ETV “Junior Secondary Science” Programme
Acids and Alkalis
Teachers’ Notes
Target Audience
Secondary 1 - 3
Duration
20 minutes
Production Aim
This programme is a teaching resource produced especially for enriching the
classroom teaching of the Syllabus for Science (Secondary 1 – 3) prepared by the
Curriculum Development Council (1998).
The content of the programme serves to illustrate the following parts of the
Syllabus:
“Unit 10: Common Acids and Alkalis
Topic 10.1 Common acids and alkalis,
Topic 10.2 Indicators for testing acids and alkalis,
Topic 10.3 Acids and corrosion,
Topic 10.4 Acid rain, and
Topic 10.7 Potential hazards related to the use of acids and alkalis”
Key Points
1.
2.
3.
4.
An explanation of the scientific method for testing acids and alkalis.
An introduction to the chemical properties of acids and alkalis.
An introduction to everyday examples of acids and alkalis.
An introduction to the relationship between acids and corrosion
Content Outline
This programme is divisible into 4 parts as follows:
(1) Lead-in
Let students think about the questions of ‘what is sour’ and ‘how sour it is’ through
the example of children eating candy bar.
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(2) Tests for acids and alkalis
 Using blue litmus paper and solution to test for acids, the paper and
solution will turn from blue to red. On the contrary, using red litmus paper
and solution to test for alkalis, they will turn from red to blue. As these
testing paper and solution can indicate the acidic or alkaline nature of
matters, they are referred to as ‘indicators’.
 Litmus paper and solution cannot ascertain the degree of acidity or
alkalinity. To achieve that, it is necessary to use the universal indicator.
Scientists have established a set of values ranging from 0 to 14 which is
called the pH value. pH 7 indicates neutral. The lower the pH value the
more acidic the substance is. On the contrary, the higher the pH value the
more alkaline the substance is.
 Digital pH-meter is used to measure the pH values of matters.
 Some plants, such as red cabbage and hydrangea, also have the function of
indicating the acidity or alkalinity of matters.
(3) Everyday examples of acids and alkalis
 Everyday examples of acid: lemon and grapefruit contain citric acid, yogurt
contains lactic acid; tea leaves contain tannic acid; apple contains malic
acid; vinegar contains acetic acid. Acids in these examples are all weak
acids.
 Common acids found in laboratories: sulphuric acid, nitric acid and
hydrochloric acid. These acids are strong acids. They are widely used
industrially such as in electroplating. Some household cleansers contain
hydrochloric acid whereas car battery contains sulphuric acid.
 Some household cleansers contain alkalis. Kitchen and glass cleansers
usually contain aqueous ammonia which is a weak alkali. Clog remover
usually contains an alkali called sodium hydroxide which is a strong alkali
and very corrosive. When it is dissolved in water, it releases a large amount
of heat. The hot alkaline solution quickly dissolves substances like hair or
fat that may block the drain, enabling liquid to flow through the drain
freely.
 Some common alkalis found in laboratories: aqueous ammonia, sodium
hydroxide, potassium hydroxide and calcium hydroxide.
 The proper procedures for handling acids and alkalis are introduced.
(4) Acid and Corrosion
 Acids can corrode matters containing calcium carbonate. Egg shell which
consists of calcium carbonate disappear when soaking in vinegar for a period of
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time. Construction materials such as marble, limestone or gravel contain a large
amount of calcium carbonate and are therefore easily corroded when coming
into contact with acids.
 Dilute acids corrode certain metals and release a colourless gas. Place a burning
splint near the gas and a ‘pop’ sound is heard showing that the gas is hydrogen.
 Rainwater usually has a pH value of around 6.5, which is slightly acidic.
However, the air in some places is highly polluted. Exhaust fumes from factories
and vehicles contain large amounts of acidic gases such as sulphur dioxide,
carbon dioxide and nitrogen oxides. When these gases dissolve in rainwater,
they increase the acidity of the rainwater substantially. As a general rule,
rainwater with a pH value lower than 5.6 is referred to as acid rain.
Suggestions for Utilization
The teacher is advised to show the programme in one lesson. The teacher may spend
5 to 10 minutes at the beginning of each lesson to lead students to discuss with
reference to the Preparation before viewing the programme part of the Suggested
Activities. The teacher may then show the programme. After the show, the teacher
may spend another 10 to 15 minutes to discuss with students with a view to
consolidating the concepts and methods illustrated in the programme. The teacher is
advised to refer to the Activities after viewing the programme part of Suggested
Activities.
Suggested Activities
(The following activities are suggested for teacher’s reference only. The teacher
may wish to use the activities according to students’ abilities, the learning
environment of the class, and the teaching time available.)
Preparation before viewing the programme
The teacher may invite students to test household materials like vinegar, lemon juice,
orange juice, soft drink, milk, sugar water, detergent, tea, coffee, glass cleaner by
using litmus paper and classify them under three categories:
a. those can turn blue litmus paper red;
b. those can turn red litmus paper blue;
c. those have no effects on litmus paper.
Teachers then explain the concept of acids and alkalis to students using the above
results.
Activities after viewing the programme
The teacher may lead students to conduct experiments on acids and alkalis, such as:
dissolving egg shell in vinegar, using red cabbage and hydrangea as natural
indicator. Teachers may also encourage students to conduct project works on acid
rain.
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