Learning Objectives:

• Describe and explain the tests for ions using sodium hydroxide solution

• Explain how precipitation reactions can be used to test for some ions

Lesson 3

Silver nitrate and sodium chloride dissolve in water. The solutions are colourless.

Mix the solutions and a white precipitate forms.

A solution of sodium chloride...

... contains sodium ions, Na+, and chloride ions, Cl-.

A solution of silver nitrate...... contains silver ions, Ag+, and nitrate

ions, NO3-.

Mixing the two solutions puts all the ions together.The silver and chloride ions immediately form a white precipitate of solid silver chloride.

The sodium and nitrate ions remain in solution.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)Silver nitrate + sodium chloride silver chloride + sodium nitrate

A chemical equation summarises this precipitation reaction.

Practical – Testing for Cations

APut 1 cm depth of each of the cation solutions into six different test tubes.

BAdd a few drops of sodium hydroxide solution to each test tube, and observe what happens.

C If a precipitate forms, add a further 1 cm of sodium hydroxide. Again observe any changes that might occur.

Cation Symbol PrecipitateAmmonium NH4

+ (aq) none

Aluminium Al3+ (aq) WhiteCalcium Ca2+ (aq) WhiteCopper(II) Cu2+ (aq) blueIron(II) Fe2+ (aq) GreenIron(III) Fe3+ (aq) brown

1. In the cation column what does the number in the bracket correspond to?2. Which of the cation(s) need further testing?

Reactions of adding Sodium hydroxideReactions of adding Sodium hydroxide

Further testingAmmonium Ions• Heat the sample with concentrated sodium

hydroxide. A smelly alkaline gas (ammonia) will be produced. It turns universal indicator paper blue/purple.

•Aluminium and Calcium Ions

If you add excess (more!) sodium hydroxide to a calcium precipitate and aluminium precipitate, different things will happen.

Aluminium precipitate

Calcium precipitate no change, will remain white

Will dissolve

Excess NaOH

Excess NaOH

Task

• Worksheet – C3.3 3b ‘More about Cations’

Only questions 1, 2 and 3.DO NOT WRITE ON THE SHEET!