Water of Crystallisation

Question 1

what are the two types of salts?

Answer 1

anhydrous and hydrated

Question 2

what do all solid salts consist of?

Answer 2

a lattice of positive and negative ions - in some salts, water molecules are incorporated in the lattice too and this is called water of crystallisation

Question 3

what is a hydrated salt?

Answer 3

a solid salt containing water of crystallisation

Question 4

what is an anhydrous salt?

Answer 4

when a salt doesn’t contain any water of crystallisation

Question 5

what is a hydrated salt lattice like?

Answer 5

• water molecules have a small positive charge on the hydrogen atoms and a small negative charge on the oxygen atoms
• this means they are attracted to the ions in the lattice and are held in place

Question 6

how to calculate how much water of crystallisation a salt contains

Answer 6

• one mole of hydrated salt always has a particular number of water of crystallisation - its formula shows how many
• many hydrated salts lose their water of crystallisation when heated, to become anhydrous - if you know the mass of the salt when its hydrated and when its anhydrous you can work out its formula:

1. work out what mass of the anhydrous and hydrated salt you have
2. calculate the number of moles of water lost
3. calculate the number of moles of anhydrous salt made
4. work out the ratio of moles of anhydrous salt to moles of water
5. X must be a whole number