We use soaps for bathing as well as washing. Soaps are slippery in nature. Do you know why? Soaps are slippery due to the presence of ‘base’. Bases are chemical substances that are corrosive and bitter in taste. A lot of bleaches, soaps, detergents, toothpaste, etc. , contain bases. In contrast to acids which release hydrogen ions in water, bases release hydroxide ions in water.
Thus, the chemical substances that release hydroxide ions when dissolved in water are called as bases. Examples: Sodium hydroxide (NaOH) and Potassium hydroxide (KOH).
Water soluble bases are called Alkalis. Bases like sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide are highly soluble in water and hence they are called alkalis. Certain chemical substances which do not release hydroxide ions when dissolved in water also behave as bases. Examples: Sodium carbonate, Sodium bicarbonate, Calcium carbonate etc.
Sodium carbonate (Na2CO3) is commercially called as washing soda. Similarly sodium bicarbonate (NaHCO3) is commercially called as baking soda. Caustic soda is sodium hydroxide (NaOH) and caustic potash is potassium hydroxide (KOH).
Classify the following substances.Sodium oxide, Potassium hydroxide, Calcium oxide, Copper oxide, Calcium hydroxide, Ammonium hydroxide, Ferric hydroxide, Zinc oxide
Properties of Bases
a. Physical properties
• Bases generally exist in solid state but some bases exist in liquid state also. E.g. Ammonium hydroxide, calcium hydroxide
• Bases give soapy touch only in aqueous medium not in dry nature.
• Bases are bitter in taste.
• Bases are corrosive in nature. When come in contact with the skin frequently they form painful blisters.
• Bases are generally colourless
• Bases also change the colour of the indicators. Red litmus paper turns blue when treated with bases. Similarly, they turn methyl orange to yellow and phenolphthalein to pink colour
• Bases also conduct electricity in aqueous solution.
b. Chemical properties
i. Reaction with metals
Generally metals do not react with bases. Metals like aluminium and zinc react with bases like sodium hydroxide forming aluminates and release hydrogen.
Aluminum + Sodium hydroxide + Water → Sodium aluminate + Hydrogen
2Al + 2NaOH + 2H2O → 2NaAlO2+ 3H2↑
ii. Reaction with non-metal oxides
All bases react with non metallic oxides to form salt and water. For example, sodium hydroxide reacts with carbon dioxide to form sodium carbonate.
Sodium hydroxide + Carbon dioxide → Sodium carbonate + Water
2NaOH + CO2 → Na2CO3 + H2O
iii. Reaction with ammonium salts
Bases react with ammonium salts to form metal salts, ammonia gas and water.
Sodium hydroxide + Ammonium chloride → Sodium chloride + Ammonia + Water
NH4Cl + NaOH → NaCl + NH3↑ + H2O
Though acids and bases have some unique properties there are certain similarities between them. Some of them are given below.
* They are corrosive in nature.
* They undergo ionization in aqueous solution
* They conduct electricity in aqueous solution.
* They undergo neutralization reaction.
* Some of the differences between acids and bases are given in Table 14. 3.
Uses of Bases
i. Potassium hydroxide is used to make bathing soaps.
ii. Sodium hydroxide is used to make washing soaps.
iii. Sodium hydroxide is also used in paper industries, textile industries and in the preparation of medicines.
iv. Calcium hydroxide is used for white washing.
v. Aluminum hydroxide and magnesium hydroxides are used in antacids to cure acidity problems.
vi. Ammonium hydroxide is used to manufacture fertilizers, nylon, plastics and rubber.