Indicators, Acids and Bases

By | October 18, 2018

Question 1 What is an indicator? Name three indicators?

Question 2 What is the use of litmus solution?

Question 3 Why does a yellow curry stain on a white shirt turn red when it is washed with soap?

Question 4 Give few differences between acids and bases?

Question 5 What are acids?Name different types of acids?Give examples ?

Question 6 What are bases? Name different types of bases?Give examples?

Question 7 What will happens if we react litmus indicator with acid and a base?

Question 8 What will happens if we react phenolphthalein indicator with acid and a base?

Question 9 What is meant by an acid rain? How is it caused? What damage is caused by acid rain?

Question 10 What are neutral substances?

Question 11 Name an acid present in stomach?

Question 12 Name an acid present in citrus fruits?

Question 13 What are organic acids? Give example?

Question 14 What are mineral acids.Give examples?

Our stomach makes an acid  (hydrochloric acid) which is necessary for the digestion of food Baking soda (Sodium hydrogen carbonate) used in baking bread is a base.

Common salt (sodium chloride) is used in cooking food is a salt.


An indicator is a ‘dye’ that changes colour when it is put into an acid or a base. An indicator gives different colours in acid and base.

A substance which contains an acid is said to be acidic whereas the substance which contains a base is said to be basic. An indicator tells us whether the substance we are testing is acidic or basic by change in its colour.

Two types of Acid-base indicators :

1) Natural indicators 

For Example: Litmus, China rose and Turmeric

2) Synthetic indicators.

For example: Phenolphthalein

(1) Litmus

Litmus is a natural indicator.
Litmus solution is a purple coloured dye which is extracted from a type of plant called lichen. Litmus has a purple colour (mauve colour) in water.

1) When litmus solution is neutral, then its colour is purple.
2) When litmus is added to an acidic solution, it turns red.
3) When litmus is added to a basic solution, it turns blue.

litmus is made into two types:

1) Blue litmus and
2) Red litmus.

Litmus can be used in the form of litmus solution (like blue litmus solution and red litmus solution) or in the form of strips of litmus paper.

Litmus is the most common indicator for testing acids and bases in the laboratory.

(1) Acids turn blue litmus to red.
(2) Bases turn red litmus to blue.

(a) If a drop of the given solution turns blue litmus paper to red, then the given solution will be acidic in nature (or it will be an acid).
For example: a drop of lemon juice turns blue litmus paper to red, so lemon juice is acidic in nature. That is, lemon juice contains an acid.

(b) If a drop of the given solution turns red litmus to blue, then the given solution will be basic in nature (or it will be a base).
For example: a drop of baking soda solution turns red litmus paper to blue, so baking soda is basic in nature. 

(2) China Rose Indicator

China rose indicator is a light pink coloured solution which is extracted from the red flowers of China rose plant with water.
(1) Acids turn China rose indicator to magenta (deep pink).
(2) Bases turn China rose indicator to green.

(a) If a drop of the given solution turns China rose indicator from light pink to magenta (deep pink) then the given solution will be acidic in nature (or it will be an acid).
For example: lemon juice turns China rose indicator to magenta (deep pink), so lemon juice is acidic in nature.

(b) If a drop of the given solution turns China rose indicator from light pink to green, then the given solution will be basic in nature (or it will be a base).
For example:  baking soda solution turns china rose indicator to green, therefore , baking soda is a base.

(3) Turmeric 

Turmeric is a bright yellow powder obtained from a plant.
Turmeric contains a yellow dye. Turmeric turns red in basic solution.
Turmeric is used as indicator in the form of turmeric paper.

Turmeric paper is yellow in colour.
(1) Turmeric paper is yellow in acid solution.
(2) Bases turn the yellow turmeric paper to red.

If a drop of the given solution turns the yellow turmeric paper to red, then the given solution will be basic in nature (or it will be a base).
For Example: If a drop of baking soda solution is put on the strip of a turmeric paper, the yellow turmeric paper turns red. This shows that baking soda solution is basic in nature.

If we put a drop of an acid on this turmeric paper (which has been turned red by a base), then the turmeric paper will change from red to yellow. This is because acid cancels the effect of base and restores the original yellow colour of turmeric paper.

A yellow stain of curry on a white shirt (which is due to the presence of turmeric in curry), turns red when soap is scrubbed on it. This is due to the fact that soap solution is basic in nature which changes the colour of turmeric in the curry stain to red.

(4) Phenolphthalein Indicator

Phenolphthalein is a synthetic (man-made) acid-base indicator.

(1) Phenolphthalein indicator is colourless in acid solution.
(2) Phenolphthalein indicator gives pink colour in basic solution.

If we add acid in the solution of a base (which has been turned pink by phenolphthalein indicator), then the solution will change from pink to colourless. The substances whose solutions do not change the colour of any indicator in any way are called neutral substances.

For example: Pure water (distilled water), common salt and sugar are neutral substances. They do not change the colour of any indicator.


The acid present in lemon which gives it a sour taste is called citric acid

Raw mango, raw grapes, lemon juice, orange juice, curd, sour milk, vinegar and tamarind (imli), etc., are sour in taste due to the presence of acids in them. 

The acids present in plant materials and animals are natural acids called organic acids.

A substance which reacts with a base to form a salt (and water) is called an acid. Acids have sour taste. Acids turn blue litmus to red.

Some of the examples of acids are : Acetic acid, Citric acid, Hydrochloric acid, Sulphuric acid and Nitric acid.

Acids are of two types : Organic acids and Mineral acids. 

Organic Acids

Organic acids are the naturally occurring acids. They are found in various types of plants and animals.

Some of the important organic acids (which occur in nature) are:

  1. Acetic acid
  2. Formic acid
  3. Citric acid
  4. Lactic acid
  5. Tartaric acid
  6. Ascorbic acid
  7. Oxalic acid

(1) Acetic acid is found in vinegar . Vinegar is used as a preservative in foods.
(2) Formic acid is present in ant’s sting. The sharp pain caused by the sting of an ant is due to the formic acid pushed into our skin during the sting.
(3) Citric acid is present in citrus fruits such as lemons and oranges.
(4) Lactic acid is present in curd and in sour milk.
(5) Tartaric acid is present in tamarind (imli), unripe grapes and unripe mangoes.
(6) Ascorbic acid is present in amla and citrus fruits (such as lemons and oranges). Ascorbic acid is commonly known as vitamin C.
(7) Oxalic acid is present in spinach (palak)

Organic acids (or naturally occurring acids) are weak acids. It is not harmful to eat or drink substances containing naturally occurring acids in them.

Mineral Acids

The acids prepared from the minerals of the earth are called mineral acids. Mineral acids are the man-made acids. Mineral acids are also known as laboratory acids because they are used in the science laboratory to perform experiments.

The three most common mineral acids are:

1) Hydrochloric acid,
2) Sulphuric acid, and
3) Nitric acid

For exampleHydrochloric acid is used in cleaning kitchen sinks and bathroom sanitary ware (like wash basin and toilet seat). Sulphuric acid is used in making storage batteries for cars, buses, trucks and inverters. Nitric acid is used by goldsmiths for cleaning gold and silver ornaments.

Concentrated mineral acids are very dangerous. These acids can burn our hands and clothes. These acids should be handled with great care. Acids are generally mixed with water to dilute them. Such acids are called dilute acids. Dilute acids are less harmful to us.

Strong Acids and Weak Acids

All the acids can be divided into two groups : strong acids and weak acids.

Hydrochloric acid, sulphuric acid and nitric acid are strong acids.

Acetic acid, formic acid, citric acid, tartaric acid and carbonic acid are weak acids.

Strong acids are very dangerous to drink. Even the dilute solutions of strong acids are extremely harmful to drink. Being weak, the organic acids like acetic acid, citric acid and tartaric acid are used as food ingredients.

Many foods like pickle and tomato ketchup contain acetic acid in the form of vinegar. Vinegar preserves fruits and vegetables.

Baking powder used in making cakes and biscuits contains tartaric acid.

Carbonic acid is used in fizzy soft drinks and soda water. It gives them a pleasant taste.

Acid Rain

The rain which contains a higher level of acid than normal is called acid rain.

Acid rain is caused by the acidic gases like sulphur dioxide, nitrogen dioxide and carbon dioxide which are released into the air as pollutants during the burning of various types of fuels.

Sulphur dioxide gas dissolves in falling rain drops to form sulphuric acid, nitrogen dioxide gas dissolves in rain drops to form nitric acid whereas carbon dioxide gas dissolves in rain drops to form carbonic acid.

The presence of sulphuric acid, nitric acid and carbonic acid in rain water makes the rain water acidic. Acid rain causes damage to aquatic animals (like fish), trees, crop plants, metal structures and stone buildings and monuments.

This happens as follows:

(1) Acid rain makes the water of lakes, ponds and rivers too acidic due to which fish and other aquatic animals get killed.
(2) Acid rain eats up the leaves of the trees gradually. By losing leaves, the trees die. Acid rain also damages crop plants in the fields.
(3) Acid rain damages the metal structures like steel bridges, etc, when it falls on them.
(4) Acid rain damages the surfaces of buildings and monuments made of stone.


The substances (such as baking soda) which are bitter in taste and feel soapy to touch are known as bases.

When bases are added to acids, they neutralise (or cancel) the effect of acids.
A substance which can neutralise an acid to form a salt (and water) is called a base.

Some of the examples of bases are:

  1. Sodium hydroxide
  2. Potassium hydroxide
  3. Calcium hydroxide
  4. Magnesium hydroxide
  5. Ammonium hydroxide (Ammonia solution)
  6. Sodium carbonate
  7. Sodium hydrogen carbonate

A base which is soluble in water is called an alkali.

(1)Sodium hydroxide and potassium hydroxide are found in soap.
(2) Calcium hydroxide is found in lime water.
(3) Magnesium hydroxide is found in milk of magnesia.
(4) Ammonium hydroxide (also called ammonia solution) is found in window cleaners.
(5) Sodium carbonate is found in washing soda.
(6) Sodium hydrogen carbonate is found in baking soda.

Strong Bases and Weak Bases

Strong Bases
Sodium hydroxide
Potassium hydroxide

Weak Bases
Calcium hydroxide
Ammonium hydroxide
Sodium carbonate
Magnesium hydroxide
Sodium hydrogen carbonate

Sodium hydroxide is commonly known as caustic soda and potassium hydroxide is commonly known as caustic potash.

Sodium hydroxide and potassium hydroxide are dangerous bases which can burn our skin. They must be handled very carefully.

Magnesium hydroxide is a very weak base which is safe to drink. It is used in milk of magnesia and other indigestion mixtures. Sodium hydrogen carbonate is a very weak base which is used as an antacid to cure indigestion.

Sugary food eaten by us is converted into acid by the bacteria present in our mouth. And this acid causes tooth decay. The toothpaste which we use for brushing and cleaning our teeth is basic in nature Since toothpaste is basic, it neutralises the acid in our mouth and  hence prevents tooth decay.

Neutral Substances

Those substances which are neither acidic nor basic in nature are called neutral substances. Being neither acidic nor basic, neutral substances do not change the colour of any indicator.

Some of the neutral substances are : Pure water (or Distilled water), Glucose, Cane sugar and Common salt.