and ethanoic acid. Write the number.
11
PSAli writes “magnesium oxide + oxygen → magnesium”. Is this word equation correct? Write yes or no.
12
PSWorking scientifically: write a word equation for a reaction where iron reacts with oxygen. Accept any equation with iron and oxygen as reactants and iron oxide as the product.
13
RChallenge: write the word equation for magnesium reacting with hydrochloric acid. Write the equation with the products named.
9.3

Signs of a chemical reaction

Year 7
C7: recognise the signs of a chemical reaction: a colour change, a gas produced, a temperature change, a precipitate or light
Video lesson: Signs of a chemical reactionComing soon
Story. Omar mixes two clear liquids in a beaker. A white solid appears at once and settles at the bottom. “Has a chemical reaction happened?” he asks. His teacher says, “A solid forming in a liquid is one of the signs. Look for all the signs before you decide.”

There are several signs that a chemical reaction has taken place. Each one is evidence that a new substance has probably formed:

SignWhat you would see
a colour changeiron turns brown as rust forms when it reacts with air and water over time
a gas is producedfizzing or bubbles, as carbon dioxide forms when vinegar meets bicarbonate of soda
a temperature changethe mixture gets warmer or colder
a solid forms in a liquida precipitate, a solid made by mixing two solutions
light or a flamethe reaction gives out light, as when magnesium burns (teacher demonstration)

A precipitate is a solid formed when two solutions are mixed. It falls to the bottom of the container. Seeing one sign is good evidence, but a fair test checks more than one observation.

Key ideas

  • A colour change may show a new substance.
  • A gas being produced shows a reaction, such as fizzing.
  • A temperature change shows a reaction, and is an energy change.
  • A precipitate is a solid formed when two solutions are mixed.
Worked example 9.3.1
Omar sees a white solid form in a clear liquid. Which sign of a reaction is this? Write the words.

Method. Match the observation to a sign.

  1. Match. A solid formed in a liquid is a precipitate.
    precipitate
  2. Name the sign. So the sign is a precipitate.
    precipitate
  3. Write the answer.
    a precipitate

Check. A precipitate is a new solid. ✓

Worked example 9.3.2
When vinegar is added to bicarbonate of soda, bubbles and fizzing appear. Which sign is this? Write the words.

Method. Match the observation to a sign.

  1. Match. Bubbles and fizzing show that a gas is being produced.
    gas
  2. Name the sign. So the sign is a gas being produced.
    a gas is produced
  3. Write the answer.
    a gas is produced

Check. The gas is carbon dioxide. ✓

Worked example 9.3.3
A beaker gets warmer during a reaction. Which sign is this? Write the words.

Method. Match the observation to a sign.

  1. Match. The beaker getting warmer is a temperature change.
    warmer
  2. Name the sign. So the sign is a temperature change.
    a temperature change
  3. Write the answer.
    a temperature change

Check. Warmer means energy is given out. ✓

Watch out

Watch out: a change in colour, or a solid forming, is not proof on its own. Look for a new substance, which needs more evidence.

Exam tip

Exam tip. Name the sign you see, and say what it tells you: a gas is produced, a precipitate forms, or the temperature changes.

Try it (with your teacher, or a grown-up at home). With a grown-up, look at a glass of cold tap water and a glass of warm tap water, each with a drop of food colouring. Is there a sign of a chemical reaction? Explain why this is a physical change, not a chemical one.

Exercise 9C  Signs of a chemical reaction

1
FWhat is a solid formed in a liquid during a reaction called? Write the word.
2
FWhat sign shows that a gas is produced in a reaction? Write the words.
3
FWhat sign shows that a reaction gives out energy as heat? Write the words.
4
FIs a colour change always a sign of a chemical reaction? Write yes or no.
5
FWhich sign would show a reaction if a pupil put two solutions together and a solid appeared? Write the word.
6
PSA pupil sees a gas being produced and the temperature rising. How many signs of a reaction are there? Write the number.
7
PSRuby says a reaction has happened because the colour of the mixture changed. Is Ruby right? Write yes or no.
8
PSBen sees a white solid form, and says it is a precipitate. Is Ben right? Write yes or no.
9
PSWorking scientifically: a pupil wants to show a gas is produced in a reaction. Name one observation to look for. Accept any one: bubbles, fizzing, or a rise in the balloon.
10
RExplain why a pupil should look for more than one sign of a reaction. Write one sentence.
11
RChallenge: explain why a precipitate is a sign of a chemical reaction, and not just a physical change. Write one sentence.
9.4

Energy changes in reactions

Year 7
C7: reactions are exothermic (energy given out, temperature rises) or endothermic (energy taken in, temperature falls) (basic only)
Video lesson: Energy changes in reactionsComing soon
Story. Sam opens a hand warmer on a cold walk. It gets hot within minutes. Later, Ruby uses a cold pack after a sports injury, and it gets colder. “Both are chemical reactions,” her teacher says. “One gives energy out, and the other takes energy in.”

An exothermic reaction gives energy out to the surroundings, so the temperature rises. A hand warmer is an example.

An endothermic reaction takes energy in from the surroundings, so the temperature falls. Some cold packs use this.

exothermicreactantsproductsenergy given out: temperature risesendothermicreactantsproductsenergy taken in: temperature falls
Energy level diagrams for exothermic and endothermic reactions.

The table gives the start and end temperatures of two reactions, and the change.

ReactionStart (degrees C)End (degrees C)Change (degrees C)Type
reaction A: a hand warmer (iron reacting with air)2027+7exothermic
reaction B: citric acid and sodium hydrogencarbonate in water2012-8endothermic

Key ideas

  • Exothermic: energy is given out, and the temperature rises.
  • Endothermic: energy is taken in, and the temperature falls.
  • Change in temperature = end temperature minus start temperature.
  • A rise is positive, and a fall is negative.
Worked example 9.4.1
In reaction A, the temperature rises from 20 degrees C to 27 degrees C. Is it exothermic or endothermic? Write the word.

Method. Look at the direction of the change.

  1. Look. The temperature rises, so energy is given out.
    rises
  2. Decide. A reaction that gives energy out is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. The temperature went up by 7 degrees. ✓

Worked example 9.4.2
In reaction B, the temperature falls from 20 degrees C to 12 degrees C. What is the change in temperature? Write the number of degrees.

Method. Subtract the start from the end.

  1. Read the temperatures. The start is 20 degrees and the end is 12 degrees.
    20 and 12
  2. Subtract. Change = end minus start = 12 minus 20.
    12 - 20 = -8
  3. Write the answer.
    -8 degrees C

Check. 20 + (-8) = 12. ✓

Worked example 9.4.3
Is reaction B exothermic or endothermic? Write the word.

Method. Use the sign of the change.

  1. Look. The temperature falls, so energy is taken in.
    falls
  2. Decide. A reaction that takes energy in is endothermic.
    endothermic
  3. Write the answer.
    endothermic

Check. Cold packs use an endothermic reaction. ✓

Watch out

Watch out: exothermic does not mean the reaction is hot at the start. It means the energy given out raises the temperature of the surroundings.

Exam tip

Exam tip. For exothermic, say “energy given out, temperature rises”. For endothermic, say “energy taken in, temperature falls”.

Try it (with your teacher, or a grown-up at home). Look at a labelled picture of a hand warmer and a cold pack. For each one, say whether the reaction is exothermic or endothermic, and say what happens to the temperature. Do not open or squeeze any real packs.

Exercise 9D  Energy changes in reactions

1
FWhat is a reaction that gives energy out called? Write the word.
2
FWhat is a reaction that takes energy in called? Write the word.
3
FIn an exothermic reaction, does the temperature rise or fall? Write the word.
4
FIn an endothermic reaction, does the temperature rise or fall? Write the word.
5
FIs a hand warmer an example of an exothermic reaction? Write yes or no.
6
FWhat is the change in temperature in reaction A, from 20 degrees C to 27 degrees C? Write the number of degrees.
7
PSWhat is the change in temperature in reaction B, from 20 degrees C to 12 degrees C? Write the number of degrees.
8
PSA reaction changes the temperature from 15 degrees C to 22 degrees C. Is it exothermic or endothermic? Write the word.
9
PSA reaction changes the temperature from 25 degrees C to 19 degrees C. Is it endothermic? Write yes or no.
10
PSAli says that an exothermic reaction makes the temperature fall. Is Ali right? Write yes or no.
11
PSWorking scientifically: a pupil says a reaction is exothermic because the flask feels cold. Is this a reasonable conclusion? Write yes or no.
12
RExplain why a cold pack is an example of an endothermic reaction. Write one sentence.
13
RChallenge: explain why the temperature change in an exothermic reaction is positive, and in an endothermic reaction is negative. Write one sentence.
9.5

Acids, carbonates and carbon dioxide

Year 7
C7: a carbonate reacts with an acid to produce a salt, water and carbon dioxide; the carbon dioxide turns limewater milky
Video lesson: Acids, carbonates and carbon dioxideComing soon
Story. Priya adds a spoonful of bicarbonate of soda to a cup of vinegar, and the mixture fizzes strongly. “What is the gas?” she asks. Her teacher says, “It is carbon dioxide. A carbonate reacts with an acid to make a salt, water and carbon dioxide.”

Many carbonates react with acids. The products are a salt, water and carbon dioxide. For example:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide

Carbon dioxide can be identified with limewater. When carbon dioxide is bubbled through limewater, the limewater turns milky. This test is a teacher demonstration.

Key ideas

  • A carbonate and an acid make a salt, water and carbon dioxide.
  • Carbon dioxide turns limewater milky.
  • Fizzing or bubbles in an acid and carbonate mixture show carbon dioxide is produced.
  • Acids are dangerous; pupils do not handle them, except safe kitchen vinegar with a grown-up.
Worked example 9.5.1
In the reaction of a carbonate with an acid, what gas is produced? Write the words.

Method. Recall the products.

  1. Recall. The products are a salt, water and carbon dioxide.
    carbon dioxide
  2. Name the gas. The gas is carbon dioxide.
    carbon dioxide
  3. Write the answer.
    carbon dioxide

Check. Carbon dioxide makes limewater milky. ✓

Worked example 9.5.2
What happens to limewater when carbon dioxide is bubbled through it? Write the words.

Method. Recall the test.

  1. Recall. Carbon dioxide turns limewater milky.
    milky
  2. Decide. So the limewater turns milky.
    turns milky
  3. Write the answer.
    it turns milky

Check. This is the test for carbon dioxide. ✓

Worked example 9.5.3
Name the three products of calcium carbonate reacting with hydrochloric acid. Write the three names.

Method. Read the equation.

  1. Read the right side. The products are calcium chloride, water and carbon dioxide.
    three products
  2. Name them. They are calcium chloride, water and carbon dioxide.
    calcium chloride, water and carbon dioxide
  3. Write the answer.
    calcium chloride, water and carbon dioxide

Check. Three products on the right of the arrow. ✓

Watch out

Watch out: limewater turns milky with carbon dioxide, not with any gas. Only carbon dioxide gives this test.

Exam tip

Exam tip. For the product of a carbonate and an acid, name the salt, water and carbon dioxide.

Try it (with your teacher, or a grown-up at home). With a grown-up, put a spoonful of bicarbonate of soda into a clean cup, then add a little vinegar. Watch the fizzing, and say which gas is produced. Do not taste or smell the mixture closely. Wash your hands afterwards.

Exercise 9E  Acids, carbonates and carbon dioxide

1
FWhat gas is produced when a carbonate reacts with an acid? Write the words.
2
FWhat does carbon dioxide do to limewater? Write the word.
3
FWhat is the name of the salt made from calcium carbonate and hydrochloric acid? Write the name.
4
FName the other product of the reaction, besides the salt and carbon dioxide. Write the word.
5
FIs a salt a product of a carbonate reacting with an acid? Write yes or no.
6
FWhich product turns limewater milky? Write the name.
7
PSIn the vinegar and bicarbonate of soda reaction, which gas makes the fizzing? Write the name.
8
PSAli says that the products of a carbonate and an acid include oxygen. Is Ali right? Write yes or no.
9
PSRuby says limewater turns clear when carbon dioxide is bubbled through it. Is Ruby right? Write yes or no.
10
PSWorking scientifically: a pupil wants to show that a gas from a reaction is carbon dioxide. Name the test, and the result. Write the test and the result.
11
RExplain why bubbling carbon dioxide through limewater is a useful test. Write one sentence.
12
RChallenge: explain why the fizzing in vinegar and bicarbonate of soda is evidence of a chemical reaction. Write one sentence.

Chapter summary

SkillMethod
Chemical reactionNew substances (products) are made from reactants. Hard to reverse.
Word equationReactants before the arrow, products after. Plus signs separate the substances.
SignsColour change, gas produced, temperature change, precipitate, or light.
EnergyExothermic: energy out, temperature rises. Endothermic: energy in, temperature falls.
Carbonate and acidSalt + water + carbon dioxide. Carbon dioxide turns limewater milky.

Can you…?

  • I can say how a chemical reaction differs from a physical change
  • I can name the reactants and products of a simple reaction
  • I can write a word equation with the arrow and the plus signs
  • I can name the signs that show a chemical reaction has happened
  • I can say whether a reaction is exothermic or endothermic from the temperature change

Key terms

chemical reaction
a change in which one or more new substances are made
reactant
a substance that is used up in a chemical reaction
product
a new substance made in a chemical reaction
word equation
a way of showing a reaction in words, with the reactants before the arrow
precipitate
a solid formed when two solutions are mixed
exothermic
a reaction that gives energy out, so the temperature rises
endothermic
a reaction that takes energy in, so the temperature falls
carbonate
a compound containing carbon and oxygen, that reacts with acids to make carbon dioxide
limewater
a solution that turns milky when carbon dioxide is bubbled through it

Exercise 9Z  Check-up

1
What are the starting substances of a reaction called? Write the word.
2
Write the word equation for magnesium and oxygen reacting. Write the product.
3
In a word equation, what does the arrow mean? Write the words.
4
Name the sign of a reaction in which a solid forms in a liquid. Write the word.
5
Is a reaction that makes the temperature rise exothermic or endothermic? Write the word.
6
What is the change in temperature when the temperature falls from 20 degrees C to 12 degrees C? Write the number of degrees.
7
What gas is produced when a carbonate reacts with an acid? Write the words.
8
Which test shows carbon dioxide is present? Write the word.
9
Is melting a chemical reaction? Write yes or no.
10
Explain why a hand warmer gets hot. Write one sentence.

Total 17 marks. Allow about 17 minutes.

Exercise 9X  Practice test questions

1
FWrite the word equation for iron reacting with oxygen to make iron oxide. Write the whole equation.[2 marks]
2
FName the product of magnesium reacting with oxygen. Write the name.[1 mark]
3
PSA reaction makes the temperature rise from 18 degrees C to 25 degrees C. Is it exothermic or endothermic? Write the word.[1 mark]
4
PSWhat is the change in temperature when the temperature falls from 18 degrees C to 11 degrees C? Write the number of degrees.[2 marks]
5
FName the three products of a carbonate reacting with an acid. Write the three names.[2 marks]
6
FWhich product turns limewater milky? Write the name.[1 mark]
7
FA pupil sees a white solid form in a liquid. What sign of a reaction is this? Write the word.[1 mark]
8
FRuby says melting ice is a chemical reaction. Is Ruby right? Write yes or no, and then the word physical or chemical for melting.[2 marks]
9
RExplain why burning magnesium is a chemical reaction. Write one sentence.[2 marks]
10
FWrite one sign that shows a gas is produced in a reaction. Write one sign.[1 mark]
11
RExplain why a cold pack gets colder. Write one sentence.[2 marks]
← Chapter 8: Mixtures and separatingChapter 10: Conservation of mass →

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