What are bases and alkalis

A base is a substance that will neutralize an acid to yield salt and water only. Oxides and hydroxides of metals are bases.
The common examples of basic oxides are sodium oxide, Na2O, potassium oxide, K2O and magnesium oxide, NaO, potassium oxide, K2O, and magnesium oxide,mgO. These oxides are formed when the metals burn in air or oxygen. e.g
4Na + O2 - 2Na2O
4K + O2 - 2K2O
2M + O2 - MgO
Most of these metallic oxides are insoluble in water. some, however, dissolve in water to form hydroxide.
An alkali is a compound that produces hydroxyl ions, OH^-
Sodium and potassium hydroxides are strong alkalis because they ionize completely in an aqueous solution to produce negatively charged hydroxide ions. Calcium hydroxide is only slightly soluble in water. A calcium hydroxide solution produces full ions, hence, it is a weak alkali.

Physical properties of bases and alkalis

• Alkalis have a bitter taste
• Alkalis turn red litmus blue
  
• Alkalis has a soapy feel
• The concentrated form of the two caustic alkalies, NaOH and KOH, has a powerful corrosive action.
  
  

Chemical properties of bases and alkalis

Reaction with acids: all bases react with acid to form salt and water only. (Neutralization reaction),
Reaction with ammonium salts: alkalis react with ammonium salts in the presence of heat, to generate ammonia gas. This reaction can be used as a test for alkalis

Preparation of bases and alkalis

Most metals combine directly with oxygen when heated to form basic oxide.
2Ca + O2 - 2CaO
2Mg + O2 - 2MgO
4Fe + 3O2 - 2Fe2O3

Soluble oxides of metals dissolve in water to give alkalis
Na2O + H2O - 2NaOH
K2O + H2O - 2KOH
CaO + H2O - Ca(OH)2

Trioxocarbonate IV, except sodium and potassium trioxocarbonate IV, decomposes on heating to yield the oxide of the metal and carbon IV oxide gas
Zn CO3 - ZnO + CO2
CuCO3 - CuO + CO2


Uses of bases and alkalis

• Sodium hydroxide, NaOH, is used in the manufacture of paper, soap, dyes, and rayon from cellulose
• Sodium hydroxide is also used for refining petroleum, production of chemicals, and as a drying agent
• Potassium hydroxide, KOH, is used in making shaving creams and soft soaps
  
• Calcium hydroxide, Ca(OH)2 is used in making mortar, plaster of Paris, whitewash, cement, sugar refining, and neutralization of soil acidity
• Ammonium hydroxide, NH4OH, is used for bleaching clothes, as a detergent and solvent for grease
  
• Magnesium hydroxide, Mg(OH)2 is used as an anti-acid in toothpaste and medically as a mild laxative

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@Henkjan de Krijger