Question

How does an indicators differentiate an acid and base?

Answer 1

NATURAL INDICATOR
Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.
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LITMUS
Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.
An acid turns blue litmus paper red.
A base turns red litmus paper blue.

TURMERIC
Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.

RED CABBAGE
The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.

OLFACTORY INDICATORS
Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.

ONION
Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.

VANILLA
The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid. Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.

SYNTHETIC INDICATOR
Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc.
Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
Methyl orange is originally orange in colour. It turns into red with acid and turns into yellow with base.
Indicator
Original colour
Acid
Base

Red litmus
Red
No change
Blue

Blue litmus
Blue
Red
No change

Turmeric
Yellow
No change
Reddish brown

Red cabbage juice
Purple
Reddish
Greenish yellow

Phenolphthalein
Colourless
Colourless
Pink

Methyl orange
Orange
Red
Yellow

Onion
n/a
No change
Smell vanishes

Vanilla
n/a
No change
Smell vanishes
UNIVERSAL INDICATOR:
Using a litmus paper, phelophthalein, methyl orange, etc. only the acidic or basic character of a solution can be determined, but use of these indicators does not give the idea about the strength of acid or base. So, to get the strength as well as acidic and basic nature of a given solution universal indicator is used.
Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Answer 2

Answer:

An acid turns blue litmus paper red. A base turns red litmus paper blue. ... Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.

Explanation:

An acid turns blue litmus paper red. A base turns red litmus paper blue. ... Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example onion, vanilla, clove, etc.