Qualitative Analysis: Tests for Ions, Gases and Functional Groups

Introduction to Qualitative Analysis

Welcome to the detective agency of Chemistry! Qualitative analysis is all about finding out what is in a sample. Unlike quantitative analysis (where we calculate "how much"), here we use our senses—watching for color changes, sniffing out gases (carefully!), and spotting precipitates.

Whether you are preparing for Unit 3 or Unit 6, mastering these tests is essential. Think of this chapter as your "Chemical Toolkit." If you can recognize these reactions, you can identify almost any unknown substance Pearson Edexcel might throw at you in an exam paper.

1. Testing for Cations (Positive Ions)

Flame Tests

Some metal ions give off a characteristic color when heated in a Bunsen burner flame. This happens because electrons get "excited" by the heat and then drop back down, releasing energy as light.

How to do it: Dip a clean nichrome or platinum wire into concentrated hydrochloric acid, then into the solid sample, and hold it in a non-luminous (blue) Bunsen flame.

The Colors You Must Know:

• Lithium \(Li^{+}\): Red
• Sodium \(Na^{+}\): Yellow/Orange
• Potassium \(K^{+}\): Lilac
• Magnesium \(Mg^{2+}\): No color (the energy released isn't in the visible spectrum!)
• Calcium \(Ca^{2+})\: Brick-red
• Strontium \)Sr^{2+}\): Red
• Barium \(Ba^{2+}\): Apple-green

Memory Tip: "Potassium is Purple-ish (Lilac)" and "Barium is Bright green (Apple)."

Testing for the Ammonium Ion \(NH_4^{+}\)

The test for \(NH_4^{+}\) doesn't involve a flame. Instead, we use chemistry to turn it into a gas we can smell.

The Process: Add aqueous sodium hydroxide \(NaOH\) to your sample and warm it gently.
Observation: Ammonia gas \(NH_3\) is released. It has a sharp, choking smell and turns damp red litmus paper blue.

Transition Metal Ions (with NaOH and \(NH_3\))

In Unit 6, you need to know how transition metal ions react with sodium hydroxide and ammonia. They often form colorful precipitates (solids that appear in a solution).

• Copper(II) \(Cu^{2+}\): Forms a blue precipitate. With excess ammonia, it dissolves to give a deep blue solution.
• Iron(II) \(Fe^{2+}\): Forms a slirty green precipitate.
• Iron(III) \(Fe^{3+}\): Forms a orange-brown precipitate.
• Manganese(II) \(Mn^{2+}\): Forms a pale pink/beige precipitate that darkens on standing.
• Chromium(III) \(Cr^{3+}\): Forms a grey-green precipitate. It dissolves in excess \(NaOH\) to give a dark green solution.
• Zinc(II) \(Zn^{2+}\): Forms a white precipitate. Because it is amphoteric, it dissolves in excess \(NaOH\) or \(NH_3\) to give a colorless solution.

Key Takeaway: If a precipitate disappears when you add more reagent, we say it "redissolves." This is a huge clue for identifying \(Zn^{2+}\), \(Cr^{3+}\), or \(Cu^{2+}\).

2. Testing for Anions (Negative Ions)

Carbonates \(CO_3^{2-}\) and Hydrogencarbonates \(HCO_3^{-}\)

The Test: Add a dilute acid (like \(HCl\)).
Observation: Effervescence (fizzing). The gas produced is carbon dioxide \(CO_2\).
The Gas Test: Bubble the gas through limewater. If it turns cloudy/milky, \(CO_2\) is present.

Sulfates \(SO_4^{2-}\)

The Test: Add acidified barium chloride \(BaCl_2\). We acidify it with \(HCl\) to remove any carbonates that might interfere.
Observation: A white precipitate of barium sulfate \(BaSO_4\) forms.

Halides (\(Cl^{-}\), \(Br^{-}\), \(I^{-}\))

This is a "two-step" test using silver nitrate and then ammonia to confirm.

Step 1: Add dilute nitric acid \(HNO_3\) (to remove impurities) followed by silver nitrate solution \(AgNO_3\).
Observations:

• Chloride \(Cl^{-}\): White precipitate (\(AgCl\))
• Bromide \(Br^{-}\): Cream precipitate (\(AgBr\))
• Iodide \(I^{-}\): Yellow precipitate (\(AgI\))

Step 2: Since white, cream, and yellow can look very similar, we add ammonia solution \(NH_3\):

• Silver chloride: Dissolves in dilute \(NH_3\).
• Silver bromide: Dissolves only in concentrated \(NH_3\).
• Silver iodide: Insoluble even in concentrated \(NH_3\).

Common Mistake: Never use Hydrochloric Acid to acidify a halide test! The \(Cl^{-}\) ions in the acid will react with the silver nitrate and give a false positive white precipitate.

3. Testing for Gases

You will often see these as part of other tests:

• Hydrogen \(H_2\): A lighted splint makes a "squeaky pop."
• Oxygen \(O_2\): Relights a glowing splint.
• Carbon Dioxide \(CO_2\): Turns limewater cloudy.
• Ammonia \(NH_3\): Turns damp red litmus paper blue.
• Chlorine \(Cl_2\): Turns damp blue litmus paper red and then bleaches it white.

4. Testing for Organic Functional Groups

Alkenes (\(C=C\) double bond)

The Test: Shake with bromine water.
Observation: The orange/brown solution turns colorless (it is decolored).

The Hydroxyl Group (-OH) in Alcohols and Carboxylic Acids

The Test: Add solid phosphorus(V) chloride \(PCl_5\).
Observation: Steamy fumes of hydrogen chloride \(HCl\) gas are produced (which turn damp blue litmus paper red).

Halogenoalkanes

The Test: Warm the sample with aqueous silver nitrate in ethanol.
Observation: The ethanol acts as a solvent so the reactants can mix. You will see precipitates (\(AgCl\), \(AgBr\), or \(AgI\)) forming at different rates (Iodides form fastest!).

Aldehydes and Ketones (Carbonyls)

First, use 2,4-dinitrophenylhydrazine (2,4-DNPH). If a bright orange/yellow precipitate forms, a carbonyl group (\(C=O\)) is present.

To tell them apart:

• Tollens' Reagent: Aldehydes form a silver mirror on the inside of the test tube. Ketones do nothing.
• Fehling’s or Benedict’s Solution: Aldehydes turn the blue solution into a brick-red precipitate. Ketones stay blue.
• Acidified Potassium Dichromate(VI): Aldehydes turn it from orange to green. Ketones stay orange.

The Iodoform Test

The Test: Add iodine and sodium hydroxide to the sample.
Observation: A pale yellow precipitate (triiodomethane) with a medicinal smell forms. This confirms the presence of a methyl ketone (\(CH_3C=O\)) or a methyl secondary alcohol (\(CH_3CH(OH)-\)).

Phenols

The Test: Add bromine water.
Observation: Unlike simple alkenes, phenol reacts to form a white precipitate and the bromine water is decolored.

Quick Review Box

Checklist for the Exam:
1. Did I mention the starting color and the final color?
2. If a gas is produced, did I name the test for the gas?
3. Did I specify if a reagent needs to be dilute, concentrated, or acidified?
4. For Unit 6, have I checked if the precipitate redissolves in excess reagent?

Note: For help with planning how to carry out these tests safely or calculating the amounts needed, see the chapters on "Planning Experiments" and "Quantitative Practical Calculations."