Drawing and Reading Chemistry Diagrams: Where 0620 Marks Are Won With a Pencil

About a fifth of your 0620 grade (Paper 6 is worth 20%) is assessed substantially through diagrams: drawing apparatus, completing them, plotting graphs and reading values off them. The theory papers add dot-and-cross diagrams, energy profiles and electrolysis cells. None of this requires drawing talent. It requires knowing the conventions, because the mark scheme checks conventions, not artistry.

Apparatus: 2D cross-sections, ruled and labelled

Cambridge wants section drawings, as if the apparatus were sliced down the middle. The conventions that carry marks:

• 2D outlines only. A beaker is a U with a flat base. A round-bottom flask is a circle with a neck. No 3D perspective, no shading.
• Ruler for every straight line. Delivery tubes, beaker walls, clamp stands. Wobbly freehand lines invite the examiner to doubt whether your tube actually enters the liquid.
• Label every component, including the substances: “dilute hydrochloric acid”, not just “acid”. Label lines should touch the thing they name and never cross each other.
• Workability is marked. A gas-collection setup drawn as a sealed system would explode; that drawing scores zero even if every part is named. Check: can gas escape or be collected? Does the delivery tube dip below the water (or stop above the liquid, where it must)? Does the thermometer bulb sit at the right height, beside the side-arm for distillation, not in the liquid?

The classic examples worth practising until automatic: gas collection over water, downward delivery, filtration, simple distillation, and heating under reflux is not on 0620. Don’t import it from A Level books. Our Paper 6 guide works through the apparatus questions that appear most.

Dot-and-cross diagrams: electrons in the right place

These appear on every theory paper. The rules differ by bond type.

Covalent (e.g. H2O, CH4, HCl, CO2): draw overlapping outer shells; shared pairs sit in the overlap, one dot and one cross per pair so the examiner can see which atom contributed which electron. Lone pairs must be shown: water needs its two lone pairs on oxygen or the diagram is incomplete. Count at the end: oxygen finishes with 8 outer electrons, hydrogen with 2.

Ionic (e.g. NaCl, MgO, CaCl2): no overlap. Draw each ion separately, in square brackets, with the charge at the top right outside the bracket. The metal ion’s outer shell is shown empty (or the next full shell, depending on what the question asks); the non-metal’s shell is full, with the transferred electron drawn as a cross among the dots to show its origin. Forgetting the brackets and charges is the single most common dropped mark here: the bonding is the attraction between charges, so missing charges means missing bonding.

For CaCl2-type compounds, draw both chloride ions; one chloride “×2” is risky unless the question’s space clearly invites it.

Energy profiles: three labels or no marks

An energy profile (reaction pathway diagram) is marked on its labels, not its elegance:

1. Axes: energy on the y-axis, progress of reaction on the x-axis.
2. Reactants and products at the correct relative heights: products lower for exothermic, higher for endothermic.
3. ΔH drawn as a vertical arrow between reactant and product levels, pointing down (exothermic) or up (endothermic).
4. Activation energy, where asked, from the reactant level to the peak, not from zero, not from the product level.

A catalyst question wants a second, lower peak on the same diagram with both curves starting and ending at the same energies. Students who can recite “a catalyst lowers the activation energy” still draw the products at a different height, and lose the mark to their own sketch.

Electrolysis cells: the diagram the examiners reuse

The electrolysis cell appears in theory papers and practical papers alike, and the marking checklist barely changes:

• Power supply with + and − terminals marked.
• Electrodes labelled anode (+) and cathode (−), connected to the matching terminals. Reversed polarity kills every downstream answer.
• Electrolyte named precisely: “concentrated aqueous sodium chloride” and “dilute sulfuric acid” give different products, so vague labels cost marks beyond the diagram.
• Electrode material stated where it matters: carbon (inert) versus copper electrodes changes the copper(II) sulfate result entirely.
• For gas products, collection tubes drawn over each electrode if the question involves measuring volumes.

Then the diagram feeds the chemistry: cations migrate to the cathode and are reduced, anions to the anode and are oxidised. If your labelled diagram is right, the half-equations almost write themselves.

Graph marks: four marks, four checkable rules

Paper 6 (and sometimes Paper 4) asks you to plot data, and the scheme is predictable:

1. Scale: linear, sensible (multiples of 1, 2, 5, 10 per square), with the plotted points filling more than half the printed grid in both directions. A scale of 3 per square is unmarkable for the plotter and the examiner alike. Never use it.
2. Axes labelled with quantity and unit: “volume of gas / cm³”, “time / s”. The independent variable goes on the x-axis.
3. Plotting accurate to within half a small square. Use a sharp pencil and small neat crosses.
4. Line of best fit: one smooth line (ruled if the trend is linear, a single confident freehand curve if not) passing through or close to the points, with scatter balanced either side. Ignore an obvious anomaly and, if asked, circle it.

Reading the graph back earns separate marks: take values with a ruler, draw the construction lines on the graph so the examiner sees your method, and quote readings to the precision of the scale.

Practise drawing under marking, not just looking

Reading a perfect diagram teaches you less than drawing a flawed one and finding out why it failed. Take questions from past papers, draw against the clock, then mark with the real scheme. The 0620 exam format guide shows where diagram marks sit in each paper, and the Paper 5 guide covers the recording conventions practical candidates need on top.

Diagram habits are the easiest mistakes for a specialist to spot and the fastest to fix: most are mechanical, not conceptual. In a free 1-hour trial lesson, our tutors mark a set of your drawings against the 0620 scheme and show you, line by line, where the pencil is leaking marks. Students are routinely surprised that the fix takes one session.