Chemistry 5070/11 — October/November 2015

Walkthrough

This question asks which process is used when preparing a soluble salt. A soluble salt dissolves in water, so after the salt has been made it is dissolved in the reaction mixture. To get the solid salt crystals you need to remove the water, and evaporation (or crystallisation) is the method used. Evaporation leaves the salt behind as the solvent is removed.

Option A, electrolysis, is used to decompose compounds, not to prepare a soluble salt directly.

Option C, melting, would require heating a solid, and would not produce a salt from solution.

Option D, precipitation, is used for insoluble salts, not soluble ones.

Key Takeaways

• Soluble salts are usually prepared by reacting an acid with a base, alkali, metal or carbonate, then evaporating or crystallising to obtain the solid salt.
• Precipitation is the method used for insoluble salts.
• Evaporation separates a dissolved solid from its solvent.

Common Mistakes

• Choosing precipitation: precipitation is used only when the product is insoluble.
• Confusing evaporation with electrolysis: evaporation is a physical separation technique; electrolysis uses electricity to cause chemical decomposition.

Things to Be Careful About

• Read the word "soluble" – it signals that the salt stays in solution and must be recovered by evaporation/crystallisation.
• The question asks for the process used to obtain the salt, not the method of making the salt itself.