Making Soluble Salts

Question 1

how do we make soluble salts?

Answer 1

by neutralising acids and crystallising the resulting solution:

acid(aq) + base(s)/alkali(aq) -> salt(aq) + water(l)

Question 2

method for making pure, dry crystals of a soluble salt

Answer 2

1. make the salt solution by using one of the two methods - the insoluble base method or the titration method
2. you crystallise the salt
   this is done in the same way, regardless of the method you use to make the salt solution

Question 3

when do we use the insoluble base method?

Answer 3

when making salts that do not contain Na+, K+ or NH4+ ions

Question 4

the insoluble base method

Answer 4

• choose the acid and the base you use depending on which salt you want to make:
• the acid is usually one of hydrochloric, sulfuric or nitric
• the base is usually a sold metal oxide
• you can also use a solid metal carbonate as your base - the only different is that you make some carbon dioxide so there is a bit of fizzing in the reaction

Question 5

how to do the insoluble base method?

Answer 5

1. heat the acid - hot acid has more energy so it reacts faster with the base
2. while stirring, add base until no more will dissolve - this guarantees the base is in excess so that we know all of the acid has been used up
3. filter out the excess base

Question 6

the titration method

Answer 6

• we use this method to make salts containing Na+, K+ or NH4+ ions, because their bases are all soluble so you wouldn’t be able to filter out the excess if you used the insoluble base method
• you need to choose an acid and alkali to titrate depending on which salt you want to make:
• the acid is usually one of hydrochloric, sulfuric, or nitric
• the alkali is usually one of sodium hydroxide, potassium hydroxide, or ammonium hydroxide

Question 7

how to do the titration method

Answer 7

1. perform a titration to measure the volumes of acid and alkali that neutralise each other
2. repeat the titration but this time: don’t put the indicator in and use the burette to add exactly the right volume for neutralisation

Question 8

crystallisation method

Answer 8

1. heat the salt solution until crystals start forming - you can tell by looking at the edges, or by occasionally dipping in a glass rod and this causes the solution to become saturated
2. allow the solution to cool in an evaporating basin for a few days - this lowers the solubility of the salt so that lots of solid crystals form
3. filter out the crystals - this removes them from the excess salt solution in the basin
4. dry the crystals by dabbing them with filter paper - this removes the last traces of water from the crystals