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(i) Put a spot of the coloured flower extract on to a filter paper and leave to dry. Put one
drop of lemon juice on to the spot then note any change of colour. Repeat the experiment
with other fruit juices and vinegar. They are acidic substances. Note any colour change with
dilute hydrochloric acid. Different colours suggest that some substances are more acidic
than others.
(ii) Put some of the original filtrate on to another piece of filter paper. When dry, note the
colours given by sodium bicarbonate solution, washing soda, limewater and a dilute solution
of sodium hydroxide. These are alkaline, or basic, substances. Note whether they all give
the same colour: Plant extracts can act as indicators to test whether a substance is acidic
or basic.
(iii) Add a few drops of sodium bicarbonate solution to 1 mL of flower extract indicator in a
test-tube. Then add lemon juice and note any colour change.
(iv) Repeat the experiment with limewater and indicator followed by dilute hydrochloric
acid. Note any colour change. Note whether you can get back the original colour by adding
more limewater. Note how many times you can change the indicator colour before the testtube
is full.
(v) Litmus, an extract of lichens, is another plant indicator. An acidic solution turns blue
litmus red. An alkaline solution turns red litmus blue.
(v) Universal Indicator can be in the form of a solution or dried on filter paper. Universal
indicator not only indicates whether a substance is acidic or basic but also how acidic it is.
Investigate the effect of Universal Indicator on the solutions above. To avoid using the
name of a colour to indicate acidity, we use a scale of numbers from 0 to 14 called the pH
scale. We can use the pH scale to express the degree of acidity. Acid solutions have a pH
value less than 7. Alkaline or basic solutions have a pH value greater than 7. Solutions with a
pH value of 7 are neither acidic nor basic, they are neutral.
On the bottle or packet of Universal indicator, a colour chart shows the colour and the pH
value associated with this colour as follows:
(a) Colour (b) pH (c) Acid/Base
(a) Red (b) 1-3 (c) very acidic
(a) Orange (b) 4-5 (c) weak acid
(a) Yellow (b) 6 (c) very weak acid
(a) Green (b) 7 (c) neutral
(a) Blue (b) 8 (c) very weak base
(a) Indigo (b) 9-10 (c) weak base
(a) Violet (b) 11- 14 (c) very basic
Use 2 drops of Universal Indicator to 10 mL of solution to be tested. Test the pH value of
lemon juice, vinegar, sodium bicarbonate solution, washing soda, lime water, sodium
hydroxide solution, tap water, distilled water.
sharon kalunda answered the question on April 12, 2019 at 07:54
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