A-level Chemistry practicals: skills and methods

What you have to be able to do

• Weighing a solid by difference and making it up to the line of a volumetric flask, with every rinsing carried into the flask
• Rinsing a pipette and a burette with the solution each will hold, and the conical flask with distilled water only
• Running a rough titration, then accurate ones added dropwise near the end point until at least two titres are within 0.10 cm³
• Reading a burette at eye level, at the bottom of the meniscus, to the nearest 0.05 cm³

The calculations involved

• Moles from n = cV, the mole ratio from the balanced equation, and the concentration from c = n ÷ V
• The mean of the concordant titres only, leaving out the rough titre
• Percentage uncertainty: a titre is the difference of two readings, so ±0.10 cm³ on 23.40 cm³ is 0.43%

Errors to avoid

• A burette rinsed with water, not the acid it will hold, dilutes the acid, so the titre is larger and the concentration comes out too low.
• A conical flask rinsed with the solution to be pipetted into it holds extra solute, so the titre is too large.
• Using the moles in the whole volumetric flask instead of one 25.0 cm³ portion gives an answer ten times too big.

Safety

• Wear eye protection throughout: sodium carbonate and dilute hydrochloric acid both irritate the eyes.
• Lower the burette in its clamp, or take it down, so that it is filled below eye level and acid cannot splash into the face.
• Never pipette by mouth. Check the CLEAPSS Hazcards and your school's risk assessment before starting.

Methods in the lessons

• Make up a standard solution and titrate it against an acid
  In Concentration, volumetric solutions and titration calculations

Worked in full

• Acid–base titration: making and using a standard solution
  Make up a standard solution of sodium carbonate and use it to find the concentration of a hydrochloric acid by titration.

Taught alongside: Amount of substance · Redox

What you have to be able to do

• Using an expanded polystyrene cup with a lid, standing in a beaker so it cannot tip
• Recording the temperature every 30 s before mixing and after it, stirring throughout
• Plotting temperature against time and extending the cooling line back to the time of mixing to read ΔT

The calculations involved

• q = mcΔT, with the mass of the solution taken from its volume
• ΔH from q and the amount of the limiting reagent, negative for an exothermic change
• The uncertainty in ΔT: a thermometer reading to 0.1 °C is ±0.1 °C on each reading, so ±0.2 °C on the difference of two readings

Errors to avoid

• Heat lost to the air and the bench makes ΔT too small and ΔH less exothermic than it should be.
• The highest reading is below the temperature the solution would have reached with no heat loss; the extrapolation adds that back.
• The density and specific heat capacity of the solution are assumed equal to water's.

Safety

• Copper(II) sulfate solution is harmful if swallowed, irritates the eyes and is toxic to aquatic life, so residues go into the container provided, not down the sink.
• Zinc powder is flammable.
• Wear eye protection, and check the CLEAPSS Hazcards and the school's risk assessment before starting.

Methods in the lessons

• Measure an enthalpy change in solution with a polystyrene cup
  In Measuring an enthalpy change: calorimetry

Worked in full

• An enthalpy change in solution: zinc and copper(II) sulfate
  Measure the temperature change when zinc displaces copper from copper(II) sulfate in a polystyrene cup, and correct it for heat loss by extrapolation.

Taught alongside: Energetics

What you have to be able to do

• Bringing each solution to the bath temperature separately before mixing, and measuring the temperature of the mixture
• Judging an end point the same way in every run, with one observer throughout
• Following a concentration with a colorimeter, zeroed on the mixture without the coloured species and calibrated against known concentrations
• Halving one concentration between runs while the total volume stays the same, so the others do not change

The calculations involved

• 1/t as a measure of rate when a fixed amount of change is timed
• Rates from the gradient of a concentration–time graph, and orders from how the rate changes when one concentration is halved
• The uncertainty in a mean time, taken as half the range of the repeats

Errors to avoid

• The mixture cools once it leaves the bath, so the hot runs happen at a lower temperature than the bath.
• A fixed error of a few tenths of a second in starting and stopping the watch is a larger percentage of the short times.
• Temperature is the error that matters most when orders are found, because k depends on it steeply.

Safety

• Sulfur dioxide from the thiosulfate reaction is toxic and irritates the airways: work in a well-ventilated room and pour each mixture away at once.
• Propanone is highly flammable, so keep it away from flames.
• Iodopropanone irritates the eyes and nose, so do not smell the mixtures; iodine solution stains skin and clothing.

Methods in the lessons

• Time a reaction at several temperatures and read the effect on rate
  In Measuring the rate of a reaction
• Find an order by initial rates and by continuous monitoring
  In Finding orders from initial rates, half-lives and graphs

Worked in full

• Rate and temperature: thiosulfate and acid
  Time how long the cross takes to disappear at five temperatures, and plot 1/t against temperature.
• Orders of reaction: iodine and propanone
  Follow the acid-catalysed reaction of propanone with iodine with a colorimeter, then halve each concentration in turn to find every order.

Taught alongside: Kinetics · Rate equations

What you have to be able to do

• Doing the anion tests in a fixed order, carbonate, then sulfate, then the halides, each on a fresh portion of the unknown
• Adding a reagent drop by drop, shaking after each drop, and then in excess
• Running the known solutions first, and holding each unknown beside its matching known
• Testing a gas: limewater for carbon dioxide, and damp red litmus held in the mouth of the tube for ammonia

The calculations involved

• The mass of a precipitate from the moles of the ion, n = cV, and the mole ratio in the equation
• The volume of a gas given off, from its moles and the molar gas volume

Errors to avoid

• Acidifying with hydrochloric acid before silver nitrate adds chloride, so a white precipitate forms every time.
• Sodium hydroxide poured quickly into an aluminium solution forms the white precipitate and redissolves it where the alkali is concentrated, so it is never seen.
• 'Goes cloudy' and 'reacts' identify nothing: write 'white precipitate', 'effervescence' or 'no visible change'.

Safety

• Concentrated ammonia gives off an irritating vapour and is used in a fume cupboard.
• Silver nitrate stains skin and is an irritant; barium chloride is harmful if swallowed.
• Sodium hydroxide damages eyes even when dilute: wear eye protection throughout.

Methods in the lessons

• Identify cations and anions in solution with test-tube reactions
  In Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water
• Identify transition metal ions with hydroxide, ammonia and carbonate
  In Reactions of aqua ions with hydroxide, ammonia and carbonate

Worked in full

• Identifying ions in solution: anions, Group 2 and ammonium
  Identify hydroxide, carbonate, sulfate and halide ions, the Group 2 ions and the ammonium ion by test-tube reactions done in a fixed order.
• Identifying metal ions in solution: iron, copper and aluminium
  Identify Fe²⁺, Cu²⁺, Fe³⁺ and Al³⁺ from what sodium hydroxide, ammonia and sodium carbonate do to each, a little at a time and then in excess.

Taught alongside: Group 2 and Group 7 · Ions in aqueous solution

What you have to be able to do

• Heating under reflux, and distilling with the thermometer bulb level with the side arm and the receiver cooled
• Filtering under reduced pressure, and recrystallising from the minimum volume of hot solvent
• Washing a liquid product in a separating funnel, venting often, and drying it with anhydrous calcium chloride
• Measuring a melting point range in a capillary tube, heating slowly near the expected value

The calculations involved

• Moles of the limiting reactant, from a mass or from a volume and a density
• Percentage yield: the mass of pure product over the theoretical mass from the limiting reactant, times 100
• The percentage uncertainty in a mass found by difference

Errors to avoid

• A product that is not fully dry weighs too much, so the yield comes out falsely high, and it melts low and over a range.
• Apparatus sealed with no opening to the air builds pressure as it is heated, and the glass can burst.
• Heating the melting point apparatus too quickly gives a melting point that reads high.

Safety

• Potassium dichromate(VI) is toxic, can cause cancer and can sensitise the skin: wear gloves and eye protection.
• The organic liquids are flammable: heat with a heating mantle or a water bath, with no naked flame anywhere nearby.
• Ethanoic anhydride, concentrated acids and the ester's vapour are handled in a fume cupboard.

Methods in the lessons

• Oxidise an alcohol and distil the aldehyde off as it forms
  In Oxidation and elimination of alcohols
• Prepare, purify and test an organic solid and an organic liquid
  In Preparing, purifying and testing an organic solid and liquid

Worked in full

• Distilling propanal from propan-1-ol
  Oxidise propan-1-ol with acidified dichromate and distil the propanal off as it forms, before it can be oxidised further.
• Organic preparation and purification: aspirin and ethyl ethanoate
  Make aspirin and purify it by recrystallisation, testing it by its melting point, then make ethyl ethanoate and purify it by washing, drying and distillation.

Taught alongside: Alkenes and alcohols · Organic synthesis

What you have to be able to do

• Bromine water for a C=C bond, and sodium hydrogencarbonate for a carboxylic acid
• Making Tollens' reagent fresh and warming the unknown with it in a water bath
• Warming with acidified potassium dichromate(VI), and reading the result alongside the aldehyde test
• Running a tube of water beside each test as a control

The calculations involved

• A precise relative molecular mass from precise isotopic masses, such as ¹H = 1.0078, ¹²C = 12.0000 and ¹⁶O = 15.9949
• Choosing between formulas with the same nominal mass by which precise mass matches the molecular ion

Errors to avoid

• A green colour with acidified dichromate means a primary alcohol, a secondary alcohol or an aldehyde, so it is read only alongside the Tollens' result.
• In a dirty tube the silver comes down as a grey or black precipitate, which is still a positive result.
• 'Changed colour' and 'it reacted' are not observations: record a colour before and a colour after.

Safety

• Tollens' reagent is made fresh and never stored, and is washed away with plenty of water as soon as the test is over.
• No flame is used anywhere: every organic liquid here is flammable.
• Bromine water and ammonia are harmful and irritant, and sodium hydroxide is corrosive to eyes: wear eye protection throughout.

Methods in the lessons

• Tell an alcohol, aldehyde, alkene and carboxylic acid apart by test
  In Test-tube reactions for functional groups, and high-resolution mass spectrometry

Worked in full

• Functional group tests: alkenes, acids, aldehydes and alcohols
  Tell an alkene, a carboxylic acid, an aldehyde and an alcohol apart with bromine water, sodium hydrogencarbonate, Tollens' reagent and acidified dichromate.

Taught alongside: Organic analysis

What you have to be able to do

• Cleaning each metal strip with emery paper just before use
• Making a salt bridge of filter paper soaked in saturated potassium nitrate solution, fresh for each cell
• Reading a high-resistance voltmeter once the reading has stopped changing, with the temperature recorded

The calculations involved

• E°cell = E°right − E°left, with the more positive half-cell on the right
• An unknown electrode potential from a measured EMF and a known E°

Errors to avoid

• A voltmeter of low resistance lets a current flow, so the reading is below the EMF.
• Oxide or grease on a metal strip gives a reading lower than expected and unsteady.
• Solutions not at 1.00 mol dm⁻³, or a temperature away from 298 K, change the EMF from E°cell.

Safety

• Copper(II) sulfate solution is harmful if swallowed and irritates the eyes and skin, and zinc sulfate irritates the eyes.
• Solid potassium nitrate is an oxidiser.
• Wear eye protection, wash off splashes, and keep solutions away from the voltmeter.

Methods in the lessons

• Build two half-cells and a salt bridge and measure the EMF
  In Electrode potentials and the standard hydrogen electrode

Worked in full

• Measuring the EMF of a cell
  Join two metal half-cells with a salt bridge, measure each cell's EMF against copper with a high-resistance voltmeter, and work out each electrode potential.

Taught alongside: Electrochemistry

What you have to be able to do

• Calibrating a pH meter with two buffer solutions at the temperature of the titration
• Adding the base in large steps where the pH changes slowly, and in drops through the steep part
• Keeping the electrode bulb covered by the liquid and clear of the stirrer follower

The calculations involved

• pKa as the pH at half the equivalence volume, and Ka = 10^−pKa
• The concentration of the acid from the equivalence volume and the concentration of the base

Errors to avoid

• A meter that is not calibrated makes every pH off by a systematic amount, so pKa is wrong although the equivalence volume is right.
• A reading taken before it has settled lags behind the additions, most of all in the steep part.
• Additions too large near equivalence leave too few points on the vertical section to place its middle.

Safety

• Sodium hydroxide at 0.100 mol dm⁻³ irritates the eyes.
• Ammonia solution gives off an irritating vapour.
• Wear eye protection and use a pipette filler.

Methods in the lessons

• Follow a titration with a pH meter and plot the curve
  In Titration curves and choosing an indicator

Worked in full

• Measuring pH curves
  Calibrate a pH meter, follow a titration with it, plot the pH curve, and read the equivalence volume and the pKa from it.

Taught alongside: Acids and bases

What you have to be able to do

• Drawing the baseline in pencil, above the level of the solvent
• Spotting small, concentrated spots, letting each dry and re-spotting
• Running the plate in a covered tank and marking the solvent front at once
• Showing colourless spots with ninhydrin, iodine vapour or ultraviolet light

The calculations involved

• Rf as the distance to the centre of a spot over the distance to the solvent front
• Recognising that Rf has no unit and always lies between 0 and 1

Errors to avoid

• Spots that are too large or too concentrated streak and overlap.
• Touching the face of the plate leaves amino acids from the skin, which show up as extra spots.
• An Rf value is comparable only on the same stationary phase, with the same solvent and at the same temperature.

Safety

• The solvent mixture is flammable and ethanoic acid is corrosive: keep it away from flames.
• Ninhydrin is harmful and is sprayed only in a fume cupboard.
• Ultraviolet light damages eyes, so never look into the lamp.

Methods in the lessons

• Separate a mixture by thin-layer chromatography and work out Rf
  In Chromatography: TLC, column and gas

Worked in full

• Thin-layer chromatography: separating amino acids
  Run an amino acid mixture beside known standards on one TLC plate, show the spots with ninhydrin, and compare Rf values.

Taught alongside: Organic analysis