Ion Tests, Gas Tests

How to test for cations?

1. Add dilute NaOH

   Few drops: white ppt; Excess: ppt redissolves → cation

   Few drops: white ppt; Excess: no change → not cation

2. Add dilute ammonia

   Few drops: white ppt; Excess: ppt redissolves → Zn

   Few drops: white ppt; Excess: no change → Al

How to test for transition metal ions?

1. Add NaOH

   Colourful ppt forms

Iron (II) ppt colour?

Green (orange-brown if left standing)

Iron (III) ppt colour?

Orange-brown (rusty)

Copper ppt colour?

Blue

How to test for Carbonate ions (CO₃^2-)?

1. Add HCl

2. CO₂ released; bubble through limewater

3. Turns milky-white

   CO₃^2-_(aq) + 2H⁺_(aq) → CO_2(g) + H₂O_(l)

How to test for Sulfate ions (SO₄^2-)?

1. Add HCl (removes CO₃^2-)

2. Add Barium chloride (BaCl₂)

3. White ppt forms

   Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_4(s)

How to test for Halide ions (Cl^-, Br^-, I^-) *Group 7?

1. Add nitric acid (HNO₃) to acidify

2. Add silver nitrate (AgNO₃)

   1. milk: chloride

   2. cream: bromide

   3. butter: iodide

3. Differentiate if white/cream by adding ammonia

After adding silver nitrate (AgNO₃), ppt is white therefore

It is chloride

After adding silver nitrate (AgNO₃), ppt is cream therefore

It is bromide

After adding silver nitrate (AgNO₃), ppt is yellow therefore

It is iodide

After adding silver nitrate (AgNO₃), ppt is white/cream; to differentiate, you add ammonia, the ppt dissolves therefore

After adding ammonia to white/cream ppt, it is chloride

After adding silver nitrate (AgNO₃), ppt is white/cream; to differentiate, you add ammonia, the ppt is insoluble therefore

After adding ammonia to white/cream ppt, it is bromide

How to test for Nitrates (NO₃^-)?

Reduce to ammonia:

1. Add NaOH

2. Add aluminium foil

3. Heat, ammonia gas releases

4. Put damp red litmus paper on top → turns blue → ammonia present

How to test for Hydrogen (H₂)?

Lighted wooden splint makes pop sound in tube

How to test for Oxygen (O₂)?

Glowing wooden splint reignites in tube

How to test for Carbon Dioxide (CO₂)?

Bubble through limewater [Ca(OH)₂] → milky-white

How to test for Ammonia (NH₃)?

Choking smell
Turns damp red litmus paper blue