Q33 of 400 Page 131

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

Dull appearance is acquired by the surface of some metals when exposed to air for a long time because a thin layer of oxides, carbonates or sulphide is formed on the metal surface. This can be seen by the action of various gases present in air.

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31 Which property of graphite is utilized in making electrodes ? 32 Name two non-metals which are both brittle and non-ductile. 34 Complete and balance the following equations:

(a)


(b)


(c)


(d)

35 Fill in the following blanks with suitable words:

(a) Magnesium liberates ________ gas on reacting with hot boiling water.


(b) The white powder formed when magnesium ribbon burns in oxygen is of ________


(c) Ordinary aluminium strips are not attacked by water because of the presence of a layer of ________ on the surface of aluminium.


(d) A metal having low melting point is ________ but a non-metal having very high melting point is ________


(e) Calcium is a ________ reactive metal than sodium.