STUDENT SPACE · TOPIC 4

Preparing soluble salt crystals

Explain the purpose of excess solid, filtration and crystallisation.

The essentials

Copper does not react with dilute sulfuric acid to release hydrogen. Use copper(II) oxide instead.
Excess insoluble solid ensures the acid has reacted completely.
Gently evaporate some water to concentrate the solution, then allow it to cool.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

Why is titration needed instead when both acid and base are soluble?

Need a hint for Question 1?

Name what is in the residue and filtrate after each step.

Check Question 1

Excess soluble reagent cannot be removed by filtration, so exact reacting volumes are needed.

Why is CuO added in excess, and why must the mixture be filtered before crystallisation?

Need a hint for Question 2?

Identify what remains dissolved and what remains solid.

Check Question 2

Excess CuO uses up the acid. Filtration removes unreacted insoluble CuO, leaving the salt solution.

Define water of crystallisation and interpret CoCl₂·6H₂O.

Need a hint for Question 3?

The dot indicates a fixed number of water molecules in the crystal.

Check Question 3

Water molecules present in hydrated crystals; six H₂O molecules per CoCl₂ formula unit.

Use Lesson 4 in your worksheet for written practice. Your teacher will discuss those answers in class.