Water of crystallisation

Question 1

What happens when salts dissolve in water?

Answer 1

Ions in its lattice separate and are surrounded by a sphere of water molecules (become hydrated ions)

Question 2

What happens during crystallisation?

Answer 2

Water molecules evaporate off, and electrostatic attraction pulls the ions together to form a crystal lattice

Question 3

What is water of crystallisation?

Answer 3

Water molecules bonded into a salt’s solid crystal structure. They did not evaporate when substance was heated, so stayed in the crystal lattice

Question 4

Why does water of crystallisation have more mass?

Answer 4

Also involves the mass of the bonded water molecules

Question 5

What is a hydrated salt?

Answer 5

A solid salt containing water of crystallisation

Question 6

What is an anhydrous salt?

Answer 6

A salt not containing water of crystallisation

Question 7

How is water of crystallisation represented?

Answer 7

a dot then H2O - that many water molecules per formula unit (x if unknown)

Question 8

How do you turn hydrated salts into anhydrous salts?

Answer 8

Vigorous heating

Question 9

How do you turn anhydrous salts into hydrated salts?

Answer 9

Cannot do it in one step because:
- It requires an exact amount of water
- Can’t ensure exact amount of added water will be evenly distributed throughout the sample
- A small amount of anhydrous salt will dissolve in the water

Question 10

What are anhydrous salts used for?

Answer 10

Drying agents (to remove some water from a sample), and to test for water - if dissolves to form blue solution or forms blue solid, sample has water. If it remains a white powder, sample does not have water