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Explain why the surfaces of some metals ...

Explain why the surfaces of some metals become dull when exposed to air for sometime.

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The surface of some metals acquires a dull appearance when exposed to air for a long time due to the formation of a thin layer of oxide, carbonate or sulphide on their surface by the slow action of the various gases present in air.
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Give reasons for the following : (i) Carbonate and sulphide ores are converted into oxides before reduction. (ii) Aluminium is rendered passive when it is dipped in conc. HNO_(3) (iii) Nonmetals do not displace hydrogen from acids. (iv) An aqueous solution of CuSO_(4) cannot be stored in an iron vessel. (v) The surface of some metals become dull when left exposed to air for a long time.

Explain why, the surface of some metals acquires a dull appearance when exposed to air for a long time.

Knowledge Check

  • Crystals of ferrous sulphate when exposed to air become

    A
    red
    B
    brown
    C
    colourless
    D
    black

  • The red or Orange coating that forms on the surface of iron when exposed to air and moisture for some time is called

    A
    galvanisation
    B
    electroplating
    C
    rust
    D
    reduction

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    Why do some metals acquire a dull appearance on exposure to air for a long time ?

    Name the chemical compounds formed on the surface of silver, copper and iron metals when exposed for sometime to atmosphere.

    What happens to the crystals of washing soda when exposed to air ?

    Metals have lustre (shine). Give reason why some metal articles become dull and loose their shine?

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