Basic 9 · Science · Materials

Acids, Bases, Salts and Chemical Bonds

In this topic you will describe the characteristics of common acids, bases and salts, and recognise that chemical bonds result from the attraction between atoms in a compound.

By the end of these notes you should be able to:

Describe the characteristics of acids and bases
Explain neutralisation and salts
Describe ionic and covalent bonds

What you already know

Binary compounds

Copy everything that has a blue line on its left into your notebook. The other parts are for reading.

Lesson 1

Strand: Diversity Of Matter
Sub-strand: Materials
Indicator: B9.1.1.1.3 – Describe the characteristics of common acids bases and salts

Introduction

Lemon juice is sour. Soap feels slippery. These are clues to a key division in chemistry.

1. Acids and Bases

Acids and bases compared

Feature	Acid	Base (alkali)
taste	sour	bitter
feel	can sting	slippery
pH	below 7	above 7
litmus	blue to red	red to blue
examples	lemon, vinegar, stomach acid	soap, baking soda, ammonia

Never taste or touch chemicals in a laboratory to find out what they are. Strong acids and bases burn skin and damage eyes. An alkali is a base that dissolves in water.

2. Common Acids and Bases

Common examples

Substance	Type	Use
citric acid	acid	fruits
ethanoic acid	acid	vinegar
hydrochloric acid	acid	stomach, industry
sulfuric acid	acid	car batteries
sodium hydroxide	base	soap making
calcium hydroxide	base	treating acidic soil
sodium hydrogencarbonate	base	baking, indigestion

3. The pH Scale

The pH scale runs from 0 to 14. A pH of 7 is neutral, as pure water is. Below 7 is acidic, and above 7 is basic. The further away from 7, the stronger the acid or base. Universal indicator changes colour to show pH. Many natural substances act as indicators too. The red juice of hibiscus (sobolo) changes colour in acids and bases.

Using pH

Lemon juice has pH 2, soap pH 10 and water pH 7. Classify each.

pH 2 is below 7.
pH 10 is above 7.
pH 7 is neutral.

Answer: Lemon juice is acidic, soap is basic and water is neutral

A glass of red hibiscus drink

Figure 1: Hibiscus drink Test household items with hibiscus juice.

4. Soil and Farming

Soil pH affects crops. Most crops grow best in soil that is slightly acidic to neutral. Farmers add lime, or calcium compounds, to reduce acidity in acidic soil. Testing the soil can help a farmer to decide what to add. Acids also affect buildings: acid rain wears away limestone and metals.

Safety: If an acid or alkali splashes on your skin, wash it with plenty of water immediately, and tell your teacher.

New words

acid: A substance with a pH below 7.
base: A substance with a pH above 7.
pH scale: A measure of acidity from 0 to 14.
indicator: A substance that changes colour with pH.

Summary

Acids have a pH below 7, and bases have a pH above 7.
Indicators show whether a substance is acidic or basic.
pH matters in the body and in farming.

Class activity: Hibiscus Indicator

Make juice from hibiscus.
Add lemon juice and soapy water to samples.
Record the colour changes.

You need: Lime juice, Dilute soapy water, Table-salt solution, Red-and-blue litmus paper

Exercises

Answer in your exercise book first, then check your answers.

1. Vinegar is…
- A. an acid
- B. a base
- C. a metal
- D. neutral
Show answer
Answer: an acidType.
2. A pH of 12 is…
- A. strongly basic
- B. strongly acidic
- C. neutral
- D. weakly acidic
Show answer
Answer: strongly basicScale.
3. Bases can feel slippery.
- A. True
- B. False
Show answer
Answer: TrueProperty.
4. An acid turns blue litmus ______.
Show answer
Answer: redTest.
5. State two properties of acids.
Show answer
Answer: They taste sour and have a pH below 7.Any two.
6. Why do farmers add lime to soil?
Show answer
Answer: Lime reduces acidity.Application.

Lesson 2

Strand: Diversity Of Matter
Sub-strand: Materials
Indicator: B9.1.1.1.3 – Describe the characteristics of common acids bases and salts

Introduction

When acid meets base, they cancel each other out. The result is water and a salt.

1. Neutralisation

Neutralisation: The reaction of an acid and a base to form a salt and water.

In chemistry, the word "salt" does not just mean table salt. A salt is any compound formed when the hydrogen of an acid is replaced by a metal. The general equation is: acid + base → salt + water. For example: hydrochloric acid + sodium hydroxide → sodium chloride + water. In symbols: HCl + NaOH → NaCl + H₂O.

2. Common Salts

Common salts and uses

Salt	Formed from	Use
sodium chloride	hydrochloric acid	food
calcium carbonate	carbonic acid	limestone, chalk
copper(II) sulfate	sulfuric acid	fungicide on cocoa
ammonium nitrate	nitric acid	fertiliser
sodium carbonate	carbonic acid	glass, washing soda

Copper-based fungicides have long been used on cocoa farms in Ghana to control black pod disease. Fertilisers are also salts, providing nitrogen, phosphorus and potassium.

3. Neutralisation in Daily Life

Neutralisation in daily life

Situation	How it works
indigestion	antacids neutralise stomach acid
bee sting	an acid sting may be eased by a mild base
acidic soil	lime neutralises acidity
toothpaste	a mild base neutralises mouth acids
factory waste	neutralised before being released

A cocoa farmer spraying trees

Figure 2: Spraying cocoa Write three examples of neutralisation.

4. Making a Salt

Measure dilute acid into a beaker.
Add a base a little at a time until the reaction stops.
Filter off any excess base.
Heat the solution gently to evaporate most of the water.
Leave it to cool, so that crystals form.
Dry the crystals with filter paper.

Copper(II) sulfate crystals can be made in this way, from copper(II) oxide and sulfuric acid. The crystals are a beautiful blue. This method is used in schools, and it uses the separation skills that you learned in Basic 8. Evaporating and crystallising show how a solid product can be taken from a solution.

Safety: Wear goggles when working with acids. All heating must be supervised by a teacher, and you must not taste the salts you make.

New words

neutralisation: Acid and base forming salt and water.
salt: A compound formed from an acid and a base.
antacid: A medicine that neutralises stomach acid.
fungicide: A chemical that kills fungi.

Summary

Acid + base → salt + water.
Salts have many uses in farming and industry.
Neutralisation is used in daily life.

Class activity: Antacid Test

Crush an antacid tablet in water.
Add vinegar with an indicator.
Observe the change.

You need: Lime juice, Dilute soapy water, Table-salt solution, Red-and-blue litmus paper

Exercises

Answer in your exercise book first, then check your answers.

1. Acid + base gives salt and…
- A. water
- B. oxygen
- C. gold
- D. sand
Show answer
Answer: waterEquation.
2. Antacids treat…
- A. indigestion
- B. fever
- C. malaria
- D. toothache
Show answer
Answer: indigestionUse.
3. Fertilisers can be salts.
- A. True
- B. False
Show answer
Answer: TrueUse.
4. A chemical that kills fungi is a ______.
Show answer
Answer: fungicideTerm.
5. Write the word equation for HCl and NaOH.
Show answer
Answer: Hydrochloric acid + sodium hydroxide → sodium chloride + water.Equation.
6. Give two examples of neutralisation in daily life.
Show answer
Answer: Antacids for indigestion and lime on acidic soil.Application.

Lesson 3

Strand: Diversity Of Matter
Sub-strand: Materials
Indicator: B9.1.1.2.1 – Recognize that chemical bond results from the attraction between atoms in a compound

Introduction

Atoms are held together in compounds by forces that we call chemical bonds.

1. What Is a Chemical Bond?

Chemical bond: The force of attraction that holds atoms together in a compound.

A bond forms when atoms lose, gain or share electrons so that they can have a full outer shell. The three main types of bond are ionic, covalent and metallic. The type of bond explains many of the properties of a substance, such as its melting point and whether it conducts electricity.

2. Ionic Bonds

An ionic bond forms between a metal and a non-metal. The metal loses electrons to become a positive ion, and the non-metal gains electrons to become a negative ion. Opposite charges attract. In sodium chloride, sodium (2, 8, 1) loses one electron to become Na⁺, and chlorine (2, 8, 7) gains it to become Cl⁻. Both now have full outer shells.

3. Covalent Bonds

A covalent bond forms between non-metal atoms, which share pairs of electrons. In water, the oxygen atom shares electrons with two hydrogen atoms. In oxygen gas, two oxygen atoms share two pairs of electrons. Covalent substances are often gases or liquids at room temperature.

Ionic and covalent compounds

Property	Ionic	Covalent
formed by	transfer of electrons	sharing of electrons
elements	metal and non-metal	non-metals
melting point	high	usually low
conducts when dissolved	yes	usually no
example	NaCl	H₂O, CO₂

A molecule model of balls and sticks

Figure 3: Molecule model Build a water molecule from clay.

4. Metallic Bonds

In a metal, the outer electrons are free to move through a lattice of positive ions. This sea of electrons holds the metal together. It also explains why metals conduct electricity, and why they can be bent and hammered without breaking.

5. Bonds and Properties

Salt has a very high melting point, because many strong ionic bonds must be broken. Its solution conducts electricity, as the ions can move. Sugar is covalent, and it melts much more easily. Its solution does not conduct electricity. This is why electrical safety matters around salty water, and why sea water causes electric shocks more easily than pure water.

New words

chemical bond: The force holding atoms together.
ion: A charged particle.
ionic bond: Attraction between opposite ions.
covalent bond: A shared pair of electrons.

Summary

Bonds form as atoms lose, gain or share electrons.
Ionic bonds involve transfer, and covalent bonds involve sharing.
Bond type explains properties.

Class activity: Bond Role Play

Learners act as atoms.
Pass or share counters as electrons.
Explain the type of bond.

You need: Coloured bottle tops, Wooden sticks, Printed sodium-chloride lattice diagram

Exercises

Answer in your exercise book first, then check your answers.

1. Sharing electrons forms a…
- A. covalent bond
- B. ionic bond
- C. metallic mixture
- D. solution
Show answer
Answer: covalent bondBond.
2. NaCl has which bond?
- A. ionic
- B. covalent
- C. metallic
- D. none
Show answer
Answer: ionicMetal and non-metal.
3. Ions are charged particles.
- A. True
- B. False
Show answer
Answer: TrueTerm.
4. Metals have a sea of free ______.
Show answer
Answer: electronsMetallic.
5. How does a sodium atom become an ion?
Show answer
Answer: It loses one electron to form Na⁺.Ions.
6. Why does salt solution conduct electricity?
Show answer
Answer: It contains ions that can move.Property.

Homework

Draw how electrons are arranged in sodium chloride and in water.