Making Use of Electricity9

9-5Activity 9.1

Acids and alkalis found in different products What acids or alkalis are found in the following products? Search for information on the Internet to

find out the answers. Write down the names of the acids or alkalis in the spaces provided.

Yoghurt Tomato sauce Kitchen cleaner Tea

Lactic acid Ethanoic acidSodium hydroxide or potassium hydroxide Tannic acid

9-6TEST YOURSELF 9.1

1. C 2. A

3. Give two examples of acids and alkalis commonly used in the laboratory.

(a) Acids commonly used in the laboratory:

(i) hydrochloric acid / sulphuric acid / nitric acid

(ii)

(b) Alkalis commonly used in the laboratory:

(i) potassium hydroxide solution / sodium hydroxide solution / ammonia solution

(ii)

9-9Makers ’ Lab 9.1

Making natural acid-alkali indicators Makers’ creation

1. Search for information from the Internet and choose two deeply coloured plant parts to make

natural acid-alkali indicators. Bring the plant parts you are going to use to the laboratory.

Plant part 1: Answers varies with students.

Plant part 2:

Unit 9 Making Use of Electricity2

Makers’ prediction

You have made your natural acid-alkali indicators. Before testing them, predict the colours of the

indicators in acidic solutions, alkaline solutions and distilled water, and complete the table below.

Plant part (name)

Colours of the natural acid-alkali indicator in

acidic solutions alkaline solutions distilled water

1 (

)

2 (

)

Makers’ findings

Observe whether there are any colour changes of the indicators in the three test tubes. Record the

results in the table in ‘Makers’ findings’

Plant part (name)

Colours of the natural acid-alkali indicator in

dilute hydrochloric acid

dilute sodium hydroxide solution

distilled water

1 (

)

2 (

)

Makers’ discussion

Why can the extracts that are made in this activity be used as natural acid-alkali indicators?

It is because they show different colours in acidic and alkaline solutions.

Answers varies with students. Two examples are shown below:

Answers varies with students.

Plant part

Colours of the natural acid-alkali indicator in

dilutehydrochloric acid

dilute sodium hydroxide solution distilled water

Red cabbage leaves pink green purple

Red rose petals red brown pink

Unit 9 Making Use of Electricity3

9-13EXPERIMENT 9.1

Observing the colour changes of litmus paper in acidic and alkaline solutions5. Repeat Steps 3 to 4 with dilute ethanoic acid (wells B), dilute sodium hydroxide solution (wells

C), dilute ammonia solution (wells D) and distilled water (wells E).

Colour changes (if any) of the litmus paper in

dilute hydrochloric

acid (wells A)

dilute ethanoic acid

(wells B)

dilute sodium hydroxide solution (wells C)

dilute ammonia solution (wells D)

distilled water (wells E)

Red litmus paper

no change no change changes from red to blue

changes from red to blue no change

Blue litmus paper

changes from blue to red

changes from blue to red no change no change no change

Discussion

1. What are the colour changes of red litmus paper in acidic solutions and alkaline solutions

respectively?

Red litmus paper does not change colour in acidic solution, but changes to blue in alkaline solution.

2. What are the colour changes of blue litmus paper in acidic solutions and alkaline solutions

respectively?

Blue litmus paper changes to red in acidic solutions, but does not change colour in alkaline solution.

9-14Thinking

How can we use litmus paper to test whether a solution is neutral?

Test the solution with both blue and red litmus paper. If the colours of both pieces of paper remain unchanged, the solution

is neutral.

Unit 9 Making Use of Electricity4

9-17EXPERIMENT 9.2

Finding the pH values of common household products

Part A Using universal indication

5. Repeat Steps 2 to 4 with other solutions of household products listed in the table.

Solution of household product

pH valueSolution of household

productpH value

(a) lemon juice ~2 (d) soy sauce Hard to observe

(b) table salt solution ~7 (e) soap solution ~10

(c) baking soda solution ~9 (f) toilet cleaner ~1

Part B Using an electronic instrument (Demonstration)

Your teacher will measure the pH values of the solutions of household products using a pH meter.

Record the results in Table (b).

Solution of household product

pH valueSolution of household

productpH value

(a) lemon juice 2.0 – 3.0 (d) soy sauce 4.4 – 5.4

(b) table salt solution 7.0 (e) soap solution 9.0 – 10.0

(c) baking soda solution 8.4 (f) toilet cleaner 1.0 – 2.0

Discussion

1. According to the pH values of the solutions listed in Tables (a) and (b),

(a) which solutions are acidic? Arrange them from less acidic to more acidic.

soy sauce, lemon juice, toilet cleaner

(b) which solutions are alkaline? Arrange them from less alkaline to more alkaline.

baking soda solution, soap solution

(c) which solution(s) is/are neutral?

table salt solution

2. Give one advantage of using electronic instruments over universal indicator.

Electronic instruments can measure the pH values of the solutions more accurately. / Electronic instruments can

determine the pH values of all solutions, including deep-coloured solutions, but universal indicator cannot.

Unit 9 Making Use of Electricity5

9-20TEST YOURSELF 9.2

1. B 2. C 3. A

9-22EXPERIMENT 9.3

Finding what new substances are formed during neutralisation5. Observe the substance left on the watch glass and state the appearance of it.

white solid

9-23EXPERIMENT 9.4

Finding if the mass of reactants and products is conserved in a neutralisation reaction3. Measure the mass of the conical flask, measuring cylinder B and the reactants using an

electronic balance (Fig.(a)).

The mass of the conical flask, measuring cylinder B and the reactants (M1)

= g (Answers varies with students.)

4. Pour the dilute barium hydroxide solution from measuring cylinder B into the conical flask to

mix the reactants (Fig.(b)). Be careful not to spill any solution. What do you observe?

White solid is formed.

5. Measure the mass of the conical flask, measuring cylinder B and the products using an

electronic balance.

The mass of the conical flask, measuring cylinder B and the products (M2)

= g (Answers varies with students.)

Discussion

1. Does M1 equal to M2?

Yes.

2. Are the total mass of reactants and products equal in the neutralisation reaction?

Yes.

Extension

Unit 9 Making Use of Electricity6

9-26Activity 9.2

Neutralisation board game2. After playing the game, write the word equations for the following neutralisation reactions.

(a) Neutralisation reaction between hydrochloric acid and barium hydroxide:

hydrochloric acid + barium hydroxide barium chloride + water

(b) Neutralisation reaction between nitric acid and sodium hydroxide:

nitric acid + sodium hydroxide sodium nitrate + water

(c) Neutralisation reaction between sulphuric acid and potassium hydroxide:

sulphuric acid + potassium hydroxide potassium sulphate + water

9-26EXPERIMENT 9.5

Finding how the pH value of solution changes when an acid is added to an alkali6. Repeat Steps 4 to 5 until 15 cm3 of hydrochloric acid has been added.

Volume of dilute hydrochloric acid added (cm3)

0 1 2 3 4 5 6 7

pH value of the solution 13 13 13 13 13 13 13 13

Volume of dilute hydrochloric acid added (cm3)

8 9 10 11 12 13 14 15

pH value of the solution 13 12 7 3 2 2 2 2

Unit 9 Making Use of Electricity7

7. Use the data recorded in the table to plot a graph on the graph paper below.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

2 4 6 8 10 12 14 15131197531Volume of dilute hydrochloric acid added (cm

3)

Change of pH value when dilute hydrochloric acid is added to dilute sodium hydroxide solution

pH

val

ue o

f the

sol

utio

n

Discussion

1. How does the pH value of the solution change when dilute hydrochloric acid is added to dilute

sodium hydroxide solution?

The pH value decreases.

2. From the graph, find the volume of dilute hydrochloric acid required to completely neutralise

10 cm3 of dilute sodium hydroxide solution.

10 cm3 (Depends on the molarities of NaOH and HCl used in the experiment.)

3. If we carry out the experiment by adding dilute sodium hydroxide solution to dilute

hydrochloric acid, how will the pH value of the solution change?

The pH value will increase.

Unit 9 Making Use of Electricity8

9-29EXPERIMENT 9.6

Studying the change in pH value during a neutralisation reaction using a data-logger (Demonstration)

6. Use the computer to plot a graph of the data collected by the data-logger, with the pH value

of the solution (vertical axis) against the volume of acid added (horizontal axis). Print out the

graph and paste it in the space below.

Discussion

1. Describe how the pH value of the solution change when dilute hydrochloric acid is added to

dilute sodium hydroxide solution.

The pH value decreases.

2. Give one advantage and one disadvantage of using a data-logger to measure the pH value of

the solution during the neutralisation reaction over the use of universal indicator.

(a) Advantage of using a data-logger:

The pH values measured are more accurate. / It is more convenient as continuous data can be collected.

(b) Disadvantage of using a data-logger:

It is more difficult to use. / It is more expensive.

Answers varies with students.

Unit 9 Making Use of Electricity9

9-33TEST YOURSELF 9.3

1. Complete the following word equation of neutralisation:

acid + alkali salt + water

2. (a) F (b) T (c) F (d) F

3. B

9-34EXPERIMENT 9.7

Studying the action of dilute acid on metals6. Repeat Steps 3 to 5 with zinc, iron and copper strips, using the acid in test tubes B to D

respectively.

MetalAre any gas

bubbles given off?What happens to the

size of the metal?Does the test tube

become warm?Is hydrogen produced?

magnesium yes decreases yes yes

zinc yes decreases yes yes

iron yes decreases yes yes

copper no remains unchanged no no

Discussion

1. Which metals react with dilute hydrochloric acid?

magnesium, zinc and iron

2. What gas is formed when metals react with dilute hydrochloric acid?

hydrogen

Unit 9 Making Use of Electricity10

Extension

9-36Activity 9.3

Writing word equations to describe the reactions between dilute acids and metalsWrite the word equations for the following reactions between dilute acids and metals.

1. Reaction between dilute hydrochloric acid and aluminium:

aluminium + dilute hydrochloric acid aluminium chloride + hydrogen

2. Reaction between dilute hydrochloric acid and zinc:

zinc + dilute hydrochloric acid zinc chloride + hydrogen

3. Reaction between dilute sulphuric acid and iron:

iron + dilute sulphuric acid iron sulphate + hydrogen

9-37EXPERIMENT 9.8

Studying the action of dilute acid on some building materials5. Repeat Steps 1 to 4, using limestone chips to replace marble chips.

What do you observe in the test tube containing Is carbon dioxide produced?the building material? lime water?

Marble chips

gas bubbles are given off, the marble chips decrease in size turns milky yes

Limestone chips

gas bubbles are given off, the limestone chips decrease in size turns milky yes

9-39Thinking

Should we use acidic cleaners to clean a kitchen benchtop made of marble? Why?

We should not because the acids in the acidic cleaners will corrode the marble surface.

Unit 9 Making Use of Electricity11

Extension

9-39Activity 9.4

Writing word equations to describe the reactions between dilute acids and metal carbonatesWrite the word equations for the following reactions between dilute acids and metal carbonates.

1. Reaction between dilute hydrochloric acid and magnesium carbonate:

magnesium carbonate + dilute hydrochloric acid magnesium chloride + water + carbon dioxide

2. Reaction between dilute sulphuric acid and sodium carbonate:

sodium carbonate + dilute sulphuric acid sodium sulphate + water + carbon dioxide

3. Reaction between dilute nitric acid and calcium carbonate:

calcium carbonate + dilute nitric acid calcium nitrate + water + carbon dioxide

9-42EXPERIMENT 9.9

Studying the effect of acid on the growth of seedlings7. After one week, measure the shoot length of the seedlings in each Petri dish and calculate the

average length of the seedlings. Also, observe the appearance of the seedlings. Record the

results in the table below.

Petri dish A (diluted lemon juice)

Petri dish B (distilled water)

Average shoot length of the seedlings after one week (mm)

Answer varies with students. Answer varies with students.

Appearance of the seedlings after one week

thin and poor thick and healthy

Discussion

1. How does diluted lemon juice affect the growth of the seedlings?

The diluted lemon juice hinders ( promotes / hinders ) the growth of the

seedlings.

2. If the diluted lemon juice represents acid rain, what can you deduce from the results?

Acid rain hinders the growth of plants.

Unit 9 Making Use of Electricity12

9-44TEST YOURSELF 9.4

1. B 2. B 3. A

9-45EXPERIMENT 9.10

Studying the corrosive effect of concentrated sulphuric acid and concentrated sodium hydroxide solution (Demonstration)

Part A Corrosive effect of concentrated sulphuric acid

2. What do you observe?

The white cloth turns brown and several holes are formed on the white cloth.

Part B Corrosive effect of concentrated sodium hydroxide solution

4. Describe the appearance of the chicken foot.

The flesh on the chicken foot becomes loose. Some has even fallen off.

9-49Activity 9.5

An acid attack1. Why did the couple get hurt when they were splashed by the drain cleaner?

It is because the drain cleaner contained concentrated sulphuric acid, which is corrosive. It corroded their skin and

flesh.

2. If you were a shopkeeper at a convenience store nearby, what would you do to help the

victims?

Use water to wash their wounds. and call an ambulance.

Unit 9 Making Use of Electricity13

9-50Activity 9.6

Studying the safety instructions of bleach1. Should Joanne mix bleach with the drain cleaner? Why?

No. It is because toxic chlorine gas may be released.

2. How can she use bleach to kill germs in the drain?

She can mix 10 mL of bleach in 1 L of cold water. Pour the diluted bleach into the drain and rinse thoroughly.

3. What necessary safety precautions should she take when using bleach?

Keep the room ventilated during use. Wear rubber gloves and avoid contact with skin and eyes during use. Wash her

hands after use.

9-51TEST YOURSELF 9.5

1. State two safety measures when handling acids and alkalis.

(a) wear safety goggles

(b) put on protective gloves

2. (a) T (b) F (c) F (d) F

9-58REVISION EXERCISE

A. Multiple-choice Questions1. A

5. A

2. B

6. B

3. D

7. C

4. A

Unit 9 Making Use of Electricity14

B. Short-answer Questions8. (a) Doris used a pH meter to measure the pH values of the following solutions. Match the

possible pH values with the solutions.

Solution Possible pH value

toilet cleaner • • 5.6

oven cleaner • • 13.0

unpolluted rain water • • 3.2

soft drink • • 1.6

sugar solution • • 7.0

(b) Hydrangeas contain a pigment which turns blue in an acidic solution and turns pink in an

alkaline solution. When we add the solutions above to the Hydrangea extract respectively,

which of the solutions will turn the extract pink?

Oven cleaner

9. Steven found that his marble table had some black stains on the surface. He used some acidic

cleaner to clean the surface of the table. He heard a fizzing sound and the black stain was

successfully removed.

(a) Why was the black stain on the table successfully removed?

The marble surface with the black stain was corroded by the acidic cleaner.

(b) Why was there a fizzing sound when he cleaned the surface of the table?

It is because carbon dioxide is given out.

(c) Write the word equation representing the above process.

calcium carbonate + acid salt + water + carbon dioxide

(d) What safety measures should he take when using the acidic cleaner? Give one example.

He should put on protective gloves.

(e) What should he do if acidic cleaner is splashed on the skin?

He should wash the affected area immediately with running water.