C7.3 Preparation of salts

What is a salt?

• Salts are compounds formed when a metal or ammonium ion (NH₄⁺) replaces the H+ ions in an acid

What is the composition of a salt?

• Have cation and anion:

  • Cation comes from the metal ions from either the metal (hydroxide, oxide, carbonate) or ammonium reactant

  • Anion comes from the acid used

What is a hydrated substance?

• A substance that is chemically combined with water

What is an anhydrous substance?

• A substance containing no water

Describe the preparation, separation and purification of soluble salts by the reaction of an acid with an alkali (soluble base) by titration

• Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange)

• Add the acid into the burette and note the starting volume

• Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour

• Note and record the final volume of acid in the burette and calculate the volume of acid added (starting volume of acid - final volume of acid)

• Add this same volume of acid into the same volume of alkali without the indicator

• Heat the resulting solution in an evaporating basin to partially evaporate, leaving a saturated solution (crystals just forming on the sides of the basin or on a glass rod dipped in and then removed)

• Leave to crystallise, decant excess solution and allow crystals to dry

Describe the preparation, separation and purification of soluble salts by the reaction of an acid with excess metal/excess insoluble base/excess insoluble carbonate

• Add dilute acid into a beaker and heat using a bunsen burner flame

• Add the insoluble metal, base or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid)

• Filter the mixture into an evaporating basin to remove the excess base

• Heat the solution to evaporate water and to make the solution saturated. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end

• Leave the filtrate in a warm place to dry and crystallize

• Decant excess solution and allow crystals to dry or blot to dry with filter paper

Describe the preparation of insoluble salts by precipitation

• To prepare an insoluble salt, a precipitation reaction is used since the salt is formed in the form of a precipitate

• Dissolve two soluble salts in water and mix together using a stirring rod in a beaker

• Filter to remove precipitate from mixture

• Wash the precipitate with distilled water to remove traces of other solutions

• Leave in an oven to dry