Acids and Bases

1

Chapter 8
Acids and
Bases

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Chapter 8: Acids and Bases

• 8.1 What Is an Acid?

• 8.2 What Is a Base?
• 8.3 How Do We Compare Relative Acidity and Alkalinity?
• 8.4 How Is the pH of Soil Controlled?

• 8.5 How Are Oxides Classified?

Open Ended

What do you know about acids?

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8.1 What Is an Acid?

Learning Outcome(s)
• Describe the meaning of the term acid in terms of the ions it produces in

aqueous solutions.

• Describe the characteristic properties as in reactions with metals, bases and

carbonates to form salts.

4

Chemistry Connect

Watch this video to find out why vultures don't get food poisoning.

Textbook
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Multiple Choice

Vultures induce vomiting. Their defensive vomit is foul-smelling enough to drive away predators. If enemies approach too closely, the high amount of acid in the vomit is strong enough to burn them as well.

What property of acids do you think allows it to be used as a defence?

• corrosive

• sour

• foul-smelling

• good conductor of electricity

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8.1 What Is an Acid?

• Acids have many uses in our daily lives.

• Meats are often marinated before cooking. Lemon juice can be added to

tenderise the meat, in addition to improving its flavour.

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8.1 What Is an Acid?

• Table 8.1 lists some common acids.

What do you notice about these acids?

• An acid is a substance that produces hydrogen ions, H+,

in aqueous solutions.

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Acid

Formula

Ions Produced in Aqueous Solution

ethanoic acid

CH3COOH

H⁺

CH3COO^–

hydrochloric acid

H⁺

Cl ^–

nitric acid

H⁺

NO3

–

sulfuric acid

H⁺

SO4

2–

Table 8.1

Multiple Choice

How is a hydrogen ion, H⁺, formed?

• Hydrogen atom gains an electron.

• Hydrogen atom loses an electron.

• Hydrogen atom shares an electron with another hydrogen atom.

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8.1 What Is an Acid?

• Recall! What is a hydrogen ion (H+)?

• A hydrogen ion is formed when a hydrogen atom loses 1 electron.

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8.1 What Is an Acid?

• What is the bonding in hydrogen chloride?

How does hydrogen chloride form H+ ions in aqueous solution?

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8.1 What Is an Acid?

Properties of Acids:

1. Acids have a sour taste.

Example: citric acid in fruits

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8.1 What Is an Acid?

Properties of Acids:

2. Acids produce ions when dissolved in

water. These ions allow the resulting
aqueous solution to conduct
electricity.

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8.1 What Is an Acid?

Properties of Acids:

3. Acids turn blue litmus paper

red.

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8.1 What Is an Acid?

Properties of Acids:

4. Acids react with reactive metals.

General Equation:

Acid + Metal ➝ Salt + Hydrogen Gas

Example:
Magnesium ribbon reacts with dilute sulfuric acid.

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Test for hydrogen gas:
Extinguishes burning splint with
“pop” sound

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8.1 What Is an Acid?

Question:

What is a salt?

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​A salt is a compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ( NH₄⁺ ) ion.

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8.1 What Is an Acid?

Properties of Acids:

5. Acids react with bases in a reaction called neutralisation.

General Equation:

Acid + Base ➝ Salt + Water

Example:
Magnesium oxide reacts with dilute sulfuric acid.

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​Qn: What is a base?

A base is a metal oxide / hydroxide.

17

8.1 What Is an Acid?

Properties of Acids:

6. Acids react with carbonates to form a salt, water and carbon dioxide gas.

General Equation:

Acid + Carbonate ➝ Salt + Water + Carbon dioxide

Example:
Calcium carbonate reacts with dilute hydrochloric acid.

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Test for carbon dioxide:
Forms white precipitate in limewater

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Let’s Investigate 8A

Check your understanding by trying
out the question(s).

Textbook
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Let’s Practise 8.1

Check your understanding by trying
out the question(s).

Textbook
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1. Solid citric acid does not conduct electricity but a solution of citric

acid in water does.
(a) What happens to solid citric acid when it dissolves in water?

Multiple Choice

Why does citric acid conduct electricity only in the aqueous

           state?

• There are mobile ions to conduct electricity in the aqueous state, but not in the solid state.

• There are mobile electrons to conduct electricity in the aqueous state, but not in the solid state.

• There are neutral molecules to conduct electricity in the aqueous state, but not in the solid state.

Multiple Choice

When solution X is added to pieces of calcium carbonate, bubbles of gas Y are given off. When Y is passed into limewater, a white precipitate forms. State the identify of gas Y.

• hydrogen

• carbon dioxide

• oxygen

• ammonia

Check your understanding by trying
out the question(s).

Textbook
Page 126

3. Write the equations for reactions between the following

substances.
(a) magnesium and hydrochloric acid

(b) lithium carbonate and nitric acid

Let’s Practise 8.1

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Acid Rain

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Summary of Lesson 8.1

• An acid is a substance that produces hydrogen ions, H+,

in aqueous solutions.

• Properties of acids

25

Chapter 8: Acids and Bases

• 8.1 What Is an Acid?

• 8.2 What Is a Base?
• 8.3 How Do We Compare Relative Acidity and Alkalinity?
• 8.4 How Is the pH of Soil Controlled?

• 8.5 How Are Oxides Classified?

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8.2 What Is a Base?

Learning Outcome(s)
• Describe the meaning of the term alkali in terms of the ions it produces in

aqueous solutions.

• Describe the characteristic properties of bases in reactions with acids and

ammonium salts.

• Describe the reaction between hydrogen ions and hydroxide ions to produce

water as neutralisation.

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Activity!

• What does soap feel like?
• What chemical ingredients do you think are mainly present in soap?

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• Bases can be found in many cleaning products.

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8.2 What Is a Base?

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• A base is any metal oxide or hydroxide.
• Bases are mostly insoluble in water.

• What ions do these bases have in common?

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8.2 What Is a Base?

Bases

Formula

sodium oxide

Na2O

zinc oxide

ZnO

copper(II) oxide

CuO

magnesium hydroxide

Mg(OH)2

aluminium hydroxide

Al(OH)3

Multiple Choice

What is the solubility rule for hydroxides?

• All hydroxides are insoluble except that of sodium, potassium, calcium and ammonium.

• All hydroxides are insoluble except that of sodium, potassium and ammonium.

• All hydroxides are soluble except that of lead and silver.

• All hydroxides are soluble except that of calcium, lead and barium.

Multiple Choice

What is the reaction between a base and an acid known as?

• acidification

• decomposition

• neutralisation

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Recall!
A base reacts with an acid in a neutralisation reaction.

General Equation:
Acid + Base ➝ Salt + Water

Example:
Copper(II) oxide reacts with dilute sulfuric acid.
CuO(s) + H2SO4(aq) ➝ CuSO4(aq) + H2O(l)

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8.2 What Is a Base?

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• An alkali is a base that is soluble in water.
• The table below lists some common alkalis.

What do you notice about these alkalis?

• An alkali is a substance that produces hydroxide ions, OH–, in aqueous solutions.

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8.2 What Is a Base?

Alkali

Formula

Ions Produced in Aqueous Solution

sodium hydroxide

NaOH

Na+

OH–

potassium hydroxide

KOH

K+

OH–

calcium hydroxide

Ca(OH)2
Ca2+

OH–

ammonia

NH3
NH4

+

OH–

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Properties of Alkalis:

1. Alkalis have a bitter taste.

Example: bitter gourds

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8.2 What Is a Base?

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Properties of Alkalis:

2. Alkalis feel slippery

and soapy.

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8.2 What Is a Base?

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Properties of Alkalis:

3. Alkalis produce ions when

dissolved in water. These ions
allow the resulting aqueous
solution to conduct
electricity.

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8.2 What Is a Base?

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Properties of Alkalis:

4. Alkalis turn red litmus paper

blue.

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8.2 What Is a Base?

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8.2 What Is a Base?

Properties of Alkalis:

5. Alkalis react with acids in a neutralisation reaction.

General Equation:

Alkali + Acid ➝ Salt + Water

Example:
Sodium hydroxide reacts with dilute hydrochloric acid.
Chemical equation: HCl(aq) + NaOH(aq) ➝ NaCl(aq) + H2O(l)

Ionic equation: H+(aq) + OH–(aq) ➝ H2O(l)

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8.2 What Is a Base?

Properties of Alkalis:

6. Alkalis react with ammonium salts.

General Equation:

Alkali + Ammonium salt ➝ Salt + Water + Ammonia

Example:
Potassium hydroxide reacts with ammonium chloride.
KOH(aq) + NH4Cl(s) ➝ KCl(aq) + H2O(l) + NH3(g)

Test for ammonia:
Damp red litmus
paper turns blue

Check your understanding by trying
out the question(s).

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1. (a) What is an alkali?

(b) Name three properties of alkalis.

An alkali is a substance that produces hydroxide ions, OH-,
in aqueous solutions.

Alkalis taste bitter. / Alkalis feel slippery and soapy. / Alkalis dissolve in
water to form solutions that conduct electricity. / Alkalis turn red litmus
paper blue. / Alkalis turn Universal Indicator violet/purple. / Alkalis react
with acids to form a salt and water. / Alkalis give off ammonia gas when
heated with ammonium salts. (Any three)

Let’s Practise 8.2

Check your understanding by trying
out the question(s).

Textbook
Page 129

Check your understanding by trying
out the question(s).

(a) What are the products formed

when an acid reacts with a base?
Salt and water.

(b) (i) Write the chemical equation, including state symbols, for the

reaction between nitric acid and potassium hydroxide.

(ii) Write the ionic equation for the reaction in (b)(i).

2. Dilute nitric acid is added to potassium

hydroxide.

Let’s Practise 8.2

Check your understanding by trying
out the question(s).

(a) Write the balanced chemical equation for the reaction

between magnesium hydroxide with hydrochloric acid.

(b) Write the name of the reaction in (a).

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3. Milk of magnesia is used to treat

indigestion. It contains magnesium
hydroxide which is a white solid that is
only slightly soluble in water. It reacts
with excess hydrochloric acid in the
stomach.

Neutralisation reaction.

Let’s Practise 8.2

Check your understanding by trying
out the question(s).

(c) Suggest why magnesium hydroxide is used to relieve

indigestion and not sodium hydroxide.

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3. Milk of magnesia is used to treat

indigestion. It contains magnesium
hydroxide which is a white solid that is
only slightly soluble in water. It reacts
with excess hydrochloric acid in the
stomach.

Sodium hydroxide is a strong alkali. It is corrosive and would
‘burn’ the stomach lining.

Let’s Practise 8.2

42

Worksheet 8A

Check your understanding by trying
out the question(s).

Theory Workbook

Pages 55–56
Pages 45–46

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Textbook
Page 128
Chemistry Connect

What particles exist in an acidic or alkaline solution?
What happens to the solution when you mix an acid with an alkali?

44

Summary of Lesson 8.2

• Bases are metal oxides or hydroxides.
• Alkalis are soluble bases that produce hydroxide ions, OH–, in aqueous solutions.
• Properties of alkalis

45

Recall!

• What are some properties of acids and bases?

46

Chapter 8: Acids and Bases

• 8.1 What Is an Acid?

• 8.2 What Is a Base?
• 8.3 How Do We Compare Relative Acidity and Alkalinity?
• 8.4 How Is the pH of Soil Controlled?

• 8.5 How Are Oxides Classified?

47

8.3 How Do We Compare Relative Acidity
and Alkalinity?

Learning Outcome(s)
• Describe neutrality and relative acidity and alkalinity, in terms of

(a) relative H+and OH–concentrations;
(b) colour in Universal Indicator; and
(c) the pH scale.

48

Activity!

• What is the pH of stomach acid?

• Antacids are a type of medicine taken

to treat gastric conditions.
How do you think antacids work?

pH = 1

They are bases that neutralise the acid
in the stomach.

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• Indicators change colour when they are added

to an acid or an alkali.

• Universal Indicator produces distinct colours in

solutions with varying pH levels.

• What are some

commonly used
indicators?

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8.3 How Do We Compare Relative Acidity
and Alkalinity?

50

• Table 8.4 shows the colour changes of some indicators.

Table 8.4

• The type of indicator to use will depend on the pH changes in the reaction.
• Indicators do not take part in the chemical reaction.

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8.3 How Do We Compare Relative Acidity
and Alkalinity?

Indicator

Colour in Acidic

Solution

pH Range at Which
Indicator Changes

Colour

Colour in Alkaline

Solution

methyl orange

red

3–5

yellow

screened methyl orange

violet

3–5

green

litmus

red

5–8

blue

thymolphthalein

colourless

9–10.5

blue

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• The colours of Universal Indicator mimic the colours of the rainbow.

Fig 8.13

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8.3 How Do We Compare Relative Acidity
and Alkalinity?

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• The pH scale ranges from 0 to 14.

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8.3 How Do We Compare Relative Acidity
and Alkalinity?

pH < 7

pH = 0

pH > 7

The concentration of H+ions
is higher than that of OH–
ions.

The concentration of H+ions
is the same as that of OH–
ions.

The concentration of H+ions
is lower than that of OH–ions.

Acidic

Neutral

Alkaline

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• A pH meter or pH sensor can also be

used to measure the pH of a solution.

Textbook
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8.3 How Do We Compare Relative Acidity
and Alkalinity?

Check your understanding by trying
out the question(s).

Textbook
Page 131

1. Describe the differences between bleach, lemon juice and water

in terms of their concentration of hydrogen ions.

Let’s Practise 8.3

Lemon juice has the highest concentration of hydrogen ions since
it is acidic, followed by water which is neutral. Bleach has the
lowest concentration of hydrogen ions since it is an alkali.

Check your understanding by trying
out the question(s).

Textbook
Page 131

2. Using the substances in Fig 8.13, name the following:

(a) the most alkaline substance

(b) the substance that would react with sodium carbonate to give

sodium chloride.

Let’s Practise 8.3

Drain cleaner

Gastric juice in stomach

Check your understanding by trying
out the question(s).

Textbook
Page 131

Check your understanding by trying
out the question(s).

(a) Suggest two pairs of solutions that would react together to

give a solution with a pH of 7.

(b) Three of the substances in Table 8.5 are aqueous ammonia,

nitric acid, and carbonic acid. Which letters correspond to
these three substances?

W and X/ U and Z

3. Table 8.5 gives the pH of various

aqueous solutions.

Table 8.5
Let’s Practise 8.3

Solution

U

V

W

X

Y

Z

pH

2

1

9

5

6

11

Aqueous ammonia: Z, nitric acid: U or V, carbonic acid: X or Y

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pH Rainbow

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Summary of Lesson 8.3

• Indicators can be used to tell the pH of a solution.
• The pH scale reflects the concentration of H+ ions in a solution.

59

Recall!

• How is pH related to acids and alkalis?

• What are some strong and weak acids?

• What are some strong and weak alkalis?

60

Chapter 8: Acids and Bases

• 8.1 What Is an Acid?

• 8.2 What Is a Base?
• 8.3 How Do We Compare Relative Acidity and Alkalinity?
• 8.4 How Is the pH of Soil Controlled?

• 8.5 How Are Oxides Classified?

61

8.4 How Is the pH of Soil Controlled?

Learning Outcome(s)
• Describe the importance of controlling the pH of soil.
• Describe how excess acidity can be treated using calcium hydroxide.

62

Activity!

• What are some factors that can influence the growth of a plant?

©️ 2023 Marshall Cavendish Education Pte Ltd

63

Activity!

• The pH of soil can affect how much nutrients a plant absorbs.

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8.4 How Is the pH of Soil Controlled?

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• “Liming” is one way to reduce the acidity of the soil.
• Slaked lime (calcium hydroxide) is added to increase the pH of the soil.

• It is important to control the pH of the soil.

Soil that is too alkaline can be unsuitable for plant growth.

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8.4 How Is the pH of Soil Controlled?

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8.4 How Is the pH of Soil Controlled?

Question:

Can a small change in the pH of blood be harmful to us?

Textbook
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Check your understanding by trying
out the question(s).

Textbook
Page 133

1. Fig 8.17 shows a plan of a plot of land. The numbers represent

the pH of the soil.

Fig 8.17

(a) Which part of the land is the most acidic?

(b) Roses grow best in soil with which is very slightly acidic.

Where should the roses be planted?

A

B

Let’s Practise 8.4

A

(pH 5.5)

B

(pH 6.5)

C

(pH 6)

D

(pH 7)

Check your understanding by trying
out the question(s).

Textbook
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2. A farmer had a poor harvest of barley last year because his soil

was too acidic. Suggest what the farmer should do to improve
the harvest of barley this year.

He can add calcium hydroxide to the soil to reduce the acidity.

Let’s Practise 8.4

69

Experiment 8C

Check your understanding by trying
out the experiment.

Practical
Workbook
Pages 45–48

70

Summary of Lesson 8.4

• There are optimum pH ranges for most plants.
• Most plants thrive in neutral or slightly acidic soil.
• Liming helps to control the acidity of soil.

71

Recall!

• What are bases?
• What is an oxide?

72

Chapter 8: Acids and Bases

• 8.1 What Is an Acid?

• 8.2 What Is a Base?
• 8.3 How Do We Compare Relative Acidity and Alkalinity?
• 8.4 How Is the pH of Soil Controlled?

• 8.5 How Are Oxides Classified?

73

8.5 How Are Oxides Classified?

Learning Outcome(s)
• Classify oxides as acidic, basic, amphoteric or neutral based on metallic/non-

metallic characteristics.

74

Activity!

Name as many oxides as possible!

75

• Oxides can be classified into four categories:

• Basic
• Amphoteric
• Acidic

• Neutral

Textbook
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8.5 How Are Oxides Classified?

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• Most metal oxides are basic oxides.

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8.5 How Are Oxides Classified?

Table 8.6

77

• Basic oxides display basic properties and are solids at

room temperature.

Example:
Magnesium oxide reacts with nitric acid.

MgO(s) + 2HNO3(aq) ➝ Mg(NO3)2(aq) + H2O(l)

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8.5 How Are Oxides Classified?

78

• However, not all metal oxides are basic.

A few of them are amphoteric.

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8.5 How Are Oxides Classified?

Table 8.7

79

• Amphoteric oxides are oxides which have both basic and

acidic properties.

Example:

Zinc oxide reacts with both hydrochloric acid and sodium hydroxide.
ZnO (s) + 2HCl (aq) ➝ ZnCl2 (aq) + H2O (l)

ZnO (s) + 2NaOH (aq) ➝ Na2ZnO2 (aq) + H2O (l)
In both reactions, a salt and water are formed.

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8.5 How Are Oxides Classified?

80

• Most non-metal oxides are known as acidic oxides.

Most acidic oxides dissolve in water to form acids.

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8.5 How Are Oxides Classified?

Table 8.8

81

• Because of the carbon dioxide dissolved in them, carbonated drinks are slightly

acidic. The carbon dioxide is added to make the drinks "fizzier”.

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8.5 How Are Oxides Classified?

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• Acidic oxides do not react with acids. They react with bases to form a salt and

water.

Example:

Sulfur dioxide reacts with sodium hydroxide.
SO2 (g) + 2NaOH (aq) ➝ Na2CO3 (aq) + H2O (l)

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8.5 How Are Oxides Classified?

83

• A few oxides of non-metals display neither basic nor acidic properties.

Hence, they are called neutral oxides.

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8.5 How Are Oxides Classified?

Table 8.9

84

How can we determine the type of oxide based on its properties?

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8.5 How Are Oxides Classified?

Check your understanding by trying
out the question(s).

Textbook
Page 137

1. Which of the elements below form oxides that dissolve in water

to give solutions with pH values of less than 7?

carbon, copper, hydrogen, magnesium, sulfur

Carbon and sulfur

Let’s Practise 8.5

Check your understanding by trying
out the question(s).

A: It is a white solid that is insoluble in water. It forms a colorless

solution with both aqueous hydrochloric acid and aqueous
sodium hydroxide.

B: It is a white solid that is soluble in water. It forms a solution

which turns blue litmus paper red.

C: It is a white solid that dissolves sparingly in water to form a

white suspension. The suspension turns red litmus paper blue.

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3. Using the observations below, classify

oxides A, B and C as either
amphoteric, basic or acidic. Suggest
possible identities for A, B and C.

A: amphoteric, B: acidic, C: basic

Let’s Practise 8.5

87

Worksheet 8B

Check your understanding by trying
out the question(s).

Theory Workbook

Pages 55–56
Pages 45–46

88

Textbook
Page 138
8.5 How Are Oxides Classified?

Let’s Review

Textbook

Pages 139–140

Check your understanding by trying
out the question(s).

Textbook

Pages 139–140

Let’s Review

Check your understanding by trying
out the question(s).

Check your understanding by trying
out the question(s).

Textbook

Pages 139–140

Let’s Review

Let’s Review

Textbook

Pages 139–140

Check your understanding by trying
out the question(s).

93

Let’s Assess
Let’s Reflect

Check your understanding by
trying out the question(s).

Theory Workbook

Page 58
Page 48

94

Stomach Acid

Why is it important to protect some types of tablets from stomach acid?

95

Summary of Lesson 8.5

• Metal oxides are mainly basic oxides, but can also be amphoteric.
• Non-metal oxides are mainly acidic oxides, but can also be neutral.