CBSE Class 10 Science Notes Chapter 2 Acids Bases and Salts
Indicators: Indicators are substances which indicate the acidic or basic nature of the solution by the colour change.
Types of Indicator: There are many types of indicators. Some common types of indicators are:
1. Natural Indicators: 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 colours- 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.
2. Olfactory Indicator: Substances which change their smell when mixed with acid or base are known as Olfactory Indicators. For example; Onion, vanilla 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 its smell does not vanish with an acid.
Olfactory Indicators are used to ensure the participation of visually impaired students in the laboratory.
3. Synthetic Indicator: Indicators that are synthesized in the 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 the red with acid and turns into yellow with 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|
|Onion||n/a||No change||Smell vanishes|
|Vanilla||n/a||No change||Smell vanishes|
Acids: Acids are sour in taste, turn blue litmus red, and dissolve in water to release H+ ions.
Example: Sulphuric acid (H2SO4), Acetic Acid (CH3COOH), Nitric Acid (HNO3) etc.
Properties of Acids:
- Acids have a sour taste.
- Turns blue litmus red.
- Acid solution conducts electricity.
- Release H+ ions in aqueous solution.
Types of Acids: Acids are divided into two types on the basis of their occurrence i.e., Natural acids and Mineral acids.
(i) Natural Acids: Acids which are obtained from natural sources are called Natural Acids or Organic Acids.
Methanoic acid (HCOOH)
Acetic acid (CH3COOH)
Oxalic acid (C2H2O4) etc.
|Organic Acids and their Sources|
|Ascorbic acid||Guava, amla|
|Citric acid||Lemon, orange and other citrus fruits|
|Lactic acid||Sour milk, curd|
|Methanoic acid||Ant sting, nettle sting|
(ii) Mineral Acids: Acids that are prepared from minerals are known as Mineral Acids Example; Inorganic acids, man-made acids or synthetic acid are also known as Mineral Acids.
Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Nitric acid (HNO3)
Carbonic acid (H2CO3)
Phosphoric acid (H3PO4) etc.
Chemical Properties of Acid:
(i) Reaction of acids with metal: Acids give hydrogen gas along with respective salt when they react with a metal.
Metal + Acid → Salt + Hydrogen
Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.