Every practical and planning question in the Sec 3–4 course rests on three choices: the right apparatus to measure with, the right method to separate a mixture, and the right test to check that a substance is pure. Experimental Chemistry gives you the toolkit for all three.
These notes cover measuring time, temperature, mass and volume to the correct precision, collecting and drying gases, laboratory hazard symbols, separation and purification methods, paper chromatography and Rf values, and using melting and boiling points to deduce both identity and purity.
What these notes cover
- •Measuring apparatus and precision
- •Measuring the volume of a liquid and a gas
- •Apparatus for measuring the rate of a reaction
- •Choosing how to collect a gas
- •Drying a gas
- •Laboratory safety and hazard symbols
- •Separation and purification methods
- •Choosing the right separation method
- •Paper chromatography and the Rf value
- •Interpreting a chromatogram and locating agents
- •Checking purity: melting and boiling points
- •Common mistakes and exam technique
Key definitions
- Parallax error
- A wrong reading caused by looking at the scale from above or below the meniscus, instead of reading the bottom of the meniscus with the eye level with it.
- Sublimation
- The change of state from solid directly to gas without passing through the liquid state. Iodine, ammonium chloride and dry ice (solid carbon dioxide) sublime.
- Saturated solution
- A solution that contains the maximum amount of solute that can dissolve in the solvent at that temperature.
- Miscible and immiscible liquids
- Miscible liquids mix completely to form one uniform layer, such as ethanol and water. Immiscible liquids do not mix and form two separate layers, such as oil and water.
- Pure substance
- A substance made up of only one substance, with nothing else mixed into it. It can be an element, such as copper, or a compound, such as water or sodium chloride.
Key formulas
Rf value
Rf = distance moved by substance ÷ distance moved by solvent front
Both distances are measured from the baseline, to the centre of the spot. Rf has no units and 0 < Rf ≤ 1 for any substance that moves. A substance has the same Rf only when the same solvent and the same type of paper are used.
Volume delivered by a burette
volume delivered = final reading − initial reading
A burette is read to the nearest 0.05 cm³ and every reading is recorded to 2 decimal places, with the second decimal place 0 or 5, e.g. 24.35 cm³.
Comparing gas density with air
Mr of gas > 29 → denser than air; Mr of gas < 29 → less dense than air
Use when the question does not give the density. Air has Mr ≈ 29. CO₂ (44) is denser, so downward delivery; NH₃ (17) is less dense, so upward delivery.
Gas-producing reaction for a rate experiment
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
The rate is followed with a stoppered conical flask, a gas syringe to measure the volume of hydrogen and a stopwatch to measure the time, recording the volume at regular intervals, e.g. every 30 s.
Why concentrated sulfuric acid cannot dry ammonia
2NH₃(g) + H₂SO₄(l) → (NH₄)₂SO₄(s)
Ammonia is alkaline, so it reacts with the acid to form a salt and no ammonia comes out of the drying bottle. Quicklime, CaO, is used instead.
Worked example
A food colouring X is tested by paper chromatography together with three reference dyes P, Q and R, all spotted on the same pencil baseline and run in the same solvent. The solvent front is 8.0 cm from the baseline. X gives two spots, at 2.4 cm and 5.2 cm from the baseline. P has a spot at 5.2 cm, Q at 3.6 cm and R at 2.4 cm. (a) Calculate the Rf values of the two spots from X. (b) Is X a pure substance or a mixture? (c) Which of the reference dyes does X contain? (d) Which dye in X is more soluble in the solvent?
- 1(a) Measure both distances from the baseline; for the lower spot, Rf = 2.4 cm ÷ 8.0 cm = 0.30.
- 2For the upper spot, Rf = 5.2 cm ÷ 8.0 cm = 0.65; both answers lie between 0 and 1 and have no units, as expected.
- 3(b) X gives two spots, so X is a mixture of at least two dyes.
- 4(c) P has a spot at 5.2 cm (Rf = 0.65), which matches the upper spot of X, so X contains P.
- 5R has a spot at 2.4 cm (Rf = 0.30), which matches the lower spot of X, so X contains R.
- 6Q has a spot at 3.6 cm (Rf = 3.6 ÷ 8.0 = 0.45), and X has no spot at this height, so X does not contain Q.
- 7Conclusion: X is a mixture that contains dyes P and R but not Q.
- 8(d) P travels further up the paper (larger Rf value), so P is more soluble in the solvent than R.
Watch out: Measure from the baseline, not the bottom edge of the paper, and put the solvent front distance at the bottom of the fraction. An Rf value greater than 1 means the two distances have been swapped.
Common mistakes
Mixing up downward and upward delivery
Wrong: Downward delivery is for gases that are less dense than air.
Right: The name describes the direction the gas moves into the gas jar. Downward delivery collects gases denser than air (e.g. chlorine, hydrogen chloride). Upward delivery collects gases less dense than air (e.g. ammonia).
Drying ammonia with the wrong agent
Wrong: Concentrated sulfuric acid can dry any gas, including ammonia.
Right: Ammonia is alkaline, so it reacts with concentrated sulfuric acid and is lost. Fused calcium chloride also reacts with ammonia. Dry ammonia with calcium oxide (quicklime).
Treating a sharp melting point as proof of identity
Wrong: A sharp melting point means it is the substance I expected.
Right: A sharp (fixed) melting point only shows the substance is pure. To confirm its identity, the melting point must also match the known value for that substance.
Questions students ask
How do you decide which method to use to collect a gas?
First ask whether the gas is soluble in water: an insoluble or only slightly soluble gas can be collected over water. If it is soluble, compare its density with air: a gas denser than air is collected by downward delivery and a gas less dense than air by upward delivery. If the volume must be measured, use a gas syringe.
What is the difference between evaporation to dryness and crystallisation?
Evaporation to dryness heats the solution until all the solvent has boiled away, and is used only for solids that are stable to heat, such as sodium chloride. Crystallisation heats the solution only until it is saturated, then lets it cool so crystals form. It is used for solids that decompose on strong heating, such as hydrated copper(II) sulfate, and gives a purer product because soluble impurities stay in solution.
Why is the baseline in paper chromatography drawn in pencil?
Pencil lead (graphite) is insoluble in the solvent, so it stays in place. Ink contains dyes that would dissolve, move up the paper and interfere with the spots from the sample. An answer such as “pencil does not smudge” does not score.
How do impurities affect melting point and boiling point?
Impurities lower the melting point and make the solid melt over a range of temperatures, and they raise the boiling point so that boiling takes place over a range. The more impurity present, the bigger the effect. For example, salt water freezes below 0 °C and boils above 100 °C.
In the full PDF
- •Full apparatus and precision table for time, temperature, mass and volume, with the burette-reading rules
- •Diagrams of the four gas-collection set-ups, a labelled chromatogram and heating curves for a pure and an impure solid
- •Two further worked examples: preparing a dry sample of ammonia, and obtaining pure dry sand and copper(II) sulfate crystals from a mixture
- •Master table of separation methods, the drying-agent table and the hazard-symbol table with precautions
- •Step-by-step methods for crystallisation, distillation and the separating funnel, including the three distillation facts tested every year
- •Remaining common mistakes, the exam technique checklist and the end-of-topic summary