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KS3 Science · Year 8Chapter 9 · Acids, alkalis and pH
Year 8 · Chapter 9

Acids, alkalis and pH

By the end of this chapter you will be able to

  • Use the pH scale to say whether a solution is acidic, neutral or alkaline
  • Use indicators, including universal indicator, to test solutions
  • Write word equations for neutralisation reactions
  • Name a simple salt from the acid and the metal
  • Describe acids and alkalis in everyday life, and their safe use

You should already know

  • Word equations and reactions (Year 7 Chapter 9)
  • Reactions of metals and hydrogen (Year 8 Chapter 8)

Where this leads

  • Rates of reaction and energy changes (Year 8 Chapter 10)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8C4 acids and alkalis: the pH scale, indicators and universal indicator; neutralisation; naming simple salts (basic); acids and alkalis in everyday life.

Exercise P  Do you remember?

1
FWhat is the name of the gas released when a metal reacts with dilute acid? Write the word.
2
FWhat is 4 times 25? Write the number.
3
FWhat is the symbol for sodium? Write the symbol.
4
FWhat is the symbol for lithium, the group 1 metal above sodium? Write the symbol.
5
FWhat is 20 plus 7 minus 9? Write the number.
9.1

Acids, alkalis and the pH scale

Year 8
8C4: the pH scale; acidic (below 7), neutral (7) and alkaline (above 7) solutions
Video lesson: Acids, alkalis and the pH scaleComing soon
Story. Ruby tests some household liquids with a universal indicator, with a grown-up. The lemon juice turns red, and the soap turns purple. “What do the colours mean?” she asks. Her teacher says, “They show the pH. Low pH is acidic, pH 7 is neutral, and high pH is alkaline.”

An acid is a substance with a pH less than 7. An alkali is a soluble base with a pH greater than 7. A neutral solution has a pH of 7, such as pure water. The pH scale runs from 0 to 14, with lower numbers more acidic and higher numbers more alkaline.

The pH scale with universal indicator colours1234567891011121314acidicneutral (7)alkalinelemon juicepure watersoapColours are approximate; the number is the pH.
The pH scale from 1 to 14 with universal indicator colours.

The table gives the approximate pH of some substances.

SubstanceApproximate pH
stomach acid1
lemon juice2
vinegar3
pure water7
sea water8
soap solution10

Key ideas

  • An acid has a pH below 7.
  • A neutral solution has a pH of 7.
  • An alkali has a pH above 7.
  • Lower pH is more acidic; higher pH is more alkaline.
Worked example 9.1.1
What is the pH of pure water? Write the number.

Method. Recall the neutral value.

  1. Recall. Pure water is neutral.
    neutral
  2. Write. A neutral solution has a pH of 7.
    7
  3. Write the answer.
    7

Check. Pure water is neutral. ✓

Worked example 9.1.2
Is vinegar an acid or an alkali? Write the word.

Method. Read the pH in the table.

  1. Read. Vinegar has a pH of 3, which is below 7.
    3
  2. Decide. A pH below 7 is an acid.
    acid
  3. Write the answer.
    acid

Check. Vinegar is ethanoic acid. ✓

Worked example 9.1.3
Which substance in the table is the most acidic? Write the name.

Method. Find the lowest pH.

  1. Look. The lowest pH in the table is 1, for stomach acid.
    1
  2. Name it. So stomach acid is the most acidic.
    stomach acid
  3. Write the answer.
    stomach acid

Check. Lower pH is more acidic. ✓

Watch out

Watch out: a higher pH is more alkaline, not more acidic. Each change of one pH unit is a tenfold change in acidity.

Exam tip

Exam tip. Say “pH below 7 is acidic, 7 is neutral, above 7 is alkaline”.

Try it (with your teacher, or a grown-up at home). Look at the pH scale picture. Say which of the labelled substances is the most alkaline, and which is neutral. Do not taste or touch any household liquid; read the picture only.

Exercise 9A  Acids, alkalis and the pH scale

1
FWhat is the pH of a neutral solution? Write the number.
2
FIs a solution with a pH of 3 acidic, neutral, or alkaline? Write the word.
3
FIs a solution with a pH of 10 acidic, neutral, or alkaline? Write the word.
4
FWhat is the pH range of the pH scale? Write the two numbers.
5
FWhat is the pH of lemon juice, from the table? Write the number.
6
FWhat is the pH of sea water, from the table? Write the number.
7
FWhich substance in the table has a pH of 7? Write the name.
8
FIs a solution of pH 1 more acidic or less acidic than one of pH 3? Write the word.
9
FWhich is the more alkaline: soap solution, at pH 10, or sea water, at pH 8? Write the name.
10
PSAli says that a solution with a pH of 9 is acidic. Is Ali right? Write yes or no.
11
PSWorking scientifically: a pupil tests three liquids with universal indicator, and gets pH values of 2, 7 and 11. Which one is neutral? Write the number.
12
RExplain why pure water is described as neutral. Write one sentence.
13
RChallenge: a solution changes from pH 2 to pH 4. Has it become more acidic or less acidic? Write the word.
9.2

Indicators and testing solutions

Year 8
8C4: indicators change colour with pH; litmus and universal indicator; testing household solutions safely
Video lesson: Indicators and testing solutionsComing soon
Story. Omar wants to know whether a cleaning liquid is acidic or alkaline. “I could taste it,” he jokes. His teacher says, “No. Use an indicator and a pH chart, and never taste any chemical.”

An indicator is a dye that changes colour with pH. Litmus turns red in acid and blue in alkali. Universal indicator gives a range of colours, so you can read the approximate pH by matching the colour to a chart.

Household solutions can be tested with universal indicator, with a grown-up. Never taste a solution, and wash hands after testing.

The table shows what a solution is when it turns universal indicator a certain colour, from the pH scale in section 9.1.

SubstanceApproximate pH
stomach acid1
lemon juice2
vinegar3
pure water7
sea water8
soap solution10

Key ideas

  • An indicator changes colour with pH.
  • Litmus turns red in acid and blue in alkali.
  • Universal indicator shows the pH by its colour.
  • Never taste a chemical; test with an indicator instead.
Worked example 9.2.1
Litmus turns blue in a solution. Is the solution an acid or an alkali? Write the word.

Method. Recall the litmus colours.

  1. Recall. Litmus turns red in acid and blue in alkali.
    blue in alkali
  2. Decide. So the solution is an alkali.
    alkali
  3. Write the answer.
    alkali

Check. Blue means alkaline. ✓

Worked example 9.2.2
Universal indicator shows green. What is the pH? Write the number.

Method. Match the colour to the chart.

  1. Match. Green on the universal indicator chart is pH 7.
    green
  2. Write. So the pH is 7.
    7
  3. Write the answer.
    7

Check. Green is neutral. ✓

Worked example 9.2.3
Which indicator gives a range of colours to show the approximate pH? Write the words.

Method. Recall the indicator.

  1. Recall. Universal indicator gives a range of colours for pH.
    range of colours
  2. Name it. So the indicator is universal indicator.
    universal indicator
  3. Write the answer.
    universal indicator

Check. Litmus only shows acid or alkali. ✓

Watch out

Watch out: a single indicator such as litmus shows only whether a solution is acidic or alkaline. Universal indicator gives an approximate pH value.

Exam tip

Exam tip. Name the colour and the pH: “green on universal indicator means pH 7”.

Try it (with your teacher, or a grown-up at home). With a grown-up, use universal indicator on two household liquids, such as diluted lemon juice and washing-up liquid. Match the colours to the chart and say whether each is acidic or alkaline. Do not taste either liquid.

Exercise 9B  Indicators and testing solutions

1
FWhat is a dye that changes colour with pH called? Write the word.
2
FWhat colour does litmus turn in an acid? Write the colour.
3
FWhat colour does litmus turn in an alkali? Write the colour.
4
FWhich indicator gives an approximate pH by its colour? Write the words.
5
FUniversal indicator shows purple. Is the solution acidic or alkaline? Write the word.
6
FUniversal indicator shows red. Is the solution acidic or alkaline? Write the word.
7
FUniversal indicator shows green. Is the pH 7? Write yes or no.
8
FWhich indicator turns blue in an alkali and red in an acid? Write the word.
9
FShould a pupil taste a household liquid to find its pH? Write yes or no.
10
PSRuby tests a liquid with universal indicator and gets an orange colour. Is the liquid acidic or alkaline? Write the word.
11
PSWorking scientifically: a pupil wants a fair test of three liquids with universal indicator. Name one thing to keep the same. Accept any one: the amount of indicator, or the amount of liquid.
12
RExplain why a pupil should compare the colour with a chart rather than guess. Write one sentence.
9.3

Neutralisation

Year 8
8C4: neutralisation: an acid reacts with an alkali to form a salt and water; the pH moves towards 7
Video lesson: NeutralisationComing soon
Story. Sam adds an alkali to a flask of acid, a little at a time, while his teacher measures the pH. “The pH is climbing towards 7,” he says. “That is neutralisation,” says his teacher. “The acid and the alkali cancel out, and a salt and water are made.”

Neutralisation is the reaction between an acid and an alkali. The products are a salt and water. The reaction is exothermic, and the pH moves towards 7.

The word equation for hydrochloric acid and sodium hydroxide is: hydrochloric acid + sodium hydroxide → sodium chloride + water.

acid+ alkalisalt + waterpH 7 when neutralNeutralisation: acid and alkali cancel each other out.
Neutralisation: an acid and an alkali react to form a salt and water.

The table gives three neutralisation reactions.

Acid and alkaliProducts
hydrochloric acid + sodium hydroxidesodium chloride + water
sulfuric acid + potassium hydroxidepotassium sulfate + water
nitric acid + sodium hydroxidesodium nitrate + water

The table gives the pH when alkali is added to 25 cm3 of acid. The pH rises towards 7 and past it.

Alkali added (cm3)pH
01
52
104
156
207
2510
pH as alkali is added to acid (example data)051015202502468101214Volume of alkali added (cm3)pH
pH rises as alkali is added to an acid, passing 7 at 20 cm3.

Key ideas

  • Neutralisation: acid + alkali gives a salt and water.
  • The pH moves towards 7 as the acid is neutralised.
  • The reaction gives out energy, so it is exothermic.
  • Neutralisation is used to treat acid in soil and stomach acid.
Worked example 9.3.1
In the reaction hydrochloric acid + sodium hydroxide, what is the salt formed? Write the name.

Method. Read the right side of the equation.

  1. Read. The products are sodium chloride and water.
    sodium chloride and water
  2. Name it. So the salt is sodium chloride.
    sodium chloride
  3. Write the answer.
    sodium chloride

Check. Water is the other product. ✓

Worked example 9.3.2
From the table, at what volume of alkali is the pH exactly 7? Write the number of cm3.

Method. Find the row where the pH is 7.

  1. Look. The row with pH 7 is at 20 cm3 of alkali.
    20
  2. Name it. So the volume is 20 cm3.
    20
  3. Write the answer.
    20 cm3

Check. Neutral is reached at 20 cm3. ✓

Worked example 9.3.3
What is the pH change from 0 cm3 to 25 cm3 of alkali, from the table? Write the number.

Method. Subtract the start pH from the end pH.

  1. Read. The pH rises from 1 to 10.
    1 and 10
  2. Calculate. The change is 10 minus 1.
    10 - 1 = 9
  3. Write the answer.
    9

Check. 1 + 9 = 10. ✓

Watch out

Watch out: a neutralisation does not always reach pH 7 exactly. The aim is a neutral solution, which needs the right amount of alkali.

Exam tip

Exam tip. Write the products as a salt and water, and name the salt from the acid and the metal.

Try it (with your teacher, or a grown-up at home). Look at the titration graph. Say at which volume the pH passes 7, and what happens to the pH after that. Do not carry out any titration yourself; your teacher does this demonstration.

Exercise 9C  Neutralisation

1
FWhat are the two products of neutralisation? Write the words.
2
FWhat is the name of the reaction between an acid and an alkali? Write the word.
3
FIn neutralisation, does the pH move towards 7 or away from 7? Write the words.
4
FIs neutralisation an exothermic reaction? Write yes or no.
5
FWhat is the salt in the reaction of nitric acid and sodium hydroxide? Write the name.
6
FWhat is the salt in the reaction of sulfuric acid and potassium hydroxide? Write the name.
7
FFrom the table, what is the pH after 15 cm3 of alkali has been added? Write the number.
8
PSAli says the pH reached 7 after 10 cm3 of alkali. Is Ali right? Write yes or no.
9
PSWorking scientifically: a pupil adds alkali to acid and records the pH each time. What is the independent variable? Write the words.
10
RExplain why the pH of the acid goes up when alkali is added. Write one sentence.
11
RChallenge: explain why the pH may pass 7 if too much alkali is added. Write one sentence.
9.4

Naming simple salts

Year 8
8C4: naming simple salts (basic): the acid gives the ending, the metal gives the first word
Video lesson: Naming simple saltsComing soon
Story. Ali reads a label on a bottle that says “copper sulfate”. “Why does the name have two parts?” he asks. His teacher says, “The first part is the metal. The second part comes from the acid it was made with.”

A salt is formed when the hydrogen of an acid is replaced by a metal (or ammonium). The name has two parts. The metal comes first, and the acid gives the ending:

The table shows how each salt is named.

AcidSalt ends inMetalSalt name
hydrochloric acidchloridesodiumsodium chloride
sulfuric acidsulfatecoppercopper sulfate
nitric acidnitratepotassiumpotassium nitrate

Key ideas

  • A salt has two parts: the metal first, then the acid ending.
  • Hydrochloric acid gives chlorides; sulfuric acid gives sulfates; nitric acid gives nitrates.
  • Each salt is made from an acid and a metal, base or alkali.
  • Salts are the product of neutralisation and of reactions with metals.
Worked example 9.4.1
What is the name of the salt made from sodium and hydrochloric acid? Write the name.

Method. Use the metal and the acid ending.

  1. Metal. The metal is sodium.
    sodium
  2. Acid. Hydrochloric acid gives the ending chloride.
    chloride
  3. Name. So the salt is sodium chloride.
    sodium chloride
  4. Write the answer.
    sodium chloride

Check. Table salt is sodium chloride. ✓

Worked example 9.4.2
What is the name of the salt made from potassium and nitric acid? Write the name.

Method. Use the metal and the acid ending.

  1. Metal. The metal is potassium.
    potassium
  2. Acid. Nitric acid gives the ending nitrate.
    nitrate
  3. Name. So the salt is potassium nitrate.
    potassium nitrate
  4. Write the answer.
    potassium nitrate

Check. Nitrate ends the name. ✓

Worked example 9.4.3
Which acid gives a salt ending in sulfate? Write the name.

Method. Recall the acid.

  1. Recall. Sulfuric acid gives salts ending in sulfate.
    sulfate
  2. Name it. So the acid is sulfuric acid.
    sulfuric acid
  3. Write the answer.
    sulfuric acid

Check. Copper sulfate is one example. ✓

Watch out

Watch out: the name of the salt does not include the word acid. Hydrochloric acid gives sodium chloride, not sodium hydrochloric acid.

Exam tip

Exam tip. Name the metal first, then the ending from the acid: chloride, sulfate or nitrate.

Try it (with your teacher, or a grown-up at home). Look at a labelled bottle of a salt, such as table salt, and name the metal and the acid it came from. Do not open or taste any chemical.

Exercise 9D  Naming simple salts

1
FWhat is the ending of a salt made from hydrochloric acid? Write the word.
2
FWhat is the ending of a salt made from sulfuric acid? Write the word.
3
FWhat is the ending of a salt made from nitric acid? Write the word.
4
FWhich part of a salt name comes first: the metal or the acid? Write the word.
5
FName the salt made from copper and sulfuric acid. Write the name.
6
FName the salt made from potassium and hydrochloric acid. Write the name.
7
FWhich acid would make a salt named sodium nitrate? Write the name.
8
FWhich metal is in the salt zinc chloride? Write the name.
9
FWhat is the salt made from calcium and sulfuric acid? Write the name.
10
PSRuby says the salt sodium chloride contains the acid name hydrochloric. Is Ruby right? Write yes or no.
11
PSWorking scientifically: a pupil names a salt made from sodium and sulfuric acid. Write the name.
12
RExplain why a salt is not named after the whole acid. Write one sentence.
13
RChallenge: name the salt made from magnesium and nitric acid. Write the name.
9.5

Acids and alkalis in everyday life

Year 8
8C4: acids and alkalis in everyday life, and their safe use; neutralising acid in the body and in soil
Video lesson: Acids and alkalis in everyday lifeComing soon
Story. Sam has heartburn after a large meal, and his mum gives him an antacid tablet. “Why does this work?” he asks. His teacher says, “The stomach contains acid. The antacid is a base, and it neutralises some of the excess acid.”

Acids and alkalis are common in daily life. Lemon juice and vinegar are acids, and they add flavour to food. Antacids are alkalis that neutralise excess stomach acid. Gardeners may add lime, an alkali, to acidic soil to raise the pH.

Strong acids and alkalis can burn the skin and eyes, and must be handled with care. Read labels, wear eye protection when your teacher asks, and never mix household chemicals unless you are told how.

The table gives some everyday acids and alkalis.

SubstanceAcid or alkaliUse or note
lemon juiceacidflavour; citric acid
vinegaracidethanoic acid, used in cooking
antacid tabletalkalineutralises excess stomach acid
soilacid or alkalimost plants grow best near neutral pH

Key ideas

  • Antacids are alkalis that neutralise excess stomach acid.
  • Lime, an alkali, can be used to raise the pH of acidic soil.
  • Acids in food, such as lemon juice and vinegar, are weak acids.
  • Strong acids and alkalis are corrosive; handle them with care.
Worked example 9.5.1
What type of substance is an antacid? Write the word: acid or alkali.

Method. Think about what it does.

  1. Think. An antacid neutralises excess acid in the stomach.
    neutralises
  2. Decide. Something that neutralises an acid is an alkali.
    alkali
  3. Write the answer.
    alkali

Check. Antacids are alkalis. ✓

Worked example 9.5.2
What does adding lime to acidic soil do to its pH? Write the word: raises or lowers.

Method. Think about the lime.

  1. Think. Lime is an alkali, so it adds alkaline material to the soil.
    alkaline
  2. Decide. An alkali raises the pH of the soil.
    raises
  3. Write the answer.
    raises

Check. Acidic soil is improved by lime. ✓

Worked example 9.5.3
Name the acid in vinegar. Write the words.

Method. Read the table.

  1. Read. Vinegar contains ethanoic acid.
    ethanoic acid
  2. Name it. So the acid in vinegar is ethanoic acid.
    ethanoic acid
  3. Write the answer.
    ethanoic acid

Check. Vinegar has pH 3. ✓

Watch out

Watch out: an antacid is not an acid. It is an alkali, and it must not be mixed with other chemicals or swallowed by anyone else.

Exam tip

Exam tip. Say what the acid or alkali does, for example “an antacid neutralises excess stomach acid”.

Try it (with your teacher, or a grown-up at home). Look at the everyday substances table. For each, say whether it is an acid or an alkali, and give one use. Do not taste, mix or touch any household chemical; talk about the labels only.

Exercise 9E  Acids and alkalis in everyday life

1
FIs lemon juice an acid or an alkali? Write the word.
2
FWhat is the name of the acid in vinegar? Write the words.
3
FWhat type of substance is used in an antacid tablet? Write the word: acid or alkali.
4
FWhat do antacids neutralise in the stomach? Write the words.
5
FWhich kind of plants grow best near a neutral pH? Write the words: most plants.
6
FDoes lime raise or lower the pH of acidic soil? Write the word.
7
FIs an antacid tablet an acid? Write yes or no.
8
FAre strong acids and alkalis safe to taste? Write yes or no.
9
FName one everyday acid in food. Accept any one: lemon juice, or vinegar.
10
PSWorking scientifically: a pupil reads a label that says “corrosive”. What does this tell the pupil about handling it? Write one sentence.
11
RExplain why an antacid can help with stomach acid. Write one sentence.
12
RChallenge: explain why a gardener might test soil with universal indicator before adding lime. Write one sentence.

Chapter summary

SkillMethod
pH scaleBelow 7 acidic; 7 neutral; above 7 alkaline; scale 0 to 14.
IndicatorsLitmus: red in acid, blue in alkali. Universal indicator: a colour for each pH.
NeutralisationAcid + alkali gives salt + water; pH moves towards 7.
SaltsMetal first, then the acid ending: chloride, sulfate or nitrate.
EverydayAntacids are alkalis; lime raises soil pH; handle strong acids and alkalis with care.

Can you…?

  • I can use the pH scale to say whether a solution is acidic, neutral or alkaline
  • I can use indicators, including universal indicator, to test solutions
  • I can write word equations for neutralisation
  • I can name a simple salt from the acid and the metal
  • I can describe acids and alkalis in everyday life and their safe use

Key terms

acid
a substance with a pH below 7
alkali
a soluble base with a pH above 7
neutral
a solution with a pH of 7
indicator
a dye that changes colour with pH
universal indicator
an indicator that shows a range of colours for the pH
neutralisation
the reaction between an acid and an alkali, giving a salt and water
salt
a compound formed when a metal replaces the hydrogen of an acid

Exercise 9Z  Check-up

1
What is the pH of a neutral solution? Write the number.
2
Is a solution of pH 4 acidic or alkaline? Write the word.
3
Litmus turns red in an acid. What colour does litmus turn in an alkali? Write the colour.
4
What are the two products of neutralisation? Write the words.
5
Name the salt made from sodium and hydrochloric acid. Write the name.
6
What is the ending of a salt made from nitric acid? Write the word.
7
Universal indicator shows green. What is the pH? Write the number.
8
From the titration table, what is the pH after 10 cm3 of alkali? Write the number.
9
Name the product, other than the salt, of a neutralisation reaction. Write the word.
10
Explain why a neutralisation reaction moves the pH towards 7. Write one sentence.

Total 14 marks. Allow about 14 minutes.

Exercise 9X  Practice test questions

1
FWhat is the pH range of the pH scale? Write the two numbers.[1 mark]
2
FName the indicator that turns red in an acid and blue in an alkali. Write the word.[1 mark]
3
FWrite the word equation for hydrochloric acid reacting with sodium hydroxide. Write the products.[2 marks]
4
FName the salt made from potassium and nitric acid. Write the name.[1 mark]
5
PSFrom the titration table, at what volume of alkali is the pH 7? Write the number of cm3.[2 marks]
6
RExplain why a pupil must not taste a household liquid to find out if it is acidic. Write one sentence.[2 marks]
7
FWhat is the pH of vinegar, from the table? Write the number.[1 mark]
8
FGive the name of the salt made from copper and sulfuric acid. Write the name.[1 mark]
9
RExplain why an antacid tablet can help with heartburn. Write one sentence.[2 marks]
10
FA solution has a pH of 12. Is it acidic, neutral or alkaline? Write the word.[1 mark]
← Chapter 8: Reactions of metalsChapter 10: Rates of reaction and energy changes →

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