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. An explanation of the scientific method for testing acids and alkalis.
  2. An introduction to the chemical properties of acids and alkalis.
  3. An introduction toeveryday examples of acids and alkalis.
  4. An introduction totherelationship 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.

(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 alkalinethe 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 inlaboratories: 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 vinegarfor a period of 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 andclassify 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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