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Indicators

The substances that show different colours in different mediums or the substances that are used to distinguish between an acid and base are called indicators.
ph Scaleph Scale

Types of Indicators

1. Natural Indicators: The indicators obtained from plants are called natural indicators.
(a) Litmus

  • Litmus is a natural indicator which is extracted from “Lichens”, a plant belonging to thallophyta group. It is purple in colour. The pH range for litmus is 4.5 - 8.3 at room temperature.
  • There are 2 types of litmus solution:
    (i) Red Litmus: It is obtained by acidifying purple litmus extract
    (ii) Blue Litmus: It is obtained by making purple litmus extract alkaline
    In the laboratory litmus is available in the form of litmus paper; Red litmus and blue litmus paper.Red and Blue Litmus Paper
    Red and Blue Litmus Paper

(b) Turmeric

  • It is yellow in colour and turns red in the base and remains yellow in acids. 

2. Synthetic Indicators: These indicators are manufactured in laboratories.
(a) Methyl Orange

  • It is orange in colour and turns red in acids and yellow in the base.

(b) Phenolphthalein

  • It is colourless and remains colourless in acids and turns pink in the base.

3. Olfactory Indicators: These indicators change their smell when dipped in acid and base.
(a) Onion Extracts

  • Retains its smell in acid and loses in base.  

(b) Vanilla Extracts

  • Retains its smell in acid and loses in base.  

Table: Indicator Change in Acidic Medium

PH Scale: Indicators & Its Types | Science Class 10


Table: Indicator Change in Basic Medium
PH Scale: Indicators & Its Types | Science Class 10


Some Solved Questions


Ques. What is an indicator?

Ans. An indicator is a dye that changes colour when it is put into an acid or a base. An indicator tells us whether a given substance is an acid or a base by change in its colour.


Ques. What is a universal indicator?

Ans. Universal indicator is a solution, which undergoes several colour, changes over a wide range of pH. The colour is used to ‘indicate’ pH directly. Universal indicators are usually mixtures of several indicators.


Ques. Name the common indicators used to test acid, bases and salts?

Ans. The three common indicators to test for acids and bases are

1) Litmus

2)Methyl Orange

3)Phenolphthalein


Ques.What are olfactory indicators. Give examples?

Ans. Those substances whose smell or odour changes in acidic or basic solution are called olfactory indicators.

Example: Onion and Vanilla extract


Ques. What is the colour of Phenolphthalein in acidic and basic medium?

Ans. It remains colourless in acids and turns pink in basic medium.

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FAQs on PH Scale: Indicators & Its Types - Science Class 10

1. What is the pH scale and how is it related to indicators?
Ans. The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 being neutral, values below 7 indicating acidity, and values above 7 indicating alkalinity. Indicators are substances that change color depending on the pH of the solution they are in. They are used to determine the pH of a solution by observing the color change of the indicator.
2. What are the different types of indicators?
Ans. There are several types of indicators, including litmus paper, phenolphthalein, bromothymol blue, methyl orange, and universal indicator. Litmus paper is a simple indicator that turns red in acidic solutions and blue in basic solutions. Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions. Bromothymol blue is yellow in acidic solutions, green in neutral solutions, and blue in basic solutions. Methyl orange is red in acidic solutions and yellow in basic solutions. Universal indicator is a mixture of several indicators and can display a range of colors to indicate the pH of a solution.
3. How does an indicator change in an acidic medium?
Ans. In an acidic medium, indicators undergo a color change. For example, litmus paper turns red, phenolphthalein remains colorless, bromothymol blue turns yellow, methyl orange turns red, and universal indicator may display various colors depending on the pH.
4. How does an indicator change in a basic medium?
Ans. In a basic medium, indicators also undergo a color change. Litmus paper turns blue, phenolphthalein turns pink, bromothymol blue turns blue, methyl orange turns yellow, and universal indicator may display various colors depending on the pH.
5. How are indicators useful in determining the pH of a solution?
Ans. Indicators are useful in determining the pH of a solution because they provide a visual indication of the solution's acidity or alkalinity. By observing the color change of the indicator when added to the solution, one can determine the approximate pH of the solution. This is especially helpful when more precise instruments like pH meters are not available.
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