Question

Give reasons for the following:
(a) Carbon can reduce zinc oxide to zinc but not calcium oxide to calcium.
(6) Copper and aluminium wires are usually used for electricity transmission.
(c) Alloys are used in electrical heating devices rather than pure metals.
(d) Gold ornaments look new even after several years of use.
(e) An iron grill should be painted frequently.​

Answer 1

Carbon can reduce zinc oxide to zinc but not calcium oxide to calcium

• BECAUSE Carbon is more reactive than zinc (which can displace zinc from the zinc oxide solution) but Carbon is less reactive than Calcium.

Copper and aluminium wires are usually used for electricity transmission

• BECAUSE  Copper and aluminium both are good electric conductor, having low resistance and considerate ductility to be drawn into transmission wires.

Alloys are used in electrical heating devices rather than pure metals

• BECAUSE Alloys have a greater thermal resistivity and high melting point in comparison to alloys, so alloys don't oxidise even on high temperature.

Gold ornaments look new even after several years of use

• BECAUSE Gold is one of the least reactive metal that doesn't react with air or moisture and do not corrode, maintaining its shine for years.

An iron grill should be painted frequently

• BECAUSE  By painting an iron grill frequently we make sure to avoid the contact of iron surface with air or moisture and prevent it from being corroded by rust.

Answer 2

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(a) Carbon can reduce zinc oxide to zinc but not calcium oxide to calcium.

Rᴇᴀsᴏɴ :

➣ Metals of low reactivity cannot displace high reactive metals from their salts.

➣ For this reason calcium oxide cannot reduced easily by carbon, because oxide of calcium is placed above carbon (in reactivity series).

➣ Zinc oxide can be reduced to zinc by carbon, because oxides of zinc is placed below carbon (in reactivity series).

(b) Copper and aluminium wires are usually used for electricity transmission.

Rᴇᴀsᴏɴ :

↝ Copper and aluminum are good conductors of electricity, because the resistivity of copper and aluminum are very low.

↝ They have very low resistance and allow the flow of current through them easily.

(c) Alloys are used in electrical heating devices rather than pure metals.

Rᴇᴀsᴏɴ :

➻ Alloys generally have higher melting point as compared to other elements.

➻ The resistivity of an alloys is more than the resistivity of a pure metal.

➻ And alloy does not burn (oxidized) easily at higher temperature.

(d) Gold ornaments look new even after several years of use.

Rᴇᴀsᴏɴ :

➛ Gold is a highly unreactive metal which remains unaffected by air, water vapour and other gases in the atmosphere.

(e) An iron grill should be painted frequently.

Rᴇᴀsᴏɴ :

➾ To prevent rusting, it should painted frequently.

➾ We know that iron is a reactive metal and it easily react with atmospheric oxygen and moisture.

➾ Rusting causes damage to iron substances by making them weak.