Acids
* Any substance that releases hydrogen ions when dissolved in water is an acid.
* Acids obtained from food like curd, lemons, grapes, raw mango, citrus fruits and gooseberry are called organic acids.
* Hydrochloric acid, sulphuric acid and nitric acid are called inorganic or mineral acids.
Acids have the following physical properties:
* Acids taste sour.
* Acids turn blue litmus red.
* Acids turn methyl orange red.
* Acids have a corrosive action on the skin.
* Acids are good conductors of electricity.
* Acids generate heat when they are diluted.
Acids have the following chemical properties:
* Acids react with metals to form metal salts.
* Acids react with metal carbonates to form corresponding salts, carbon dioxide and water.
* Acids react with metal hydrogen carbonates and form corresponding salts, carbon dioxide and water.
* Acids react with metal oxide to form salt and water.
* Methyl orange turns red in acid and yellow in a base.
* Phenolphthalein does not change colour in an acid. It turns pink in a base.
* Sulphuric acid is used in the manufacture of fertilizers, paints, dyes, chemicals, plastics, synthetic fibres and in car batteries.
* Nitric acid is used in the manufacture of fertilizers, explosives like TNT, dyes and drugs.
Hydrochloric acid is used:
* before galvanizing, to remove oxide film from steel
* as a descaling agent for boilers
* in the textile, leather and food industry
Bases
* A substance that produces hydroxide ions is known as a base.
Bases are:
* Bitter to taste
* Soapy to touch
* Change red litmus to blue
* Strong alkalis react with metals like zinc, aluminium etc. to liberate hydrogen.
* Salts and water are products, when bases react with non-metal oxides.
* Mild bases are used as antacids.
* A base like sodium hydroxide is used in the manufacturing of soaps, paper and synthetic fibre.
* Calcium hydroxide is used in the manufacture of bleaching powder.
* Bases are also used in the preparation of fertilisers like ammonium phosphate.
Strength of Acids and Bases
* Neutralisation is a chemical reaction in which an acid reacts with a base to form salt and water.
* An acid or base is strong or weak depending on the concentration of hydrogen and hydroxide ions.
* The concentration of hydrogen ion in acids and bases can be measured using the pH scale.
On the pH scale:
* A neural solution has a pH value of 7
* A solution with pH value of less than 7 is acidic
* A solution with pH value of more than 7 is basic
Salts and their Properties
* A compound formed by the reaction between an acid and base is known as a salt.
* Salts are solids and ionic compounds.
* Based on the strength of the reacting acid and base, salts are classified into neural, acidic and basic salts.
* Sodium chloride is obtained from the sea and used to preserve pickles, to manufacture soaps, to melt ice and in the preparation of other compounds.
* Salts which have fixed number of water molecules as an essential part of their crystal are called hydrated salts.
* Plaster of paris is used as a bandage, proofing material and a sealing agent and for making statues, toys and decorative articles.