- Acids are sour in taste and change the colour of blue litmus to red, whereas, bases are bitter and change the colour of the red litmus to blue. Litmus is a natural indicator.
- Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water.
- The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as –
- Base + Acid → Salt + Water
- The general reaction between a metal oxide and an acid can be written as –
- Metal oxide + Acid → Salt + Water
- Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic in nature.
- Bases generate hydroxide (OH– ) ions in water. Bases which are soluble in water are called alkalis.
- Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+).
- A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali. Generally paper impregnated with the universal indicator is used for measuring pH.
- The strength of acids and bases depends on the number of H+ ions and OH– ions produced, respectively. If we take hydrochloric acid and acetic acid of the same concentration, say one molar, then these produce different amounts of hydrogen ions. Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids.
- The salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. The common salt thus obtained is an important raw material for various materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.
- When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.
- Chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This chlorine gas is used for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2]. Bleaching powder is represented as CaOCl2, though the actual composition is quite complex.
- Bleaching powder is used – (i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry; (ii) as an oxidising agent in many chemical industries; and (iii) for disinfecting drinking water to make it free of germs.
- Another chemical that can be obtained from sodium chloride is Na2CO3.10H2O (washing soda). recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.
- Uses of washing soda- (i) Sodium carbonate (washing soda) is used in glass, soap and paper industries. (ii) It is used in the manufacture of sodium compounds such as borax. (iii) Sodium carbonate can be used as a cleaning agent for domestic purposes. (iv) It is used for removing permanent hardness of water.
- Water of crystallisation is the fixed number of water molecules chemically attached to each formula unit of a salt in its crystalline form.
- Salts have various uses in everyday life and in industries.
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