Qualitative tests and purification techniques

Chemistry B (Salters) - H433 · Cambridge OCR A Level · 6 min read · Updated: Sep 7, 2026

Introduction: Playing Chemical Detective

Welcome to one of the most hands-on parts of the OCR Chemistry B (Salters) course! Think of this chapter as your "chemistry toolkit." In the lab, we often face two big questions: "What is this mystery substance?" (Qualitative Analysis) and "How do I make it pure?" (Purification Techniques).

Whether you are trying to identify a gas in the "Ozone Story" or purifying a drug you’ve synthesized in "What's in a Medicine?", these skills are the bread and butter of a working chemist. Don't worry if the long list of tests seems daunting—most follow very logical patterns that we will break down together!

Part 1: Qualitative Tests – Identifying Inorganic Ions

Qualitative analysis is all about observations. We aren't measuring "how much" (that’s quantitative); we are looking for color changes, bubbles, or solids forming.

1. Flame Tests (Cations)

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

Lithium (\(Li^{+}\)): Red
Sodium (\(Na^{+}\)): Yellow/Orange
Potassium (\(K^{+}\)): Lilac
Calcium (\(Ca^{2+}\)): Brick-red (orange-red)

2. Testing for Anions (Negative Ions)

You will frequently encounter these in the "Elements from the Sea" (ES) module:

Halides (\(Cl^{-}\), \(Br^{-}\), \(I^{-}\)): Dissolve the sample in water, add nitric acid (to remove impurities), then add silver nitrate solution (\(AgNO_{3}\)).
- Chloride: White precipitate (\(AgCl\))
- Bromide: Cream precipitate (\(AgBr\))
- Iodide: Pale yellow precipitate (\(AgI\))
Carbonates (\(CO_{3}^{2-}\)): Add a dilute acid (like \(HCl\)). If you see effervescence (fizzing), and the gas produced turns limewater cloudy, you've found a carbonate! The gas is \(CO_{2}\).
Sulfates (\(SO_{4}^{2-}\)): Add barium chloride solution (\(BaCl_{2}\)). A white precipitate of barium sulfate (\(BaSO_{4}\)) forms if sulfate ions are present.

Quick Tip: Always use nitric acid before silver nitrate, never hydrochloric acid. Why? Because hydrochloric acid contains chloride ions, which would give you a "false positive" white precipitate!

Part 2: Qualitative Tests – Organic Functional Groups

In organic chemistry (like in the "Developing Fuels" or "What's in a Medicine?" modules), we use specific reagents to "spot" functional groups.

Alkenes (\(C=C\)): Shake with bromine water. The orange color turns colorless (decolorizes).
Phenols: Unlike carboxylic acids, phenols are weakly acidic but won't fizz with carbonates. They often react with Iron(III) chloride to give a purple color.
Carboxylic Acids (\(-COOH\)): Add sodium carbonate. Just like the inorganic test, you will see fizzing (\(CO_{2}\) gas).
Alcohols (\(-OH\)): Warm with acidified potassium dichromate(VI) (\(K_{2}Cr_{2}O_{7}/H_{2}SO_{4}\)). Primary and secondary alcohols turn the solution from orange to green. Tertiary alcohols stay orange.
Haloalkanes: Heat with aqueous sodium hydroxide (to release the halide ion), then perform the silver nitrate test mentioned above.

Key Takeaway

Qualitative tests are "Yes/No" tests. If the observation matches (e.g., orange to green), the group is present. If nothing happens, it’s absent!

Part 3: Purification Techniques for Solids

If you’ve just synthesized a solid, like aspirin, it will be "crude" (impure). We use recrystallization to clean it up.

Step-by-Step: Recrystallization

Dissolve: Use the minimum volume of hot solvent. You want a saturated solution.
Filter Hot: Use fluted filter paper to remove any insoluble impurities. Fluting the paper increases the surface area, making it faster so the solution doesn't cool down and crystallize yet.
Cool: Let the solution cool slowly. The pure crystals will grow, while soluble impurities stay dissolved in the "mother liquor."
Filter Cold: Use reduced pressure filtration (Buchner funnel). This uses a vacuum to pull the liquid through quickly, leaving dry-ish crystals.
Wash and Dry: Wash the crystals with a tiny bit of ice-cold solvent to remove surface impurities, then pat dry.

Checking Purity: Melting Point

How do we know if our recrystallization worked? We use a melting point apparatus.

Pure substances: Melt at a sharp, specific temperature (matching the data book).
Impure substances: Melt over a wide range of temperatures and at a lower temperature than the pure substance.

Part 4: Purification Techniques for Liquids

When dealing with liquids (like in "Developing Fuels"), we have different tools.

1. Distillation

This separates liquids based on their boiling points. The liquid with the lowest boiling point evaporates first, moves into the condenser (where cold water cools it), and drips into a collection flask.
Note: To keep a reaction going without losing your volatile liquids to the air, we use heating under reflux. This boils the liquid but condenses the vapors back into the same flask.

2. Separating Funnels

If you have two liquids that don't mix (like oil and water), they are immiscible.
1. Pour the mixture into the funnel.
2. Let the layers settle (the denser liquid sinks to the bottom).
3. Open the tap to run off the bottom layer into a beaker.
4. You can "wash" the product by adding water or sodium hydrogencarbonate to the funnel to remove acidic impurities.

3. Drying Agents

After using a separating funnel, your organic liquid might still have traces of water. We add an anhydrous salt (like \(MgSO_{4}\) or \(CaCl_{2}\)). It clumps together when wet and flows like "snow" when the liquid is dry. Then, just filter the salt out!

Part 5: Chromatography

Chromatography separates mixtures based on how they distribute themselves between a stationary phase and a mobile phase.

Thin Layer (TLC) / Paper Chromatography:
- Stationary phase: Silica plate or paper.
- Mobile phase: A solvent.
- Calculation: \(R_{f} = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}\).
Gas-Liquid Chromatography (GLC): Used for volatile liquids.
- Stationary phase: A high-boiling liquid held on a solid support inside a long tube.
- Mobile phase: An inert carrier gas (like Nitrogen).
- Retention Time: The time it takes for a substance to travel through the tube. This is used to identify the substance.

Did you know? In TLC, if your spots are invisible (like most organic compounds), you can use UV light or a chemical "developer" like iodine vapor to make them show up!

Summary & Quick Review

Common Mistakes to Avoid:
- Forgetting to use "minimum volume" in recrystallization (too much solvent means your crystals won't form!).
- Mixing up "reflux" and "distillation" (Reflux keeps everything in the flask; distillation takes one part out).
- Not using \(HNO_{3}\) before \(AgNO_{3}\) in halide tests.

Key Takeaways Table

Cations: Flame tests (Li=Red, Na=Yellow, K=Lilac, Ca=Brick-red).
Anions: \(CO_{3}^{2-}\) (fizzing), \(SO_{4}^{2-}\) (white ppt with \(BaCl_{2}\)), Halides (white/cream/yellow ppt with \(AgNO_{3}\)).
Organic: \(C=C\) (Bromine water), \(-COOH\) (Carbonates), Alcohols (Dichromate).
Purity: Recrystallization for solids; Distillation/Separating funnel for liquids; Melting point/TLC for checking purity.

For more information on how to design these experiments or handle the data from them, see the chapters on "Planning and experimental design" and "Analysis, graphs and significant figures".

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the characteristic flame test colors for lithium, sodium, potassium, and calcium ions?

Lithium (\(Li^{+}\)) is red, sodium (\(Na^{+}\)) is yellow/orange, potassium (\(K^{+}\)) is lilac, and calcium (\(Ca^{2+}\)) is brick-red.

How do you test for halide ions (\(Cl^{-}\), \(Br^{-}\), \(I^{-}\)) in a solution?

Add dilute nitric acid followed by silver nitrate solution (\(AgNO_{3}\)); chloride forms a white precipitate, bromide forms cream, and iodide forms pale yellow.

What is the qualitative test for carbonate ions (\(CO_{3}^{2-}\))?

Add a dilute acid (such as \(HCl\)); if carbonate ions are present, effervescence occurs as \(CO_{2}\) gas is produced, which turns limewater cloudy.

How can you confirm the presence of sulfate ions (\(SO_{4}^{2-}\)) in a solution?

Add barium chloride solution (\(BaCl_{2}\)); a white precipitate of barium sulfate (\(BaSO_{4}\)) will form if sulfate ions are present.

What observation indicates the presence of an alkene (\(C=C\)) when reacting with bromine water?

The orange bromine water turns colorless (decolorizes).

How can primary and secondary alcohols be distinguished from tertiary alcohols using chemical tests?

Warm the sample with acidified potassium dichromate(VI); primary and secondary alcohols change the solution from orange to green, while tertiary alcohols cause no color change.

Why is it critical to use the "minimum volume of hot solvent" during the recrystallization of a solid?

To ensure the solution is saturated so that the maximum amount of pure product crystallizes out as the solution cools.

How does the melting point of a substance indicate its purity?

A pure substance melts at a sharp, specific temperature; an impure substance melts over a wide range and at a lower temperature than the pure substance.

What is the difference between heating under reflux and distillation?

Reflux returns condensed vapors to the reaction flask to prevent the loss of volatile reactants; distillation separates and collects a liquid based on its boiling point.

How do you use a separating funnel to purify an organic liquid?

Place the immiscible liquids in the funnel, allow layers to settle, then open the tap to remove the denser (usually aqueous) bottom layer.

What is the purpose of adding an anhydrous salt like \(MgSO_{4}\) to an organic liquid?

It acts as a drying agent to remove traces of water from the organic product.

How is the \(R_{f}\) value calculated in Thin Layer Chromatography (TLC)?

\(R_{f} = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}\)

What determines the retention time in Gas-Liquid Chromatography (GLC)?

Retention time is the time taken for a substance to travel through the column, determined by its distribution between the mobile phase (carrier gas) and stationary phase (high-boiling liquid).

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