Analytical Chemistry — Part 1: Colours, sodium hydroxide and ammonium salts: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Detective work with test tubes

Part 1 — Analytical chemistry finds out what a substance is made of. In qualitative analysis we identify the ions in a salt from simple observations: its colour, and the precipitate it gives with an alkali.

In this lesson you will

State the colours of common salts of copper, iron, zinc and lead and of their solutions
Predict the precipitate formed when sodium hydroxide is added to a salt solution, and what excess alkali does
Write balanced equations for the precipitation reactions
Describe how warming with sodium hydroxide shows that a salt is an ammonium salt

Table of four clues used in qualitative analysis and what each tells us
Clue	What it tells us
Colour of the solid or solution	Narrows down the metal (blue often means copper)
Colour of a hydroxide precipitate	Identifies the cation
Whether the precipitate dissolves in excess alkali	Separates look-alike cations (zinc, lead)
A gas given off on warming with alkali	Shows ammonium ions

Concept

Colours of salts and their solutions

Salts of sodium, potassium, calcium, zinc, lead and ammonium are white solids and give colourless solutions. Salts of some transition metals are coloured, and the colour of the solid often differs from that of the solution.

Table of the colours of common salts as solids and in solution
Substance	Solid	Solution in water
Copper(II) sulphate (hydrated)	Blue crystals	Blue
Copper(II) chloride	Bluish-green	Green to blue-green
Iron(II) sulphate (hydrated)	Pale green crystals	Pale green
Iron(III) chloride	Yellowish-brown	Yellowish-brown
Zinc sulphate, lead nitrate	White	Colourless
Potassium permanganate	Purple-black	Purple (pink when dilute)

Concept

Coloured oxides and other compounds

Some insoluble compounds have colours worth remembering, because they appear as residues and precipitates in many reactions.

Table of the colours of oxides and carbonates of copper, zinc, lead and iron
Compound	Colour
Copper(II) oxide, CuO	Black
Copper(II) carbonate	Green
Zinc oxide, ZnO	White when cold, yellow when hot
Lead(II) oxide, PbO	Yellow
Red lead, Pb₃O₄	Bright red
Lead(IV) oxide, PbO₂	Dark brown
Iron(III) oxide, Fe₂O₃	Reddish brown

Cookies