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Acids, Bases and Indicators: Simple acid-base indicators

3.0 Acids, Bases and Indicators

3.1 Simple acid-base indicators

What is an acid-base indicator?

Earlier, we classified matter as solids, liquids and gases. But matter can also be classified as acids, bases, and neutral substances. This requires use of acid-base indicators, such as litmus paper.

Do you know any useful acid in our stomach, and any base commonly used in the kitchen? What about a substance normally used to neutralize excess acid in the stomach (stomach acidity)?

simple acid base indicators high school chemistry

Open the link, acid-base indicator: litmus

(courtesy YouTube-Acids and Base: The litmus Test (Activity 3) by Gareth Evans), watch the demonstration, and answer Questions 3.0.

Questions 3.0

From the observations and the table above:

  1. Is sodium hydrogencarbonate solution (called bicarbonate of sodium in the video clip) an acid or base? Explain your answer.
  2. Is lemon juice a base or an acid?
  3. Where would you classify water? Explain your answer.

Answers to Questions 3.0

We can use blue and red litmus paper to test many other substances around us and classify them as acids, bases, and neutral substances.

Questions 3.1(a)

In an experiment, eight (8) different solutions labelled A to H were tested to find out if they were acidic, basic or neutral. A pair of red and blue litmus paper were partially dipped into each solution and the results were as shown in the diagram that follows.

  1. Which of the substances labelled A, B, C, D, E, F, G and H in the diagram
    1. turn blue litmus paper red but have no effect on red litmus paper?
    2. turn red litmus paper blue but have no effect on blue litmus paper?
  2. Substance A is an acid found in our stomach and G, the base often used to neutralize it. From this information, and the observations already made
    1. Which of the other substances are acids?
    2. Which of the other substances are bases?
  3. Litmus paper has enabled us to classify substances as acids and bases because its colour in acids is different from its colour in bases. It is therefore used as an acid-base indicator. Define an acid-base indicator.
  4. Substances which do not change the colour of litmus are neutral.
    Which of the substances A to H are neutral?

Answers to Questions 3.1(a).


For curiosity

A=Hydrochloric acid
B=Sodium chloride solution
C=Sodium hydroxide solution
D=Lemon juice
E=Calcium hydroxide solution (lime water)
F=Vinegar (used to preserve food)
G=Sodium hydrogencarbonate solution
H=Fresh water

How can we obtain (extract) natural acid-base indicators from plants?

Open the video below, how to extract acid-base indicator from plants.

(courtesy Youtube - make your own pH Indicator from red cabbage by Thoisoi2 - Chemical Experiments! ) to observe how an acid-base indicator can be extracted from red cabbage.

Questions on red cabbage acid-base indicator

  1. What is the neutral colour of red cabbage indicator?
  2. What is the colour of red cabbage indicator in a strong acid of pH =1?
  3. State the colour of red cabbage indicator in a strong base of Ph =14.

Answers to Questions on red cabbage acid-base indicator

Other than by heating, acid-base indicators can also be extracted from plants by crushing fresh leaves or flowers. See the set-up that follows.

Set-up acid base indicators, high school chemistry

Questions 3.1(b)

  1. Explain why the leaves (or flowers) are crushed.
  2. Why is water added to the crushed leaves (or flowers)?
  3. Which method is used to separate the solution from leaf (or flower) chaff?
  4. What is the natural colour of the leaf (or flower) extract?
  5. How can we know if the extract can serve as an acid-base indicator?

Answers to Questions 3.1(b)

For curiosity

Other suitable sources of indicators include red cabbage, beetroot, bougainvillea and rose flower.

All the substances around us, including various types of food, can be classified into acids, bases, and neutral substances. Blood, for example, is weakly basic.

The main problem of indicators extracted from plants is that they expire so fast. They can only be used when freshly prepared. We need an indicator with longer life, the universal indicator.