Class 10 Science

Acid Base Indicators

On this page, you will learn about meaning of acid base indicator. You will also learn about natural indicators and synthetic indicators such as litmus, phenolphthalein and methyl orange. These notes are based on the chapter Acids Bases and Salts from NCERT book based on CBSE syllabus.

Substances which show the acidic or basic behavior of other substance by change in colour are known as indicators or acid base indicators.

Types of Indicators

There are many types of indicators. Some common types of indicators are

  1. Natural
  2. Olfactory Indicator
  3. Synthetic Indicator
  4. Universal Indicator

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.

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. Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.

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.

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.

IndicatorOriginal colourAcidBase
Red litmusRedNo changeBlue
Blue litmusBlueRedNo change
TurmericYellowNo changeReddish brown
Red cabbage juicePurpleReddishGreenish yellow
Methyl orangeOrangeRedYellow
Onionn/aNo changeSmell vanishes
Vanillan/aNo changeSmell 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.

ph color chart universal indicator

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