Question 1

Why is a saturated solution cooled slowly for crystallisation?

Accepted Answer

Most solids become more soluble as temperature rises, so a hot saturated solution can be cooled below its preparation temperature without losing solvent. Slow cooling produces fewer nuclei and gives each nucleus more time to grow into a larger, better-formed crystal.

Question 2

What happens to an insoluble impurity during crystallisation?

Accepted Answer

It is removed by hot filtration before the clear filtrate is allowed to cool. A soluble impurity may remain in the mother liquor and is separated from the crystals after crystallisation.

Question 3

Why is NaCl not described as an octahedral crystal like potash alum?

Accepted Answer

NaCl forms a cubic rock-salt lattice in which each ion is surrounded by six oppositely charged ions. Potash alum commonly forms octahedral macroscopic crystals, so their crystal habits and structures must be kept distinct.

Question 4

How is a solid purified by sublimation?

Accepted Answer

The solid is heated in a dish beneath an inverted funnel whose cool inner surface is covered with filter paper. Vapour travels to the cooler surface and deposits there as a pure crystalline solid, while a non-volatile impurity remains in the dish.

Question 5

Why is ammonium chloride's white smoke a special case?

Accepted Answer

On strong heating, NH4Cl dissociates into NH3 and HCl. When the vapours cool, they re-associate to form NH4Cl smoke, so the process is not direct sublimation of one chemically unchanged substance.

Crystallisation & Sublimation Chamber — Grow a Lattice, Then Lift a Solid Into the Vapour

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Practice