Required Practicals: Analytical Chemistry

The AQA A-Level Chemistry specification includes several required practicals related to analytical chemistry. This lesson covers the key practical techniques: distillation and reflux, tests for ions and gases, making and purifying organic liquids and solids, and thin-layer chromatography. Understanding these techniques is essential for the practical endorsement and for exam questions about practical skills.

---

Distillation

Simple Distillation

Simple distillation is used to separate a liquid from dissolved solids (e.g., purifying water) or to separate liquids with widely different boiling points (difference > 25 °C).

Apparatus: Round-bottomed flask, distillation head, thermometer, condenser (Liebig condenser), receiving flask, anti-bumping granules, heat source (heating mantle or water bath).

Method:

1. Place the mixture in the round-bottomed flask with anti-bumping granules.
2. Heat gently. The liquid with the lowest boiling point evaporates first.
3. The vapour passes into the condenser, where cold water flowing through the outer jacket cools it.
4. The condensed liquid (distillate) is collected in the receiving flask.
5. Monitor the temperature at the top of the distillation head — the thermometer should read the boiling point of the liquid being collected.

Fractional Distillation

Fractional distillation is used to separate liquids with similar boiling points (difference < 25 °C).

Key difference: A fractionating column (packed with glass beads or filled with glass helices) is placed between the flask and the distillation head. This provides a large surface area for repeated vaporisation and condensation, improving separation.

The column creates a temperature gradient: hottest at the bottom, coolest at the top. Only the most volatile component reaches the top and passes into the condenser.

Exam Tip: Always explain why a fractionating column improves separation: it provides a large surface area for multiple vaporisation–condensation cycles (theoretical plates), effectively performing many simple distillations in series.

---

Heating Under Reflux

Reflux is used when you need to heat a reaction mixture for a prolonged time without losing volatile reactants or products.

Apparatus: Round-bottomed flask, reflux condenser (vertical), heat source, anti-bumping granules.

Method:

1. Place the reactants in the flask with anti-bumping granules.
2. Attach the reflux condenser vertically. Cold water flows upward through the condenser jacket.
3. Heat the flask gently. Vapours rise, condense in the condenser, and drip back into the flask.
4. Continue heating for the required time (often 20–30 minutes or longer).

Key point: The condenser prevents loss of volatile substances by condensing them and returning them to the reaction flask. This allows the reaction to proceed at the boiling point of the solvent/reagent without losing material.

When to Use Reflux vs Distillation

• Reflux: When you want to keep the reaction going at high temperature and collect the product afterwards.
• Distillation: When you want to separate and collect the product as it forms (removing it from the reaction mixture).

Example: Oxidation of a primary alcohol.

• To make an aldehyde: use distillation apparatus to remove the aldehyde as it forms (before it can be further oxidised).
• To make a carboxylic acid: use reflux to keep the aldehyde in the flask so it is further oxidised.

---

Tests for Ions

Tests for Cations

Ion	Test	Observation
NH₄⁺	Add NaOH(aq), warm gently	Pungent gas produced that turns damp red litmus paper blue (ammonia, NH₃)
Fe²⁺	Add NaOH(aq)	Green precipitate of Fe(OH)₂
Fe³⁺	Add NaOH(aq)	Brown (rust-coloured) precipitate of Fe(OH)₃
Cu²⁺	Add NaOH(aq)	Blue precipitate of Cu(OH)₂
Al³⁺	Add NaOH(aq)	White precipitate of Al(OH)₃, soluble in excess NaOH (amphoteric)
Ca²⁺	Add NaOH(aq)	White precipitate (only from concentrated solutions)

Exam Tip: To distinguish Al³⁺ from Mg²⁺ (both give white precipitates with NaOH), add excess NaOH. The Al(OH)₃ precipitate dissolves (amphoteric) but Mg(OH)₂ does not.

Tests for Anions

Ion	Test	Observation
CO₃²⁻	Add dilute acid	Effervescence; gas turns limewater milky (CO₂)
SO₄²⁻	Add dilute HCl then BaCl₂(aq)	White precipitate of BaSO₄ (insoluble in HCl)
Cl⁻	Add dilute HNO₃ then AgNO₃(aq)	White precipitate of AgCl (soluble in dilute NH₃)
Br⁻	Add dilute HNO₃ then AgNO₃(aq)	Cream precipitate of AgBr (soluble in conc. NH₃)
I⁻	Add dilute HNO₃ then AgNO₃(aq)	Yellow precipitate of AgI (insoluble in NH₃)

Why add acid first?

• For sulfate test: dilute HCl removes carbonate ions (which would also form a white precipitate with Ba²⁺).
• For halide test: dilute HNO₃ removes carbonate and sulfite ions (which would also form precipitates with Ag⁺).

---

Tests for Gases

Gas	Test	Positive Result
CO₂	Pass through limewater (Ca(OH)₂(aq))	Limewater turns milky (white precipitate of CaCO₃)
H₂	Hold a lit splint to the mouth of the tube	Squeaky pop
O₂	Hold a glowing splint in the gas	Splint relights
Cl₂	Hold damp litmus paper in the gas	Litmus paper is bleached white (turns red first if blue litmus, due to acidity, then white)
NH₃	Hold damp red litmus paper in the gas	Litmus turns blue
HCl	Hold damp blue litmus paper; also hold a glass rod dipped in NH₃(aq) near the gas	Litmus turns red; white fumes of NH₄Cl with ammonia
SO₂	Pass through acidified K₂Cr₂O₇	Orange dichromate turns green (Cr₃⁺)

---

Making and Purifying an Organic Liquid

General Procedure (e.g., Making a Halogenoalkane or Ester)

1. Reaction: Mix the reagents in a round-bottomed flask with appropriate conditions (e.g., reflux with a catalyst).

2. Distillation: Distil the product from the reaction mixture.

3. Washing: Transfer the distillate to a separating funnel. Wash with:

   • Sodium hydrogencarbonate solution (to remove acidic impurities).
   • Distilled water (to remove water-soluble impurities).
   • Allow the layers to separate. Drain off the aqueous layer (determine which layer is organic by density — if denser than water, it will be the lower layer).