3.4.5 Making soluble salts

Question 1

How to make soluble salts?

Answer 1

By neutralising acid and crystallising the resulting solution.

Question 2

How to make soluble salt crystals using an insoluble base?

Answer 2

• Add excess base to hot acid. Hot acid has more energy so it will react faster with the base.
• Filter out the excess base. The salt solution is the filtrate and is collected in an evaporating basin.
• Heat the salt solution until the crystals start to form around the edges. This saturates the solution so that crystals form more efficiently.
• Leave the salt solution to crystallise. Evaporation of water allows crystals to form.
• Dry the crystals by dabbing them with a paper towel.

Question 3

How to make soluble salts using a soluble base?

Answer 3

• Do a titration to measure the volumes of acid and alkali that neutralise each other:
  - Use a volumetric pipette to measure out 25cm cubed of an alkali into a conical flask.
  - Add a few drops of phenolphthalein indicator to the flask. Universal indicator doesn’t work because it changes colour too gradually.
  - Fill a burette with an acid and clamp it above the conical flask.
  - Measure the initial volume of the acid on the burette to the nearest 0.05cm cubed.
  - Use the burette to add the acid into the conical flask, one drop at a time. Swirl the flask as you add each drop.
  - Close the burette tap as soon as the colour in the conical flask changes.
  - The start colour will be pink and the end colour will be colourless.
  - Measure the final volume of known acid on the burette, to the nearest 0.5cm cubed.
  - Calculate titre (final volume - initial volume).
• Repeat the titration, but this times don’t put the indicator in. Use the burette to add exactly the right volume for neutralisation.
• Heat the salt solution until the crystals start to form around the edges. This saturates the solution so that crystals form more efficiently.
• Leave the salt solution to crystallise. Evaporation of water allows crystals to form.
• Dry the crystals by dabbing them with a paper towel.