Acids, Bases and Salts

IMPORTANT NOTES

1. Acids generally have a watery touch and sour taste. Some acids are highly corrosive in nature and can cause severe burns.

2. Bases generally have a soapy touch and bitter taste. Soluble bases have a corrosive action on the skin and can cause severe burns.

3. Aqueous solutions of acids and bases are good conductors of electricity.

4. An acid is a compound, which on dissolving in water gives hydronium ions or H+(aq) ions as the only positively charged ions.

5. An alkali is a compound, which on dissolving in water gives hydroxyl or OH– (aq) ions as the only negatively charged ions.

6. An oxide or a hydroxide of a metal which reacts with acids to form salt and water as the only products, is called a base.

7. All bases/alkalises have a positively charged metallic ion except ammonium hydroxide [NH4OH] which has a positive NH4 + radical.

8. The separation of H+(aq) ions from an acid takes place only in the presence of water. The separated H+(aq) ion can exist independently and hence combines with water molecule to form hydronium ion [H3O+].

9. An acid containing least possible amount of water is called a concentrated acid.

10. An acid containing fairly large amount of water is called a dilute acid.

11. The process of mixing water in an acid is called dilution of acid. While diluting an acid always add acid slowly to water and continuously stir the mixture. It is because the reaction is generally exothermic and can cause spurting.

12. An alkali containing least possible amount of water is called a concentrated alkali.

13. Dilution of an acid/ alkali, lowers the concentration of H+(aq)/OH–(aq) ions per unit volume.

14. pH scale measures the concentration of H+(aq) ions in a particular solution. In the word pH, p stands for “potenz” meaning power and H for the H+(aq) ions.

15. On the pH scale, the concentration of H+(aq) ions is measured from 0 to 14. Zero (0) is for highly acidic solution and 14 is for highly alkaline solution.

16. pH of distilled water and neutral salt solution is 7.

17. If the pH of a solution is less than 7, then it is an acidic solution. When the pH of a solution decreases from 7 to 0, the concentration of H+(aq) ions in it goes on increasing and so does the acidic character of the solution.

18. If the pH of a solution increases from 7 to 14, the concentration of H+(aq) ions decreases and that of OH–(aq) ions increases. So, the neutral solution becomes more and more alkaline, till at pH 14 it is highly alkaline.

19. pH is generally measured by pH paper, which is prepared by impregnating a filter paper in a solution of universal indicator and then drying.

20. The colours produced on the pH paper at different values are listed below. pH Colour pH Colour 0 Dark red 8 Greenish blue 1 Red 9 Blue 2 Red 10 Navy Blue 3 Orange red 11 Purple 4 Orange 12 Dark Purple 5 Orange yellow 13 Violet 6 Greenish yellow 14 Deep violet 7 Green

21. Acid-base indicators are organic dyes derived from plant materials which shows the presence of acids and bases.

22. Litmus is a natural indicator, extracted from a plant, belonging to the thallophyta family.

23. The dyes from the plants, such as red cabbage leaves, coloured petals of plants such as Petunia, and Turmeric are other natural indicators.

24. Phenolphthalein and methyl orange are synthetic indicators which show the presence of acids and bases.

25. Blue litmus solution turns red in acidic solutions, but is not affected in basic solutions.

26. Red litmus solution turns blue in basic solutions, but is not affected in acidic solutions.

27. Phenolphthalein solution turns pink in basic solutions, but turns colourless in acidic solutions.

28. Methyl orange solution is yellow in basic solutions and pink in acidic solutions.

29. Turmeric solution turns brown in basic solutions, but remains yellow in acidic solutions.

30. The substance whose smell changes in acidic or basic medium are called olfactory indicators.

31. Acids react with active metals to form the salts of the metals and liberate hydrogen gas. Active metals are sodium, potassium, calcium, magnesium, aluminium, zinc and iron.
Active metal + Acid ⎯→ Metal salt + Hydrogen(g) 

32. Acids react with metal carbonates to form their respective metal salts, water and carbon dioxide gas.
Metal carbonate + Acid ⎯→ Metal salt + Water + Carbon dioxide gas.

33. Acids react with metal hydrogencarbonates (metal bicarbonates) to form their respective metal salts, water and carbon dioxide gas.
Metal hydrogencarbonate + Acid ⎯→ Metal salt + Water + Carbon dioxide. 

34. Acids react with metal oxides to form their respective salts and water as the only products.
Metal oxide + Acid ⎯→ Metal salt + water. 

35. Acids react with metal hydroxides to form their respective salts and water as the only product.
Metal hydroxide + Acid ⎯→ Metal salt + Water 

36. Substances which react with acids to form salt and water as the only products are called basic substances. Thus, oxides and hydroxides of metals are bases.

37. A chemical reaction in which an acid reacts completely with a base to form salt and water as the only products, is called a neutralisation reaction.

38. All oxides of metals are insoluble in water.

39. All hydroxides of metals are insoluble in water, except the hydroxides of sodium, potassium, calcium and magnesium.

40. Soluble hydroxides of metals are called alkalises.

41. An acid solution which produces more H+(aq) ions for an aqueous solution of 1 molar concentration is called a strong acid.

42. An acid which produces few H+(aq) ions for an aqueous solution of 1 molar concentration is called a weak acid.

43. Sulphuric acid, hydrochloric acid, nitric acid and phosphoric acid are the examples of strong acids.

44. Carbonic acid, sulphurous acid, acetic acid, etc., are the examples of weak acids.

45. An alkali which produces more OH–(aq) ions in an aqueous solution of 1 molar concentration, is called a strong alkali. Sodium hydroxide and potassium hydroxide are the examples of strong alkalies.

46. An alkali which produces few OH–(aq) ions for an aqueous solution of 1 molar concentration, is called a weak alkali. Calcium hydroxide, magnesium hydroxide and ammonium hydroxide are examples of weak alkalies.

47. Some animals and plants employ acids/alkalises for their self defence.

48. An ionic compound containing a positive ion other than hydrogen ion and a negative ion other than hydroxyl ion is called a salt.

49. All the salts having the same negative ion/ radical, but different metallic ions is called a family of salts. Conversely, all the salts having the same positive metallic ion and different non-metallic ions/radicals is called a family of salts.

50. The salts formed by the action of strong acids with strong bases, are called normal salts.

51. The salts formed by the action of strong acids and weak bases are called acid salts.

52. The salts formed by the action of weak acids and strong alkalises are called basic salts.

53. Common salt is the most important and most abundant salt in nature. In addition to its use as an edible salt it is also a raw material for producing chemicals, such as chlorine, hydrogen, sodium and hydrochloric acid.

54. When saturated common salt is electrolysed, the products are sodium hydroxide, hydrogen and chlorine gas.

55. Hydrogen gas is used in :
(i) oxy-hydrogen flame
(ii) hydrogenating vegetable oils,
(iii) manufacture of ammonia and hydrochloric acid,
(iv) as a rocket fuel.

56. Chlorine gas is used in : 
(i) disinfecting water
(ii) in the manufacture of bleaching powder, hydrochloric acid and pesticides
(iii) in bleaching wood pulp and cotton fabrics
(iv) in the manufacture of polyvinyl chloride and chlorofluorocarbons.

57. Sodium hydroxide is used : 
(i) in the manufacture of all kinds of soaps and detergents
(ii) making paper pulp in paper industry
(iii) making artificial fibres like rayon and nylon
(iv) for de-greasing surface of metals
(v) and making bleaching agents such as sodium hypochlorite.

58. Bleaching powder is prepared by passing chlorine gas through freshly prepared slaked lime paste, till it stops reacting.

59. Bleaching powder is used : 
(i) for bleaching cotton-fabrics and wood pulp
(ii) making unshrinkable wool
(iii) in the manufacture of chloroform
(iv) in sterilisation of drinking water, and
(v) in disinfecting laboratories, drains, ditches, etc.

60. Baking soda is prepared industrially from common salt, by passing carbon dioxide gas through saturated ammonical common salt solution.
NaCl(s) + H2O(l) + NH3(g) + CO2(g) ⎯→ NH4Cl(aq) + NaHCO3(s) 

61. Baking soda is used : 
(i) in making baking powder
(ii) as a constituent of antacids and
(iii) in fire extinguishers.

62. Soda ash (anhydrous sodium carbonate) is prepared by strongly heating baking soda.
2NaHCO3(s) ⎯→ Na2CO3(s) +H2O(l) + CO2(s) 

63. Washing soda (hydrated sodium carbonate) is prepared by dissolving soda ash in water and then crystallising it.
Na2CO3(s) + 10H2O(l) ⎯→  Na2CO3.10H2O. 

64. Washing soda and soda ash is used :
(i) in softening of hard water
(ii) in the manufacture of glass and dry soaps
(iii) as a common cleansing agent in the household
(iv) in the manufacture of caustic soda, boron and sodium phosphate.

65. The fixed number of water molecules, which are in loose combination with one molecule of a salt, is called water of crystallisation.

66. Plaster of Paris is chemically calcium sulphate hemihydrate [(CaSO4)2.H2O or CaSO4. 1 2 H2O]. It is prepared by prolonged heating gypsum at a controlled temperature of 100°C.
CaSO4.2H2O(s) heat 100 C CaSO4.⎯→  1 2 H2O(s) + 3 2 H2O(g) 

67. Plaster of Paris reacts with water to give a hard mass called gypsum.
(CaSO4)2.H2O(s) + 3H2O(l)  ⎯→ 2CaSO4. 2H2O(s) 

68. Plaster of Paris is used : 
(i) for keeping fractured bones in position
(ii) in making decorative toys and panelling ceiling
(iii) in making fire proof materials and
(iv) in making blackboard chalk.