Form 1:
Classification of substances

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Sublimation/deposition

Some solids on heating do not melt to a liquid but change directly to a gas.

The process by which a solid changes to a gas is called sublimation.

The gas cools back and changes directly to a solid.

The process by which a gas changes to a solid is called deposition. Sublimation and deposition therefore are the same but opposite processes.

Some common substances that undergo sublimation/ deposition include:

(i)Iodine
(ii)Carbon(IV)oxide
(iii)Camphor
(iv)ammonium chloride
(v)Iron(III)chloride
(vi)Aluminum(III)chloride
(vii) benzoic acid

If a mixture has any of the above as a component, then on heating it will change to a gas and be deposited away from the source of heating.

Procedure

Place about one spatula full of ammonium chloride crystals into a clean dry 100cm3 beaker.

Add equal amount of sodium chloride crystals into the beaker.

Swirl to mix.

Place the beaker on a tripod stand.

Put about 100cm3 of water into another beaker.

Place carefully the beaker containing water on top of the beaker containing the solid mixture.

Light/ignite a burner and heat the solid.

Set up of apparatus:


Observation

(i)With ammonium chloride/common salt mixture

White fumes produced.
White sublimate deposited
Colourless residue left

(ii)With Iodine/common salt mixture

Purple fumes produced.
Dark grey sublimate deposited
Colourless residue left

Explanation

(i)On heating a mixture of ammonium chloride and common salt, white fume of ammonium chloride is produced. The white fumes solidify as white sublimate on the cooler parts. Common salt remains as residue.

Chemical equation:

(ii)On heating a mixture of Iodine and common salt, a purple fume of Iodine vapour is produced. The purple fumes solidify as dark grey sublimate on the cooler parts. Common salt remains as residue.

Chemical equation:
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