Introduction to Acid and Base Science Notes

Introduction to Acid and Base Science Notes

Important Points:

→ Acid: Compounds imparting sour taste are called acids. Acids are soluble in water and they are corrosive in nature. Animals and plants also possess acids in them.

→ Acids present in food stuffs are called natural acids or carbonic acids. These acids are weak acids. Some acids are strong in nature. They are caustic/ inflammatory, e.g. sulphuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3). These acids are also called ‘mineral acids’.
→ Base: Substances having astrigent/ bitter taste and slippery to touch are called bases.
→ Examples: Lime water Ca(OH)2, baking soda (NaHCO3), caustic soda (NaOH) and soaps.

CBSE Library


Substances which change their colour in presence of acid or base are called indicators.

Name of Indicator Colour of the Indicator Colour in Acid Colour in alkali
Litmus paper Blue Red Blue (Remain same)
Litmus paper Red Red (Remain same) Blue
Methyl orange Orange Pink Yellow
Phenolphthalein Colourless Colourless Pink
Methyl red Red Red Yellow

→ Natural acids (organic): A few substances having natural acids (organic) are given in the following table:

Substance/Source Acids (Natural/Organic)
Vinegar Acetic acid
Orange Citric acid
Tamarind Tartaric acid
Tomato Oxalic acid
Curd Lactic acid
Lemon Citric acid

Properties of acids

  • Acids are sour in taste.
  • Acid molecules contain hydrogen ion (H+) as a main constituent.
  • Acid reacts with metal to form hydrogen gas.
  • Acid reacts with carbonates and liberates CO2 gas.
  • Blue litmus turns red in acid.

CBSE Library

→ Use of acids

  • Acids are used in the production of chemical fertilizers.
  • Acids are used in the production of explosives, oil purification, medicines, dyes and paints.
  • Hydrochloric acid is used for the preparation of different types of chloride salts.
  • Dil. H2S04 acid is used in the batteries (electric cell).
  • Dil. HCI is used for sterilization of water.
  • Acid is used for making of white paper from wood pulp.

→ We have seen that mineral acids are harmful to body. However several organic acids are present in our body and in plants which are useful to us.

  • DNA (Deoxyribo Nucleic Acid) is an acid present in our body, it decides heredity properties.
  • Proteins which are part of our body cell are made up of amino acids.
  • Fat of our body is formed by fatty acids.

CBSE Library


Alkali is a substance whose water solution gives hydroxide (OH) ion e.
Introduction to Acid and Base Science Notes 1

→ Formulae of alkali and their uses:

Name of alkali Formula Use
Sodium hydroxide / Caustic soda NaOH Washing soap
Potassium hydroxide/ Potash KOH Soap/shampoo
Calcium hydroxide/lime water Ca(OH)2 lime/whitewashing
Magnesium hydroxide/milk of magnesia Mg(OH)2 Acidity resistant medicines (Antacids)
Ammonium hydroxide NH4OH for production of fertilizers


We have seen that acid contains (H) hydrogen ions and alkali contains (OH-) hydroxide ions. The reaction between an acid and an alkali to form the salt and water is termed as neutralization.
Introduction to Acid and Base Science Notes 2

→ Hydrochloric acid is present in our stomach. It helps to ease digestion process. However excessive acid leads to indigestion. Antacids are used to control this hyperacidity.
→ This medicine contains milk of magnesia (Mg(OH)2). Such alkaline medicine neutralizes excess acid present in stomach.

CBSE Library

→ The acidity of soil increases due to excessive use of chemical fertilizers. Under the guidance of agriculturist, lime stone or lime water is mixed in the acidic soil. Such alkaline substances neutralizes excess acid present in the soil.

Science Notes

Leave a Comment