ACIDS,BASES And SALTS
1. Acids are sour in taste and change the colour of blue litmus to red; bases are bitter in taste and change the colour of red litmus to blue.2. Acid-base indicators are used to identify different acids and bases.
E.g. (i) natural indicators (litmus, turmeric), (ii) synthetic indicators (methyl orange, phenolphthalein), (iii) olfactory indicators (onion, vanilla, clove).
3. Acids react with metal to form salt and produce hydrogen gas.
4. Bases also react with metal to form salt and produce hydrogen gas. However, such reactions are not possible with all metals. E.g. `2NaOH+Zn\rightarrow Na_2ZnO_2+H_2`.
5. Metal carbonates & hydrogen carbonates react with acids to form salt, water & `CO_2`.
6. Carbon dioxide gas turns lime water milky. On passing excess carbon dioxide through lime water, milky colour disappears due to formation of calcium hydrogen carbonate.
7. The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. `Acid\;+Base\;\rightarrow Salt\;+Water\;`
8. Metallic and non-metallic oxides are basic and acidic in nature, respectively.
9. Acids produce hydrogen ions (`H^+`) in solution, which are responsible for acidic properties. Separation of `H^+` from acid cannot occur in absence of water. E.g. `HCL+H_2O\rightarrow H_3O^++Cl^-`
10. Hydrogen ions can only exist after combining with water molecules as hydronium ions (`H_3O^+`). E.g.`H^++H_2O\rightarrow H_3O^+`
11. Mixing of acid or base with water results in decrease in the concentration of ions ( `H_3O^+` / `OH^-`) per unit volume. Such a process is called dilution.
12. While diluting an acid, always add acid to water, and not water to acid.
13. pH scale measures the hydrogen ion concentration in a solution. If pH = 7→ neutral solution, pH < 7 → acidic solution, pH > 7→ basic solution.
14. Salts of a strong acid and a strong base are neutral (pH value 7); strong acid - weak base are acidic (pH value < 7); strong base - weak acid are basic (pH value > 7).
15. Common salt is used as an important raw material for many products.
16. Sodium hydroxide is formed by decomposition of sodium chloride when electricity is passed through its solution. This process is called chlor-alkali process.
17. Bleaching powder (calcium oxychloride) is produced by action of chlorine on dry slaked lime [`Ca(OH)_2`].
18. Baking soda (sodium hydrogen carbonate) is formed by passing `CO_2` on ammoniacal brine. It is mild, non-corrosive basic salt, so it can be used to neutralise excess acid in the stomach and provides relief.
19. Baking powder is mixture of baking soda and a mild edible acid such as tartaric acid.
20. Sodium carbonate (washing soda) is obtained by heating baking soda; recrystallisation of sodium carbonate gives washing soda. It is a basic salt.
21. Water of crystallisation is fixed number of water molecules present in one formula unit of a salt. `CuSO_4.5H_2O`, 5 molecules of water are present in one formula unit of copper sulphate.
22. On heating Gypsum (`CaSO_4.2H_2O`) at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate `\left(CaSO_4.\frac12H_2O\right)` which is called Plaster of Paris.
