Edexcel International A Level Chemistry: Practical Skills in Chemistry I (YCH11)

This guide teaches Unit 3: Practical Skills in Chemistry I, as set out in the YCH11 specification from Pearson (Edexcel International Advanced Subsidiary/Advanced Level in Chemistry, Issue 1, September 2017). Unit 3 is an International AS (IAS) unit with no new theory: it examines the skills and techniques you build in Units 1 and 2, including Core Practicals 1 to 8, and can set them in unfamiliar practical situations.

Use it with the Unit 3 revision notes and the Unit 3 practice questions. See also the course hub, printable checklist and free diagnostics.

What the specification says about Unit 3

Feature	What the specification states
Format	Written examination, 1 hour and 20 minutes, 50 marks
Weighting	20% of the IAS, 10% of the full International A Level
Questions	All compulsory; short-open, open-response and calculation
Maths	A minimum of 6 marks target mathematics at Level 2 or above
Calculators	May be used
Availability	January, June and October

The paper assesses problem solving in a practical context, scientific method (controlling variables, presenting and evaluating data, uncertainty, commenting on a method), numeracy (graphs, data processing, significant figures, accuracy and precision) and apparatus (choice, use, range, resolution, safety). Expect tests for ions, gases and functional groups, titration, thermochemistry and kinetics data, and comments on preparations. You should also follow instructions with little help, work safely, plan experiments, justify apparatus choices and write laboratory reports in correct scientific language.

Where the core practicals sit

Core Practicals 1 to 8 are outcomes 1.11, 6.7, 8.21, 8.23, 10.11, 10.14, 10.20 and 10.23. You also need the linked outcomes 6.8 (errors, assumptions, cooling corrections), 8.22 (uncertainty in volumetric analysis) and 10.19 (reflux, separating funnel, distillation, drying, boiling temperature).

Skills that run through every practical

Accuracy and precision. An accurate result is close to the true value; precise results are close to each other.

Random and systematic error. Random errors scatter readings; averaging repeats reduces their effect. A systematic error, such as heat lost from an uninsulated cup, shifts every reading the same way; repeats do not remove it.

Uncertainty. Percentage uncertainty = (uncertainty ÷ quantity measured) × 100. Titres, masses by difference and temperature changes each rest on two readings, so double the single-reading uncertainty. Add percentage uncertainties to estimate the overall uncertainty.

Significant figures. Match the least precise measurement used.

Range and resolution. Pick apparatus whose range fits the quantity and whose resolution is fine enough, such as a 100 cm³ gas syringe for about 70 cm³ of gas.

Graphs. Independent variable on the x-axis, axes labelled with units, points spread over most of the grid, a best-fit line, and a large triangle for a gradient.

Safety. Name the specific hazard and a matching precaution: for a flammable solvent, “heat with a water bath, not a flame”.

Core Practical 1: molar volume of a gas

Add a weighed metal or carbonate to an excess of acid, collect the gas in a syringe, and note the final volume, temperature and pressure.

Worked example 1. Five pieces of magnesium ribbon react separately with excess dilute hydrochloric acid at 21 °C and 101 kPa: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Use Ar(Mg) = 24.3.

Mass of Mg / g	0.015	0.030	0.045	0.060	0.075
Volume of H₂ / cm³	14.6	29.4	44.3	58.9	73.8

1. Convert each mass to amount (0.075 ÷ 24.3 = 3.09 × 10⁻³ mol, and so on); 1 mol Mg gives 1 mol H₂.
2. Plot volume against amount of H₂. The best-fit line passes through the origin and through (3.00 × 10⁻³ mol, 71.7 cm³).
3. Gradient = 71.7 ÷ 0.00300 = 23 900 cm³ mol⁻¹, so the molar volume is 23.9 dm³ mol⁻¹.
4. Check against pV = nRT: V for 1 mol = RT ÷ p = 8.31 × 294 ÷ 101 000 = 0.0242 m³ = 24.2 dm³. The result is about 1.2% low.

A graph uses all the data and shows up an anomalous point. Low results come from gas escaping before the bung is in place, an oxide coating on the ribbon, or a sticking syringe.

Core Practical 2 and outcome 6.8: Hess’s Law in a cup

A thermal decomposition cannot be measured directly, so measure two reactions in an insulated cup that end in the same products, then combine them.

Worked example 2. Target: 2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(l). Each solid is added to 40.0 cm³ of 2.00 mol dm⁻³ hydrochloric acid (excess) in a polystyrene cup. Treat the solution as 40.0 g with specific heat capacity 4.18 J g⁻¹ °C⁻¹. Use Mr: NaHCO₃ = 84.0, Na₂CO₃ = 106.0.

Run 1: 2.52 g NaHCO₃ gives an extrapolated fall of 4.8 °C. Run 2: 2.65 g Na₂CO₃; the temperature was 21.0 °C up to mixing at 3 minutes. Readings after the peak were 25.6, 25.4, 25.2 and 25.0 °C at 5, 6, 7 and 8 minutes. The cooling line falls 0.2 °C per minute; extrapolated back to 3 minutes it gives 26.0 °C. So ΔT = 26.0 − 21.0 = 5.0 °C. The highest reading would understate the rise, because heat leaks out before the reaction finishes.

1. Run 1: energy = 40.0 × 4.18 × 4.8 = 803 J. n(NaHCO₃) = 2.52 ÷ 84.0 = 0.0300 mol. Temperature fell, so ΔH₁ = +0.803 ÷ 0.0300 = +26.8 kJ mol⁻¹.
2. Run 2: energy = 40.0 × 4.18 × 5.0 = 836 J. n(Na₂CO₃) = 2.65 ÷ 106.0 = 0.0250 mol. ΔH₂ = −0.836 ÷ 0.0250 = −33.4 kJ mol⁻¹.
3. Both routes from 2NaHCO₃ to the final solution must have the same total: ΔH(target) = 2ΔH₁ − ΔH₂ = 53.6 + 33.4 = +87 kJ mol⁻¹ (2 s.f., matching ΔT).
4. The acid is in excess: 0.0800 mol HCl against 0.0300 and 0.0500 mol needed.

With a thermometer read to ±0.2 °C, a ΔT of 5.0 °C carries ±0.4 °C, or 8%, far more than the mass or volume, so a bigger temperature change helps most. Assumptions: the solution behaves like water, the cup absorbs no heat, and all the solid reacts.

Core Practicals 3 and 4: titrations (8.20 to 8.23)

Technique. Rinse the burette and pipette with the solutions they will hold; rinse the conical flask with distilled water only, which does not change the amount pipetted in. Fill the jet, remove the funnel, read the meniscus bottom at eye level, and add dropwise near the end point. Methyl orange is red in acid and yellow in alkali; phenolphthalein is colourless in acid and pink in alkali.

Worked example 3 (Core Practical 3). 25.0 cm³ of 0.105 mol dm⁻³ sodium hydroxide is pipetted into a flask with phenolphthalein, and hydrochloric acid is run in from a burette until the pink colour just disappears. Titres: rough 23.85, then 23.30, 23.55 and 23.40 cm³.

1. Concordant titres (within 0.10 cm³) are 23.30 and 23.40; mean = 23.35 cm³.
2. n(NaOH) = 0.105 × 0.0250 = 2.625 × 10⁻³ mol = n(HCl), since the ratio is 1 : 1.
3. c(HCl) = 2.625 × 10⁻³ ÷ 0.02335 = 0.112 mol dm⁻³, which is 0.112 × 36.5 = 4.10 g dm⁻³.

Worked example 4 (Core Practical 4). Sulfamic acid, H₂NSO₃H (Mr 97.1), is a monoprotic solid acid. A weighing bottle with the acid weighs 14.682 g; after tipping the acid into a beaker it weighs 11.817 g. The acid is dissolved, transferred with rinsings to a 250 cm³ volumetric flask and made up to the mark. 25.0 cm³ portions need a mean titre of 26.85 cm³ of sodium hydroxide.

1. Mass by difference = 14.682 − 11.817 = 2.865 g; n = 2.865 ÷ 97.1 = 0.02951 mol in 250 cm³.
2. n in 25.0 cm³ = 2.951 × 10⁻³ mol = n(NaOH).
3. c(NaOH) = 2.951 × 10⁻³ ÷ 0.02685 = 0.110 mol dm⁻³.
4. Uncertainties given: balance ±0.005 g per reading, flask ±0.3 cm³, pipette ±0.06 cm³, burette ±0.05 cm³ per reading. Percentages: mass 0.010 ÷ 2.865 = 0.35%; flask 0.12%; pipette 0.24%; titre 0.10 ÷ 26.85 = 0.37%. Total about 1.1%, so c = 0.110 ± 0.001 mol dm⁻³.

Core Practicals 5 and 6: halogenoalkanes

Core Practical 5 (10.10, 10.11). Warm separate tubes of halogenoalkane and of silver nitrate solution in ethanol in a water bath, mix, and time how long a precipitate takes to appear. Water is the nucleophile; the halide ion released forms a silver halide: AgCl white, AgBr cream, AgI yellow. Ethanol is a common solvent. Control temperature, volumes and amounts. Relative rate = 1 ÷ time. If 1-iodobutane gives a precipitate after 42 s and 1-bromobutane after 380 s, the rates are 0.0238 s⁻¹ and 0.00263 s⁻¹: the iodo compound reacts about 9 times faster. Of the three, C–I has the smallest bond enthalpy and breaks most readily. Among isomers, tertiary reacts fastest and primary slowest.

Core Practical 6 (10.14). (CH₃)₃COH + HCl → (CH₃)₃CCl + H₂O. In a separating funnel, shake the tertiary alcohol and concentrated hydrochloric acid together, venting often. The less dense chloroalkane forms the upper layer. Run off the aqueous layer, shake with sodium hydrogencarbonate solution to remove remaining HCl (venting off CO₂), then with water. Dry with anhydrous sodium sulfate until clear, then distil near the product’s boiling temperature. Concentrated hydrochloric acid is corrosive and gives off irritant fumes, so work in a fume cupboard.

Core Practical 7: oxidising propan-1-ol (10.18 to 10.20)

Acidified potassium dichromate(VI) turns from orange to green as it oxidises the alcohol.

• Propanal: heat with the alcohol in excess and distil the product off as it forms, so it escapes before further oxidation. Collect it in an ice-cooled receiver. CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O. Benedict’s or Fehling’s solution gives a red precipitate on warming.
• Propanoic acid: heat with excess oxidising agent under reflux, then distil. CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O. Sodium carbonate or hydrogencarbonate gives effervescence.

Reflux uses a vertical condenser and anti-bumping granules, never a sealed system. When distilling, place the thermometer bulb opposite the side arm. A narrow boiling range near the expected boiling temperature suggests a pure product. Potassium dichromate(VI) is toxic, so wear gloves.

Core Practical 8: identifying unknowns

Test	Positive result
Flame test	Li red, Na yellow, K lilac, Ca orange-red, Sr red, Ba pale green
Dilute acid, gas into limewater	Effervescence; limewater turns milky: CO₃²⁻ or HCO₃⁻
Acidified barium chloride	White precipitate: SO₄²⁻
Sodium hydroxide, warm	Gas turns damp red litmus blue, white smoke with HCl fumes: NH₄⁺
Nitric acid then silver nitrate	White, cream, yellow: Cl⁻, Br⁻, I⁻; then ammonia solution to confirm
Glowing splint	Relights: O₂. A brown, acidic gas is NO₂
Bromine water	Orange to colourless: C=C
Phosphorus(V) chloride	Steamy fumes of HCl: –OH group
Acidified dichromate(VI), warm	Orange to green: primary or secondary alcohol, or aldehyde
Benedict’s or Fehling’s	Red precipitate: aldehyde
Sodium hydrogencarbonate	Effervescence: carboxylic acid

Common errors

• Averaging every titre, including the rough one or a non-concordant one.
• Using the uncertainty of one reading for a titre or temperature change made from two.
• Using the highest reading for ΔT instead of extrapolating the cooling line.
• Writing a sealed reflux apparatus, or a thermometer dipped into the liquid during distillation.
• Giving only the final colour: write “orange to green”, not “turns green”.

Related: the Topic 1 study guide (moles for Core Practical 1) and exam preparation.

Official syllabus

Source: the YCH11 specification, Issue 1 (September 2017), Pearson Education Limited, for the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry. Unit 3: Practical Skills in Chemistry I, with Core Practicals 1 to 8 from Units 1 and 2.