Use of Ammonium Hydroxide and Sodium Hydroxide

Determination of chemical components of a substance is called analysis of that substance. The analysis can be either Quantitative or Qualitative. Quantitative analysis involves determination of composition of a mixture, whereas, qualitative analysis involves identification of an unknown substance in a mixture. Identification of unknown substance during the qualitative analysis is done with the help of reagents. Alkalis are the important laboratory reagents. The most commonly used reagents are: sodium hydroxide which is a strong alkali and ammonium hydroxide which is a weak alkali. These alkalis react with the solution of metal salts to form precipitates of different coloured hydroxides, which may be soluble or insoluble in excess of these alkalis. Formation of precipitates of different coloured hydroxides helps in identifying the metal ion (cation) present in the salt. The salts on dissolving in water form salt solutions which are of different colours. The colour of the salt solution depends upon the cation and anion present in solution. Hot and concentrated alkalis react with certain metals like zinc, aluminium and form corresponding soluble salts and liberate hydrogen gas, which burns with pop sound. Most of the metal oxides are basic in nature, they dissolve in water forming hydroxides. These metal oxides and their hydroxides neutralise acids but do not react with bases. Some of the metallic oxides and hydroxides react with both acids and alkalis to form salt and water. Thus, they are amphoteric in nature.

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  • Q1

    The colour of the precipitate formed on adding NH4OH solution to iron (II) sulphate solution is

    Marks:1
    Answer:

    dirty green.

    Explanation:

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  • Q2

    A solution of a metal salt reacts with caustic soda solution to obtain a white precipitate. This precipitate is sparingly soluble in excess of caustic soda solution. If the metal salt is colourless, then the metal present in the salt is

    Marks:1
    Answer:

    calcium.

    Explanation:

    Iron and copper salts are coloured. Calcium and zinc salts are colourless. Solution of calcium and zinc salt both gives white precipitate of calcium hydroxide and zinc hydroxide respectively on reacting with caustic soda solution. The precipitate of calcium hydroxide is sparingly soluble in excess of caustic soda solution whereas zinc hydroxide precipitate is soluble completely in excess alkali.

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  • Q3

    A metal oxide which produces salt and water on reaction with alkali as well as with acid is

    Marks:1
    Answer:

    ZnO

    Explanation:

    Amphoteric oxides react with both acids and alkalis to form salt and water. ZnO is an amphoteric oxide and all others are basic oxides.

     

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  • Q4

    Aluminium reacts with sodium hydroxide and water to form sodium meta aluminate. The sodium hydroxide used in this case is

    Marks:1
    Answer:

    boiling.

    Explanation:

    Aluminum reacts with boiling sodium hydroxide and water to form sodium meta aluminate.

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  • Q5

    To a yellow coloured salt solution, NaOH is added. It forms reddish brown precipitate which is insoluble in excess NaOH. Pick up the correct pair of salt and the precipitate formed.

    Marks:1
    Answer:

    salt= FeCl3 , precipitate = Fe(OH)3

    Explanation:

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