STUDENT SPACE · TOPIC 6

Precipitation and method selection

Prepare an insoluble salt and choose a method from solubility.

The essentials

Mix two soluble salts that supply the positive and negative ions of the required insoluble salt.
Only the ions forming the solid appear in the net ionic equation.
Filter to collect the insoluble salt as the residue. Rinse it with distilled water.

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

Where is the target product after filtering a precipitation mixture, and why wash it?

Need a hint for Question 1?

Decide whether the target salt is soluble before choosing reagents.

Check Question 1

It is the residue. Washing removes soluble impurities.

Choose two soluble reagents for barium sulfate and name the ions that form the precipitate.

Need a hint for Question 2?

Choose soluble compounds supplying the two required ions.

Check Question 2

For example, BaCl₂(aq) and Na₂SO₄(aq); Ba²⁺ and SO₄²⁻ form BaSO₄(s).

Choose a preparation method for sodium sulfate from sulfuric acid and sodium hydroxide. Explain why filtration cannot remove excess NaOH.

Need a hint for Question 3?

A dissolved excess cannot be removed using ordinary filtration.

Check Question 3

Use titration, repeat without indicator, then crystallise. Dissolved NaOH passes through filter paper.

Use Lesson 6 in your worksheet for written practice. Your teacher will discuss those answers in class.