what is base

Acid base and salt of Class 8


Bases are hydroxides of metals.

A base is a compound which contains one or more hydroxyl ions and on reacting with an acid produces a salt.

e.g. Sodium hydroxide NaOH

Calcium hydroxide Ca(OH)2

Aluminium hydroxide AI(OH)3

Alkalies: Bases which dissolves in water are called alkalies. e.g., KOH, NaOH.







Sodium hydroxide

Caustic soda


In manufacture of soap, paper, pulp, rayon, refining of petroleum etc.

Potassium hydroxide

Caustic potash


In alkaline storage batteries, manufacture of soap, absorbing CO2 gas etc.

Calcium hydroxide

Slaked lime


In manufacture of bleaching powder, softening of hard water etc.

Magnesium hydroxide

Milk of magnesia


As an antacid to remove acidity from stomach.

Aluminium hydroxide


As foaming agent in fire extinguishers.

Ammonium hydroxide


In removing grease stains from clothes and in cleaning window panes.

Preparation of Bases:

By direct combination of metals with oxygen:

Most of the metals when heated, catch fire and burn to form their oxides.

The oxides of metals are commonly called basic oxides, because they react with acids to form salt and water as only products.

(i) By direction combination with oxygen: When metals burn in oxygen they form metal oxides which are basic in nature.


(ii) By dissolving metal oxides in water: On dissolving in water these oxides form hydroxides.

Strong and weak bases:

  • The oxides and hydroxides of sodium and potassium are strong bases. They are very soluble in water.
  • The oxides and hydroxides of all other metals are weak bases. They are very soluble in water.
  • The oxides and hydroxides of all other metals are weak bases.
  • They are sparingly soluble in water or insoluble in water. Ammonium hydroxide  obtained by dissolving ammonia gas in water is also a weak base.

Physical Properties of Base

  • Taste : All bases/alkalis have a bitter taste.
  • Slippery touch: All bases and alkali solutions have a soupy touch like that of soap.
  • Corrosive action: The bases/alkalis, on coming in contact with  skin, produce very painful blister. Sodium hydroxide and potassium hydroxide are extremely corrosive in nature and should not be touched with bare hands.

General chemical properties of bases/alkalies:

Action with heat:

  • Sodium hydroxide (NaOH) and potassium hydroxide (KOH) do not decompose on heating. However, they melt, when strongly heated.
  • Ammonium hydroxide decomposes rapidly on warming to form ammonia gas and water.
  • Action with metals: Metals like zinc, tin and aluminium react with strong alkalieslikeNaOH (caustic soda), KOH (caustic potash) to evolve hydrogen gas..

  • Action with non-metallic oxides: Acids react with metal oxides, but bases react with oxides of non-metals to form salt and water.To score More in your class 8 refer NCERT solutions for class 8 .
  • .eg. 2NaOH (aq) + CO2(g) Na2CO3(aq) + H2O(l)

    Ca(OH)2(s) + SO2(g) CaSO3(aq) + H2O(l)

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