WHAT IS INDICATOR?
If we cannot taste every substance, how do we find its nature?
Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, China rose petals (Gudhal), etc., are some of the naturally occurring indicators.
Source: This topic is taken from NCERT TEXTBOOK
NATURAL INDICATOR – LITMUS
The most commonly used natural indicator is litmus. It is extracted from lichens (Fig. 5.1a). It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper (Fig. 5.1b).
Figure 5.1: (a) Lichens, and (b) Red and blue litmus paper
Activity 5.1:
* Mix some water with lemon juice in a plastic cup/tumbler/test tube.
* Put a drop of the above solution on a strip of the red litmus paper with the help of a dropper.
* Is there any change in colour?
* Repeat the same exercise with the blue litmus paper.
* Note down if there is any change in colour.
Perform the same activity with the following substances:
Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water. If possible make solutions in distilled water. Record your observations as in Table. 5.2.
S. No. |
Test solution |
Effect on red litmus paper |
Effect on blue litmus paper |
Inference |
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Table 5.2:
In your Table, are there any substances on which litmus had no effect? Name those substances.
The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
To prepare limewater, take some water in a tumbler and add some lime (chuna) into it. Stir the solution and keep it for some time. Pour a little from the top. This is lime water.
Source: This topic is taken from NCERT TEXTBOOK
NATURAL INDICATOR – TURMERIC
Activity 5.2:
* Take a tablespoonful of turmeric powder. Add a little water and make a paste.
* Make turmeric paper by depositing turmeric paste on blotting paper/filter paper and drying it. Cut thin strips of the yellow paper obtained.
* Put a drop of soap solution on the strip of turmeric paper.
* What do you observe?
S. No. |
Test solution |
Effect on turmeric solution |
Remarks |
1. |
Lemon juice |
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2. |
Orange juice |
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3. |
Vinegar |
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4. |
Milk of magnesia |
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5. |
Baking soda |
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6. |
Lime water |
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7. |
Sugar |
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8. |
Common salt |
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Table 5.3:
Similarly, test the solutions listed in Table 5.3 and note down your observations. You may try solutions of other substances also.
Now you understand why a turmeric stain on your white shirt is turned to red when it is washed with soap. It is because the soap solution is basic.
You can prepare a greeting card for your mother on her birthday. Apply turmeric paste on a sheet of plane white paper and dry it. Draw a beautiful flower with soap solution with the help of a cotton bud. You will get a beautiful greeting card.
Source: This topic is taken from NCERT TEXTBOOK
NATURAL INDICATOR – CHINA ROSE
Activity 5.3:
* Collect some China rose (Gudhal ) petals and place them in a beaker. Add some warm water.
* Keep the mixture for some time till the water becomes coloured.
* Use the coloured water as an indicator.
* Add five drops of the indicator to each of the solutions given in Table 5.4.
S. No. |
Test solution |
Initial colour |
Final colour |
1. |
Shampoo (dilute solution) |
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2. |
Lemon juice |
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3. |
Soda water |
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4. |
Sodium hydrogen carbonate solution |
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5. |
Vinegar |
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6. |
Sugar solution |
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7. |
Common salt solution |
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Table 5.4:
What is the effect of the indicator on acidic, basic, and neutral solutions? China rose indicator (Fig. 5.3) turns acidic solutions to dark pink (magenta) and basic solutions to green.
Figure 5.3: China rose flower and indicator prepared from it
Activity 5.4:
CAUTION:
Great care should be taken while handling laboratory acids and bases because these are corrosive in nature, irritating and harmful to the skin.
* The teacher is requested to get the dilute solution of the following chemicals from his/her school laboratory or from a nearby school: hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, calcium hydroxide (lime water).
* Demonstrate the effect of the three indicators on each of these solutions.
* Record your observations in Table 5.5.
S. No. |
Name of acid |
Effect on litmus paper |
Effect on turmeric paper |
Effect on China rose solution |
1. |
Dilute hydrochloric acid |
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2. |
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3. |
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Table 5.5:
Acid rain
Are you familiar with the term acid rain? Have you ever heard about the damaging effect of acid rain? As the name indicates the rain containing an excess of acids is called acid rain. Where do these acids come from? The rain becomes acidic because carbon dioxide, sulphur dioxide, and nitrogen dioxide (which are released into the air as pollutants) dissolve in raindrops to form carbonic acid, sulphuric acid, and nitric acid respectively. Acid rain can cause damage to buildings, historical monuments, plants, and animals.
Source: This topic is taken from NCERT TEXTBOOK
PHENOLPHTHALEIN
We have learnt that acids turn blue litmus red and bases turn red litmus blue. Let us see what happens when an acid is mixed with a base.
We are going to use an indicator you have not used so far. It is called phenolphthalein.
Activity 5.5:
To be demonstrated by the teacher in the class.
* Fill one-fourth of a test tube with dilute hydrochloric acid.
* Note down its colour.
* Note down the colour of the phenolphthalein solution also.
* Add 2–3 drops of the indicator to the acid.
* Now shake the test tube gently. Do you observe any change in colour of the acid?
* Add to the acidic solution a drop of sodium hydroxide solution by a dropper
Stir the tube gently. Is there any change in the colour of the solution? Continue adding the sodium hydroxide solution drop by drop while stirring till the pink colour just appears.
Now add one more drop of dilute hydrochloric acid. What do you observe? The solution again becomes colourless. Again add one drop of sodium hydroxide solution. Is there any change in colour? The solution again becomes pink in colour.
It is evident that when the solution is basic, phenolphthalein gives a pink colour. On the other hand, when the solution is acidic, it remains colourless.
Source: This topic is taken from NCERT TEXTBOOK