Acids and Bases
Physical Sciences Grade 11
Acids and Bases
Teacher Guide
Teacher Guide
Series Overview
This series of eight lessons explores acids and bases. The first three episodes the learners find out how to
classify different substances into categories of acidic, basic or neutral using different chemical indicators. They
discover that not all acids are the same and how the pH scale can be used to work out how strong or weak
acids or bases are. Once some of the concepts are in place, they investigate acids and bases in different
contexts. For example, they test soil samples and find out about the impact of acid rain. They also discover the
chemistry behind how caves were formed. An investigation into the molecular structure of water leads to an
understanding of how we define acids and bases according to how they react. The learners also explore the role
of acids and bases in the home, for example, in the use of drain cleaner and soap. They also investigate the role
of these substances in food and eating. Note that another series called Acids, Bases and Salts follows on from
this series. This series focuses on the reactions of acids and bases and explores different ways of making salts.
Curriculum Links
This series of lessons links to the core knowledge area of Chemical Change and addresses the theme Types of
Reactions. The content of the series is made real through a range of contexts in the home, food, medicine, soil
and in cave formation
The lessons address the Assessment Standards related to:
LO 1: Scientific Inquiry and Problem-solving Skills
LO 2: Constructing and Applying Scientific Knowledge
LO 3: The Nature of Science and its Relationship to Technology, Society and the Environment
E Educational Approach
The series relates scientific concepts and skills to the lived experience of the learner and they are challenged
to apply their understanding in a broad range of different contexts. This should lead to heightened awareness
of the role and relevance of chemistry in everyday life. The content covered in each episode also motivates the
learners to apply their knowledge in their own environment and prepares them to carry out a series of different
experiments. These tasks challenge them for example, to test the pH of soil in their environment or find out
what kind of toothpaste is most effective in preventing tooth decay. In each experiment they are asked to record
and analyze their results. The emphasis on practical investigations should give many opportunities to assess
learners’ progress towards achieving the assessment standards for learning outcome 1.
These investigations also provide a concrete starting point for learners to construct knowledge about the nature
of both acids and bases. We show the how the concentration of hydronium ions, the concentration of hydroxide
ions and the pH scale are related to each other in a qualitative way without requiring any formal calculations. We
also emphasise the importance of terminology and require learners to explain the difference between the words
“strong” and “concentrated” and “weak” and “dilute”.
Our investigation of toothpaste may give learners an opportunity to research and discuss indigenous methods
for cleaning teeth. These methods can be assessed scientifically to show the validity of indigenous knowledge
systems.
Physical Sciences Grade 11
Teacher Guide
Acids and Bases
Teacher Guide
Series at a Glance
Lesson Title
Lesson Outcomes
Lesson 1
Introducing Acids and
Bases
• Distinguish between acids and bases by using chemical indicators
Lesson 2
Are all acids and bases
the same?
• Define the terms concentrated and dilute
• Classify acids and bases as weak or strong
Lesson 3
The pH scale
• Explain the pH scale
• Use the pH scale and Universal Indicator to analyse samples
Lesson 4
Acids in Africa
• Describe how acids and bases are used in the real world
• Conduct an investigation on soil samples
Lesson 5
Cave Chemistry
• Explain the role acids played in the formation of caves
• Write chemical equations for the reactions taking place
• Critically evaluate human impact on a cave environment
Lesson 6
Defining acids and
bases
• Recall the structure of a water molecule
• Define an acid and a base
• Apply your knowledge of acid – base reactions to the reaction of dry
ice and water
Lesson 7
Acids in the home
• Classify common substances as acids or bases
• Explain how soap works
• Follow a set of instructions to make soap
Lesson 8
Acids in food
• Plan an investigation to test the pH of different substances found at
home
• Present your results to your class
• Recognize that acids and bases play a useful role in our lives
By the end of this lesson, the learner should be able to:
Physical Sciences Grade 11
Acids and Bases
Teacher Guide
Teacher Guide
G Teaching Guidelines
Lesson 1: Introducing Acids and Bases
The first lesson introduces learners to the focus of this series: acids and bases. It also establishes important
differences between these two kinds of substances. Before you start the video challenge them to reflect on what
they know about this topic; after watching the video they will be able to check if these ideas are valid. In this
lesson the learners observe a series of experiments that involve familiar, easily accessible and safe materials
from a kitchen. The learners discover, for example, that when a natural substance such as tea or beetroot juice
is added to either acid or base solutions they change colour. The learners find out that as the chemicals in them
change in these consistent ways they are called indicators. Later they explore other ways of distinguishing
between acids and bases, for example, taste and feel. Consider stopping the video before this experiment and
give them an opportunity to reflect on what they know about the characteristics of these substances and what
can therefore be deduced about acids and bases. In this way you will affirm their prior knowledge and involve
them more actively in the learning process.
Task assesses LO 1 AS 1; LO 1 AS 2 and LO 1 AS 4
The first task requires learners to make their own natural indicator and test it with different household
substances such as vinegar, lemon juice, water, dish washing soap and bicarbonate of soda. Most leaves
and petals of plants contain chemical indicators but some have more noticeable colour changes than others.
Challenge your learners to try out different plants to see which ones change colour the most. Their results
should be recorded in a table.
Lesson 2: Are all acids and bases the same?
The second lesson the learners make comparisons between substances that are all acids or all bases. They
discover that metal oxides in Group I of the Period Table are strong bases and that as we move across the
Periodic Table from Group I to Group IV the metal oxides become weaker bases. They move on to explore
similar trends with acids, non-metal oxides are acidic and if we move across the Periodic Table from Group IV
to VII we find they get stronger. Some oxides of elements in between Group III and IV can act as either acids
or bases; these include some neutral substances. The strength of an acid or base depends upon it’s chemical
content rather than its relative concentration or dilution.
Task assesses LO 1 AS 3; LO 2 AS 1 and LO 2 AS 2
For the second task learners write down an example of a strong concentrated acid, a weak dilute acid, a weak
concentrated base, and a strong dilute alkali.
Lesson 3: The pH scale
The third lesson begins by reviewing the previous lesson and discusses the answers to the related task. The
learners discover how to work out the concentration of an acid or base using the equation: C = n ÷ V. You will
need to provide your learners with examples after watching the video to apply and consolidate this knowledge.
This lesson also leads them to recognise that there are not just three categories, acid, neutral and bases but a
whole range of acidity or basic levels. They are introduced to a universal indicator and measure acidity on the
pH scale. They discover that this scale is called pH because in acids the number of hydrogen ions was very high
and there is therefore a mathematical relationship between the pH value and the hydrogen ion concentration.
Ideally, your learners should have an opportunity to test and measure the pH of a range of acids and bases after
viewing this lesson so as to consolidate this learning.
Task assesses LO 1 AS 2; LO 1 AS 3 and LO 2 AS 2
The task requires learners to look at a series of test tubes and use the indicator reference strip on screen to
determine what the pH of each solution is; they record this information in a table.
Physical Sciences Grade 11
Acids and Bases
Teacher Guide
Teacher Guide
Lesson 4: Acids in Africa
This lesson applies what the learners have discovered about acids and bases in the environment. It focuses on
soil and introduces the importance of growing plants in soil that is suited to its needs. The presenter undertakes
a simple test with soil samples and checks the pH, the learners find out that many plants don’t grow in soil with
a high acid content. They also develop insight into why many people are becoming concerned about the impact
of ‘acid rain’ and how pollution is changing our soil. The learners discover that when soil is too acidic farmers
add lime which is a base and neutralizes the acid in the soil. Farmers also need to rotate crops and prevent
all the minerals and soil nutrients from being leached out of the soil. A knowledge of acids and bases is also
applied in the mining industry. The learners observe how fine crushed rocks need to be placed in an acid bath to
free the metal from the rock. Then how once the metal ions are separated from the rock they are recovered and
then purified through a refining process.
Task assesses LO 1 AS 1; LO 1 AS 2 and LO 1 AS 3
The task requires learners to collect soil samples from two different areas, near their home or school and
test them using a natural indicator. They are asked to tabulate their results and write a conclusion from this
investigation.
Lesson 5: Cave Chemistry
In this lesson the learners find out how the Sterkfontein Caves were formed from a layer of rock called dolomite
that contains the compound calcium carbonate. Carbonic acid, formed when carbon dioxide dissolves in water,
filtered through and began to dissolve the dolomite layer forming underground caves with no large openings
to the surface. The video footage takes them down into the caves and they have an opportunity to experience
the impact of this chemistry in an immediate way. They are also able to observe and appreciate how calcium
carbonate was deposited in thin crystalline formations or icicles from the roof of the cave.
Task assesses LO 2 AS 1; LO 2 AS 3 and LO 3 AS 3
Most caves have a very moist environment. This task requires learners to explain, using chemistry, why is it
necessary to limit the number of people visiting caves.
Lesson 6: Defining acids and bases
In this lesson the learners investigate the bonding that takes place in the water molecule. Animated graphics
help to explain visually that when we bring two water molecules close to each other there are forces of attraction
between the molecules. The learners discover that this attraction is sometimes strong enough to pull the
hydrogen away from the original water molecule, forming hydroxide ion and hydronium ions. You may wish
to replay parts of the video to consolidate these insights and therefore secure the understanding of how the
exchange of hydrogen ions is the basis of modern acid and base theory. The learners should be able to clearly
understand that we define an acid and a base in these terms: an acid is a proton donor and a base is a proton
acceptor. This lesson also provides an opportunity to observe and understand how hydrochloric acid can be
neutralised with sodium hydroxide. They also see how when dry ice, or solid carbon dioxide, is added to a basic
solution the carbon dioxide dissolves in the water and forms carbonic acid.
Task assesses LO 2 AS 1; LO 2 AS 2 and LO 2 AS 3
The task requires learners to look carefully at a series of pictures of the earlier observed bubbling reaction with
dry ice. They consider what they can say about the relationship between pH, concentration of hydrogen ions
and concentration of hydroxide ions in each case. These answers are tabulated.
Lesson 7: Acids in the home
This lesson gives learners insight into how a good knowledge of acids and bases can be applied in and around
the home. Use this lesson to open up broad, active participation in learning. You will be able to use many of
their observations and experiences to enhance and consolidate understanding. Through watching the lesson
the learners discover how unsafe water can be purified by adding a small amount of chlorine bleach. They learn
exactly how bleach removes the stains from clothes (hypochlorous acid reacts with these materials releasing
oxygen and hydrochloric acid). They also find out that the one end of the soap molecule, coming from the fat, is
very similar to oil. In this way soap molecules are able to hook fat and grease and help them dissolve in water.
After this, they should then be able to anticipate that drain cleaners made with sodium hydroxide use a similar
principle. Challenge them to explain why soap feels ‘greasy’ and why scum is left around the rim of a basin or
bath. They may also be able to share important observations and experiences of water in swimming pools and
how it needs to be tested and monitored carefully.
Physical Sciences Grade 11
Teacher Guide
Acids and Bases
Teacher Guide
Task assesses LO 1 AS 1
The task requires learners to follow simple steps and make different types of soap! They can add fragrances
and colour at the same time as adding lemon juice. They could also try using different oils – even melting butter
or fat.
Lesson 8: Acids in food
As in the previous lesson, this last lesson opens up lots of opportunities for your learners to participate in
learning and see chemistry applied in everyday life. When ever possible, stop the video to give your learners a
chance to answer questions and predict outcomes. The lesson begins by looking at how the tongue’s taste buds
can be used to indicate whether a substance is acid or base. Milk is, for example, slightly basic but when it goes
sour it’s pH drops and becomes more acidic. Various acids like lemon juice and vinegar can be used to preserve
food. Vitamin C in ascorbic acid in lemon juice also helps the body’s immune system. The lesson then moves on
to looking at how saliva neutralizes the acids produced when bacteria break down left over food in the mouth.
The learners discover, however, that a build up food in the mouth can prevent the saliva from fulfilling this role
and the resulting acid can cause teeth decay. Finally the learners become involved in an experiment which tests
if bicarbonate soda is more effective than toothpaste in preventing tooth decay.
Task assesses LO 1 AS 1; LO 1 AS 2; LO 1 AS 4 and LO 3 AS 2
The task requires learners to analyze the results found in an experiment that attempts to determine which
substance (toothpaste or bicarbonate of soda) neutralizes the acid in the mouth more effectively. They are also
challenged to do further similar investigations with mouthwash, chewing sugar free gum, different brands of
toothpaste or even ash. Their results should be recorded in a similar way.