Ch.9.2

Question 1

Why do we have indicators?

Answer 1

This is because many acids and alkalis used in the laboratory are colourless, corrosive liquids. Hence, we cannot distinguish them based on their appearances, or by tasting or touching. Therefore, we can use indicators to show the presence of acids and alkalis.

Question 2

Where do many acid-alkali indicators come from?

Answer 2

They are extracted from plants in Nature.

Question 3

Give an example of a natural indicator and explain why it can be used as one.

Answer 3

Red cabbage extract. It can be used as a natural indicator because red cabbage contains pigments called anthocyanins, which change colour in the presence of an acid or alkali.

Question 4

Name 5 examples of plants that contain the same pigment as red cabbage extracts.

Answer 4

Red rose flower, blueberry, strawberry, red radish and butterfly pea flowers.

Question 5

Can all plant extracts be used as an indicator for distinguishing acids and alkalis? Why?

Answer 5

No, because not all plants contain pigments which change colour in acids and alkalis.

Question 6

Name one plant which contains pigments that are not those found in red cabbages,, but also change colour in the presence of an acid or an alkali. Name the pigment.

Answer 6

Beetroot,, betalains.

Question 7

Apart from natural indicators,, name one commonly used alternative that can be used as an acid-alkali indicator.

Answer 7

Litmus.

Question 8

What is litmus composed of?

Answer 8

It is a mixture of pigments extracted from lichen.

Question 9

Why can litmus distinguish between acidic and alkaline solutions?

Answer 9

This is because it shows different colours in acidic and alkaline conditions.

Question 10

Litmus is available in two forms and two colours::

forms: : __________________ and __________________
colours: : __________________ and __________________

Answer 10

solution,, paper; red,, blue

Question 11

How is litmus paper prepared?

Answer 11

They are prepared by soaking strips of filter paper in litmus solution and then drying them before use.

Question 12

__________________/ __________________ litmus is used for testing __________________/ __________________ solutions.

Answer 12

blue + acidic

red + alkaline

Question 13

Acidic solutions turn _____________ litmus _____________.

Alkaline solutions do the opposite.

Answer 13

blue,, red.

Question 14

How do we know if a solutions is neutral?

Answer 14

If the solution doesn’t react to either litmuses, it is neutral.

Question 15

What is the major disadvantage of using natural indicators and/ or litmus?

Answer 15

They can only help distinguish whether a substance is acidic or alkaline. They cannot show how acidic or alkaline a substance is.

Question 16

Name the indicator that we should use when we want to know how acidic or alkaline a substance is.

Answer 16

A universal indicator.

Question 17

What is a universal indicator composed of?

Answer 17

It is a mixture of several indicators.

Question 18

How does a universal indicator work?

Answer 18

It shows a range of different colours depending on the degrees of acidity or alkalinity of the substance mixed with it.

Question 19

Universal indicator is available in _________________ and ________(a)________ forms. The (a) form of universal indicator is called __________________________.

Answer 19

solution, paper; pH paper.

Question 20

There is a colour chart on the _______________________________________ (for liquid form) or the _______________________________________ (for other form). Each colour on the chart _______________________________________.

Answer 20

bottle of universal indicator solution, box of pH paper, corresponds to a pH value

Question 21

The pH value is expressed as _______________________________________. This range of pH values is known as the _________________________.

Answer 21

a number from 0 to 14, pH scale.

Question 22

On the range of pH values:
—an acidic substance has a pH value ____________________ :: the ____________________ the pH value,, the more acidic the substance.
—an alkaline substance has a pH value ____________________ :: the ____________________ the pH value,, the more alkaline the substance.
—neutral substances have a pH value of ____________________.

Answer 22

less than 7,, lower,, greater than 7,, higher,, exactly 7.

Question 23

pH values of common substances::
0 —
1 —
2 —
3 —
4 —
5 —
6 —
7 —
8 —
9 —
10 —
11 —
12 —
14 —

Answer 23

acid in car battery
sulphuric acid
cola
orange juice / vinegar
tomato juice
black coffee
tap water / tea
distilled water
toothpaste
detergent
baking soda
glass cleaner
limewater
drain cleaner

Question 24

What is the full name of pH?

Answer 24

power of Hydrogen.

Question 25

Is pH value of less than 0 or greater than 14 possible? Do pH values have to be whole numbers?

Answer 25

Yes. No.

Question 26

Universal indicator can give only a ________________________________ of the pH value of a substance. What can we do to measure pH values more accurately?

Answer 26

rough measurement; we can use an electronic instrument — pH meter.

Question 27

What does a pH meter consist of?

Answer 27

It consists of a probe connected to a digital display.

Question 28

What is the accuracy of a pH meter?

Answer 28

It is accurate to two decimal places.

Question 29

What are the steps to using a pH meter?
1.
2.
3.

Answer 29

1. calibrate the pH meter
2. put its probe into a solution
3. the pH value of the solution will be displayed on its screen

Question 30

Besides the pH meter, we can use a _______________________________________________ to measure the pH value of a solution.

Answer 30

pH sensor connected with a data-logger.

Question 31

What is the benefit of using a data-logger?

Answer 31

The readings from the data-logger can be recorded and analysed in a computer. This way, the data-logger system can help monitor the change in pH value of a solution during an experiment.

Question 32

Name three advantages of a pH meter (compared to universal indicator solution and/ or pH paper).

Answer 32

—a pH meter can measure the pH value of a substance more accurately than universal indicator solution and pH paper
—a pH meter can be reused while pH paper cannot
—a pH meter can be used to measure the pH value of a liquid with a deep colour

Question 33

Name four disadvantages of a pH meter (compared to universal indicator solution and/ or pH paper).

Answer 33

—a pH meter is more expensive than universal indicator solution and pH paper
—the size of a pH meter is larger than that of pH paper. It is less convenient to carry around
—calibration is needed, making it less user friendly and more difficult to operate.
—we need to handle pH meters with care, its probe is made of glass.