← Back to Chemistry
Cambridge IGCSE Chemistry 0620 · Extended · Paper 4

Drawing Skills Practice Sheet

Dot-and-cross diagrams · energy level diagrams · apparatus · electrolysis cells · graphs and tables. These are the marks a screen cannot test — print this and draw them by hand.

Print it, then draw every diagram in pencil with a ruler — model answers are hidden from the printout, so check them on screen afterwards.
How to use this sheet. Work in pencil so you can rub out. Give yourself the mark allocation as a time guide (roughly 1½ minutes per mark). The number of marks tells you how many separate features the examiner is looking for — a 3-mark diagram needs three distinct correct things, not one pretty picture. Do not open the model answer until your own attempt is finished.

Section 1 · Dot-and-cross diagrams

Show outer shell electrons only. Use dots for one atom's electrons and crosses for the other's. Ions need square brackets and a charge.

1.1 Sodium chloride, NaCl, is an ionic compound. Draw a dot-and-cross diagram to show the electron arrangement in the ions present in sodium chloride. Show outer shell electrons only. Include square brackets and the charge on each ion. (Na is in Group I, Cl is in Group VII.) [3]
sodium ion chloride ion given: outlines only — add symbols, electrons, brackets and charges
Model answer & marking points
+ Na outer shell now EMPTY − Cl × 7 dots (Cl) + 1 cross (the electron from Na) = 8
Marking points
  • 1 mark — Na+ drawn with no electrons in the outer shell (the 3s electron has been transferred away).
  • 1 mark — Cl− drawn with 8 outer electrons, one of which is clearly shown as a cross (i.e. it came from the sodium).
  • 1 mark — square brackets round each ion with the correct charge written outside the bracket, top right: + on sodium, − on chloride.
  • Common losses: showing all 11 electrons of sodium; forgetting the brackets; putting the charge inside the bracket; giving Cl− only 7 electrons.
1.2 Magnesium burns in oxygen to form magnesium oxide, MgO. Draw a dot-and-cross diagram of magnesium oxide. Show outer electrons only, with brackets and charges. State the charge on each ion. [3]
Mg is in Group II · O is in Group VI draw both ions above this line
Model answer & marking points
2+ Mg both outer electrons lost 2− O × × 6 dots (O) + 2 crosses (from Mg) = 8
Marking points
  • 1 mark — Mg2+ with an empty outer shell.
  • 1 mark — O2− with 8 outer electrons, two of them crosses to show they came from the magnesium.
  • 1 mark — brackets on both ions with charges 2+ and 2− outside.
  • Write the number before the sign: 2+ and 2−, not +2 and −2.
1.3 Calcium fluoride has the formula CaF₂. Draw a dot-and-cross diagram for calcium fluoride. You must show the correct number of each ion. Outer electrons only, with brackets and charges. [3]
Hint: how many electrons must calcium lose, and how many can each fluorine accept?
Model answer & marking points
2+ Ca − F × − F × × 2 Ca gives one electron to each of TWO fluorine atoms
Marking points
  • 1 mark — Ca2+ with an empty outer shell and charge 2+.
  • 1 mark — two fluoride ions drawn (or one drawn and clearly labelled ×2), each with 8 outer electrons.
  • 1 mark — each F− has one cross (the electron gained from calcium) and a charge of −; brackets on all three ions.
  • Charges must balance: (2+) + 2(−) = 0. If you only draw one fluoride the diagram is chemically wrong.
1.4 Water, H₂O, is a simple covalent molecule. Draw a dot-and-cross diagram for a molecule of water. Show outer shell electrons only. Use dots for oxygen electrons and crosses for hydrogen electrons. [2]
given: the oxygen atom — add the two hydrogen atoms and all electrons remember the two lone pairs on oxygen
Model answer & marking points
O H H × × 2 bonding pairs + 2 lone pairs on O · O has 6 outer electrons, each H has 1
Marking points
  • 1 mark — two shared pairs, each drawn in the overlap between O and an H, each containing one dot and one cross.
  • 1 mark — two lone pairs shown on the oxygen, so oxygen has 8 electrons around it in total and each hydrogen has 2.
  • Only outer electrons — do not draw oxygen's inner shell of 2.
  • Count before you stop: O contributes 6 dots, the two H contribute 1 cross each → 8 electrons around O, 2 around each H.
1.5 Draw a dot-and-cross diagram for a molecule of methane, CH₄. Show outer shell electrons only. [2]
given: the carbon atom — carbon has 4 outer electrons how many hydrogen atoms overlap it?
Model answer & marking points
C H H H H × × × ×
Marking points
  • 1 mark — four shared pairs, one to each hydrogen, drawn in the overlap regions.
  • 1 mark — each shared pair contains one dot and one cross; carbon ends with 8 outer electrons and each hydrogen with 2.
  • There are no lone pairs on carbon in methane — all four outer electrons are used in bonding.
1.6 Draw a dot-and-cross diagram for a molecule of ammonia, NH₃. Nitrogen is in Group V. Show outer shell electrons only and label the lone pair. [3]
draw the whole molecule — nothing given
Model answer & marking points
N H H H × × × lone pair
Marking points
  • 1 mark — three shared pairs, one to each hydrogen, each with one dot and one cross.
  • 1 mark — one lone pair of electrons shown on the nitrogen (both electrons must be dots — they are nitrogen's).
  • 1 mark — lone pair correctly labelled; nitrogen surrounded by 8 electrons, each hydrogen by 2.
  • N has 5 outer electrons: 3 used in bonding + 2 as the lone pair. If you draw 4 bonds you have made NH₄+, not NH₃.
1.7 Carbon dioxide contains two carbon–oxygen double bonds. Draw a dot-and-cross diagram for CO₂. Each double bond is two shared pairs, i.e. four electrons in the overlap. Show outer electrons only. [3]
given: the carbon atom in the middle — add both oxygen atoms O has 6 outer electrons · C has 4
Model answer & marking points
C O O × × × × × × × × × × × × C: 4 dots, all bonding · each O: 2 crosses bonding + 4 crosses as 2 lone pairs
Marking points
  • 1 mark — four electrons (2 shared pairs) shown in each C—O overlap, i.e. a genuine double bond on both sides.
  • 1 mark — two lone pairs shown on each oxygen atom.
  • 1 mark — correct dot/cross split: all 4 of carbon's electrons in bonds, each oxygen using 2 of its 6 in the bond; carbon ends with 8, each oxygen with 8.
  • The classic error is drawing only single bonds — then carbon only has 4 electrons around it and the molecule is wrong.
1.8 Ethene, C₂H₄, is an unsaturated hydrocarbon. Draw a dot-and-cross diagram for a molecule of ethene. Use dots for the electrons of the carbon atoms and crosses for the electrons of the hydrogen atoms. [3]
4 C—H single bonds and 1 C≈C double bond nothing given — draw the whole molecule
Model answer & marking points
C C H H H H × × × × Each C: 8 electrons · each H: 2 electrons · C≈C = 4 electrons shared
Marking points
  • 1 mark — four electrons shown in the C—C overlap (the double bond).
  • 1 mark — four C—H shared pairs, each with one dot (carbon) and one cross (hydrogen).
  • 1 mark — every atom has a full outer shell: 8 around each carbon, 2 around each hydrogen; no lone pairs anywhere.
  • Count the electrons in your finished diagram: 2(4) + 4(1) = 12 outer electrons in total.
1.9 Nitrogen gas exists as N₂ molecules containing a triple bond. Draw a dot-and-cross diagram for a nitrogen molecule. Use dots for one nitrogen atom and crosses for the other. Do not forget the lone pairs. [2]
given: the two overlapping atoms — add the symbols and all 10 outer electrons
Model answer & marking points
N N × × × × × 6 shared electrons (3 pairs) + 1 lone pair on each N = 8 around each atom
Marking points
  • 1 mark — six electrons (3 shared pairs) in the overlap, three dots and three crosses.
  • 1 mark — one lone pair on each nitrogen atom, so both atoms are surrounded by 8.
  • Total outer electrons must be 5 + 5 = 10. If your diagram shows more or fewer, you have made a counting slip.

Section 2 · Energy level (reaction pathway) diagrams

Axes must be labelled Energy (y) and Progress of reaction (x). Ea runs from the reactant level to the top of the hump. ΔH runs between the reactant and product levels.

2.1 The combustion of methane is exothermic. On the axes below, draw an energy level diagram for this reaction. Label the reactants, the products, the activation energy Ea and the enthalpy change ΔH. Show the sign of ΔH. [4]
Energy Progress of reaction draw the reaction pathway here
Model answer & marking points
Energy Progress of reaction E a ΔH (negative) reactants products products are LOWER than reactants → energy released → ΔH is negative
Marking points
  • 1 mark — product level drawn below the reactant level, with both levels drawn as horizontal lines and labelled “reactants” and “products”.
  • 1 mark — a hump (the energy barrier) drawn between them whose peak is above the reactant level.
  • 1 mark — Ea arrow drawn from the reactant level up to the top of the hump and labelled.
  • 1 mark — ΔH arrow drawn between the reactant and product levels, pointing downwards, labelled ΔH and identified as negative.
  • Do not measure Ea from the x-axis or from the product level. Do not draw ΔH from the peak.
2.2 The thermal decomposition of calcium carbonate is endothermic. Draw an energy level diagram for this reaction on the axes below. Label reactants, products, Ea and ΔH, and state the sign of ΔH. [4]
Energy Progress of reaction this time the products must end up higher
Model answer & marking points
Energy Progress of reaction E a ΔH (positive) reactants products products are HIGHER than reactants → energy absorbed → ΔH is positive
Marking points
  • 1 mark — product level drawn above the reactant level; both levels horizontal and labelled.
  • 1 mark — hump drawn with its peak above the product level (not just above the reactants).
  • 1 mark — Ea arrow from reactant level to the peak, labelled.
  • 1 mark — ΔH arrow between the two levels pointing upwards, labelled and identified as positive.
  • In an endothermic reaction Ea is always bigger than ΔH — the arrow lengths in your diagram should reflect this.
2.3 A catalyst is added to an exothermic reaction. On one set of axes, draw the reaction pathway with and without the catalyst. Label both curves. Mark Ea (uncatalysed), Ea (catalysed) and ΔH. Your diagram must make clear what a catalyst does and does not change. [4]
Energy Progress of reaction reactants (given) products (given) join them with TWO different humps
Model answer & marking points
Energy Progress of reaction Ea without catalyst Ea with catalyst (lower) ΔH unchanged reactants products —— without catalyst · – – – with catalyst
Marking points
  • 1 mark — two curves drawn on the same axes, starting at the same reactant level and finishing at the same product level.
  • 1 mark — the catalysed curve has a lower peak (smaller Ea).
  • 1 mark — both activation energies marked with arrows from the reactant level to their own peak, and clearly labelled which is which.
  • 1 mark — ΔH marked and stated to be the same for both routes — a catalyst changes only the activation energy.
  • Killer error: drawing the catalysed products at a different energy. The catalyst is not used up and does not change the energy of reactants or products.

Section 3 · Apparatus diagrams

Draw apparatus in section (as if sliced down the middle), leave vessels open at the top only where they really are open, seal every joint where a gas must not escape, and label everything.

3.1 A student reacts excess copper(II) oxide with dilute sulfuric acid. Draw a labelled diagram of the apparatus used to separate the unreacted copper(II) oxide from the copper(II) sulfate solution. Label the residue and the filtrate. [3]
given: beaker add the funnel, the filter paper and the labels
Model answer & marking points
filter funnel filter paper residue (excess CuO) filtrate (copper(II) sulfate solution) beaker
Marking points
  • 1 mark — a cone of filter paper drawn inside a filter funnel (the paper must be inside the funnel, not lying flat across the top).
  • 1 mark — the funnel stem sitting inside a beaker/flask that catches the liquid.
  • 1 mark — correct labels: residue (the solid left in the paper) and filtrate (the liquid that passes through), plus funnel and filter paper.
  • Never draw the mixture being poured straight into the beaker; the paper must lie between the mixture and the beaker.
3.2 The copper(II) sulfate solution is then crystallised. Draw a labelled diagram of the apparatus used to evaporate the solution to the point of crystallisation. Then, underneath your diagram, write the two remaining steps needed to obtain dry crystals. [4]
given: tripod and bench add the container, the solution, the gauze and the heat source
Model answer & marking points
evaporating basin saturated solution gauze tripod Bunsen burner (gentle heating) Heat until about half the water has evaporated / crystals start to form at the edge
Marking points
  • 1 mark — solution in an evaporating basin (open dish), not a sealed vessel.
  • 1 mark — basin supported on gauze on a tripod with the heat source underneath, all labelled.
  • 1 mark — heat only until the solution is saturated (crystallisation point) — do not evaporate to dryness or you decompose/dehydrate the salt.
  • 1 mark — remaining steps: leave the saturated solution to cool slowly so crystals form, then filter off the crystals and dry them between filter papers (or in a warm oven).
3.3 Draw a labelled diagram of the apparatus used to titrate dilute hydrochloric acid against aqueous sodium hydroxide. Your diagram must include the piece of apparatus used to measure 25.0 cm³ of alkali accurately, and the piece used to add acid a little at a time. [4]
given: clamp stand add the burette, tap, conical flask, white tile and pipette
Model answer & marking points
burette (acid) tap conical flask: 25.0 cm³ alkali + a few drops of indicator white tile pipette (with filler) graduation mark clamp
Marking points
  • 1 mark — burette drawn vertically, held in a clamp and stand, with a tap at the bottom, labelled with what it contains.
  • 1 mark — conical flask (not a beaker) positioned directly under the burette jet.
  • 1 mark — pipette shown/named as the apparatus used to measure exactly 25.0 cm³ of alkali, with its graduation mark.
  • 1 mark — white tile under the flask (so the colour change of the indicator is easy to see) and indicator mentioned.
  • A measuring cylinder is not accurate enough for the 25.0 cm³ portion — that answer scores zero.
3.4 Magnesium reacts with dilute hydrochloric acid to produce hydrogen. Draw a labelled diagram of the apparatus you would use to collect the hydrogen over water and measure its volume. The apparatus must be gas-tight. Show clearly where the delivery tube ends. [4]
given: trough add the reaction flask, bung, delivery tube and the inverted measuring cylinder
Model answer & marking points
bung (gas-tight) magnesium + dilute HCl delivery tube measuring cylinder full of water hydrogen collected end of delivery tube UNDER the water, inside the mouth of the cylinder
Marking points
  • 1 mark — reaction vessel sealed with a bung and a delivery tube — no gaps anywhere for gas to escape.
  • 1 mark — a measuring cylinder (or gas jar/burette) filled with water and inverted in a trough of water.
  • 1 mark — the end of the delivery tube drawn below the water surface and under the mouth of the inverted cylinder.
  • 1 mark — everything labelled: reactants, bung, delivery tube, trough of water, gas collected.
  • Hydrogen is only slightly soluble in water, so this method works. Do not use it for very soluble gases such as ammonia or hydrogen chloride.
3.5 A student investigates the rate of reaction between calcium carbonate and dilute hydrochloric acid by measuring the volume of carbon dioxide produced every 30 seconds. Draw the apparatus, using a gas syringe. Label all apparatus and state the two measuring instruments needed. [3]
bench level nothing given — draw the flask, the bung, the tube and the syringe
Model answer & marking points
bung — no gaps excess dilute HCl + calcium carbonate chips delivery tube gas syringe (reads volume of CO₂ in cm³) stopwatch syringe supported in clamps and stands
Marking points
  • 1 mark — sealed flask: reactants in a conical flask closed with a bung, delivery tube leading out. No gaps.
  • 1 mark — gas syringe connected to the delivery tube, drawn horizontally and supported, labelled with the quantity it measures (volume of gas / cm³).
  • 1 mark — both instruments named: gas syringe for volume and stopwatch/timer for time.
  • The bung must be pushed in as soon as the acid is added, or gas is lost before you start timing — a very common written follow-up mark.
3.6 Draw a labelled diagram of the apparatus used to obtain pure water from sea water by simple distillation. Show where the thermometer bulb must be, and clearly label the water in and water out of the condenser. [4]
flask → thermometer → condenser (sloping down) → receiver bench level
Model answer & marking points
water out water in thermometer bulb level with the side arm sea water Liebig condenser pure water heat
Marking points
  • 1 mark — flask containing the mixture with a heat source, sealed by a bung carrying the thermometer.
  • 1 mark — thermometer bulb level with the side arm (so it measures the temperature of the vapour going into the condenser), not dipping into the liquid.
  • 1 mark — condenser sloping downwards to the receiver, with water in at the lower end and water out at the upper end (counter-current), both labelled.
  • 1 mark — receiving vessel labelled with the distillate (pure water) and the apparatus not sealed at the receiver end — if it is sealed, pressure builds up.
  • Draw the condenser as an outer jacket with an inner tube. A single tube scores nothing for the condenser mark.
3.7 Ethanol (b.p. 78°C) and water (b.p. 100°C) are miscible. Draw the apparatus used to separate ethanol from a mixture of ethanol and water in the laboratory. Your diagram must show and label the extra piece of apparatus that makes this different from simple distillation. [4]
given: flask what goes between the flask and the condenser?
Model answer & marking points
water out water in thermometer reads 78°C while ethanol distils over fractionating column packed with glass beads (large surface area) ethanol/water mixture ethanol Liebig condenser
Marking points
  • 1 mark — fractionating column drawn vertically between the flask and the condenser, and labelled.
  • 1 mark — column shown packed (glass beads / rings) or with a clear temperature gradient, and the thermometer bulb at the top of the column level with the side arm.
  • 1 mark — condenser with water in at the lower end, water out at the upper end, sloping down to the receiver.
  • 1 mark — all labels present, including which liquid distils first and at what temperature (ethanol, 78°C, because it has the lower boiling point).
3.8 Draw a labelled diagram to show how paper chromatography is set up to separate the dyes in a sample of black ink. Label the baseline, the solvent, the solvent front and the chromatography paper. Explain in one line why the baseline is drawn in pencil. [4]
given: beaker add the paper, the baseline, the spots, the solvent and the lid
Model answer & marking points
lid (stops the solvent evaporating) solvent front separated dyes pencil baseline — ABOVE solvent level solvent chromatography paper Pencil is insoluble in the solvent, so the baseline does not run and contaminate the results
Marking points
  • 1 mark — paper suspended in a container with solvent in the bottom, container covered with a lid/watch glass.
  • 1 mark — baseline drawn above the level of the solvent, with the spot(s) placed on the baseline. If the baseline is under the solvent the spots dissolve into it and the experiment fails.
  • 1 mark — solvent front marked and separated spots shown at different heights up the paper.
  • 1 mark — baseline drawn in pencil because pencil is insoluble in the solvent and will not run (ink would separate too).
  • Rf = distance moved by the spot ÷ distance moved by the solvent front, measured from the baseline in both cases.

Section 4 · Electrolysis cells

Cathode = negative and attracts positive ions (cations). Anode = positive and attracts negative ions (anions). Electrons move through the wires; ions move through the electrolyte. Never draw electrons moving through the solution.

4.1 Molten lead(II) bromide is electrolysed using inert graphite electrodes. Draw a fully labelled diagram of the apparatus. Label the cathode and the anode with their charges, show the direction of movement of each ion, and name the product formed at each electrode. [5]
given: container add the electrodes, the d.c. supply, the heat source, all arrows and labels
Model answer & marking points
− + d.c. power supply e− e− Pb²⁺ Br⁻ heat — the lead(II) bromide must be MOLTEN so the ions can move CATHODE negative electrode (graphite) ANODE positive electrode (graphite) molten lead formed at cathode red-brown bromine vapour at anode molten lead(II) bromide (the electrolyte)
Marking points
  • 1 mark — two electrodes dipping into the molten electrolyte, connected to a d.c. supply (a battery, not a.c.), circuit complete.
  • 1 mark — cathode labelled negative and anode labelled positive, matching the terminals of the supply.
  • 1 mark — ion movement shown: Pb2+ moves to the cathode, Br− moves to the anode, with arrows in the electrolyte.
  • 1 mark — products named at the right places: lead (a molten grey bead) at the cathode, bromine (red-brown vapour) at the anode.
  • 1 mark — heat shown/stated so the compound is molten, and the electrolyte labelled.
  • Half-equations if asked: cathode Pb2+ + 2e− → Pb (reduction); anode 2Br− → Br₂ + 2e− (oxidation).
4.2 Concentrated aqueous sodium chloride (brine) is electrolysed using inert electrodes. Draw a labelled diagram of apparatus that allows both gaseous products to be collected separately. Name the product at each electrode and state what remains in solution. [5]
given: cell and solution level add electrodes, gas collecting tubes, the power supply and all labels
Model answer & marking points
− + d.c. supply H⁺ (and Na⁺) Cl⁻ (and OH⁻) CATHODE (−) hydrogen collected — squeaky pop ANODE (+) chlorine collected — bleaches damp litmus paper concentrated aqueous sodium chloride sodium hydroxide left in solution Chlorine wins over oxygen at the anode because the solution is CONCENTRATED
Marking points
  • 1 mark — two inert electrodes in brine, connected to a d.c. supply, cathode marked negative and anode positive.
  • 1 mark — an inverted tube over each electrode so the two gases are collected separately (they must not mix).
  • 1 mark — hydrogen at the cathode (H+ is discharged in preference to Na+ because sodium is more reactive than hydrogen).
  • 1 mark — chlorine at the anode, because the chloride solution is concentrated.
  • 1 mark — sodium hydroxide identified as remaining in solution, plus the ion movement arrows.
  • If the solution were dilute, oxygen would be given off at the anode instead. Always read the concentration in the question.
4.3 A steel spoon is to be electroplated with copper. Draw a labelled diagram of the apparatus. State which electrode the spoon must be, what the other electrode is made of, and name a suitable electrolyte. [4]
given: beaker + liquid level which side is the spoon? what is the electrolyte?
Model answer & marking points
− + d.c. supply Cu²⁺ CATHODE (negative) = the steel spoon it GAINS mass as copper is deposited ANODE (positive) = pure copper it LOSES mass as it dissolves into solution aqueous copper(II) sulfate (a soluble copper salt) Cathode: Cu²⁺ + 2e⁻ → Cu · Anode: Cu → Cu²⁺ + 2e⁻ · concentration of the solution stays the same
Marking points
  • 1 mark — the object to be plated is the cathode (negative electrode) — positive Cu2+ ions are attracted to it.
  • 1 mark — the anode is made of the plating metal, i.e. pure copper.
  • 1 mark — electrolyte is a solution of a soluble salt of the plating metal, e.g. aqueous copper(II) sulfate.
  • 1 mark — complete circuit with a d.c. supply, both electrodes dipping into the electrolyte, everything labelled.
  • The anode loses mass by exactly the amount the cathode gains, so the concentration of the electrolyte does not change.
4.4 Draw a labelled diagram of a hydrogen–oxygen fuel cell. Show the gases entering, the electrolyte, the direction of electron flow in the external circuit and the only product. Write the overall equation underneath. [4]
given: cell body add the two electrodes, gas inlets, the external circuit and the product
Model answer & marking points
lamp / electric motor e− flow e− hydrogen in (the fuel) oxygen in (from the air) OH⁻ ions move through the electrolyte water out — the only product NEGATIVE electrode (porous, hydrogen side) POSITIVE electrode (porous, oxygen side) electrolyte (aqueous potassium hydroxide)
Marking points
  • 1 mark — hydrogen fed to one electrode and oxygen to the other, both electrodes in contact with the electrolyte.
  • 1 mark — electrolyte labelled (aqueous alkali such as KOH) and the electrodes labelled positive/negative — hydrogen at the negative electrode.
  • 1 mark — electron flow arrow in the external circuit from the hydrogen (negative) electrode, through the lamp/motor, to the oxygen (positive) electrode. Electrons never travel through the electrolyte.
  • 1 mark — water shown as the only product, and the overall equation: 2H₂ + O₂ → 2H₂O.
  • Advantages to quote: only product is water (no CO₂, no pollutants) and it is more efficient than a petrol engine. Disadvantages: hydrogen is hard to store and is often made from fossil fuels.

Section 5 · Graphs and tables

Label both axes with the quantity and its unit. Curves must be smooth — never join the points with a zig-zag of straight lines. Table headings always carry units, and every reading in a column has the same number of decimal places.

5.1 Excess magnesium is added to 50 cm³ of hydrochloric acid. The experiment is repeated with the same volume of acid at three different concentrations, 2.0, 1.0 and 0.5 mol/dm³. Sketch, on the same axes, the three curves obtained. The same amount of acid is used each time. Label each curve with its concentration. [3]
Volume of hydrogen / cm³ Time / s draw three smooth curves — think about the gradient AND the final volume
Model answer & marking points
Volume of hydrogen / cm³ Time / s 2.0 mol/dm³ 1.0 mol/dm³ 0.5 mol/dm³ same final volume? Higher concentration → steeper initial gradient → finishes sooner
Marking points
  • 1 mark — all three curves start at the origin and are smooth, steep at first and levelling off to a horizontal plateau.
  • 1 mark — the higher the concentration, the steeper the initial gradient and the sooner the curve flattens; curves in the correct order and labelled.
  • 1 mark — because magnesium is in excess, the acid is the limiting reactant. Halving the concentration halves the moles of acid, so 1.0 mol/dm³ gives half the final volume of 2.0 mol/dm³, and 0.5 gives a quarter. Curves must plateau at different heights.
  • Watch the wording. If instead the acid were in excess and the mass of magnesium was the same each time, all three curves would plateau at the same volume — only the steepness would differ. Decide which reactant is limiting before you draw.
5.2 Hydrogen peroxide decomposes slowly to give oxygen. The experiment is repeated with the same amount of hydrogen peroxide but with manganese(IV) oxide added as a catalyst. On the axes, sketch and label both curves. Then state, in one sentence, why the two curves finish at the same height. [3]
Volume of oxygen / cm³ Time / s
Model answer & marking points
Volume of oxygen / cm³ Time / s with catalyst without catalyst same final volume A catalyst speeds the reaction up; it does not change how much product is made
Marking points
  • 1 mark — catalysed curve is steeper at the start and reaches its plateau earlier.
  • 1 mark — both curves start at the origin and plateau at exactly the same volume.
  • 1 mark — explanation: the same amount (moles) of hydrogen peroxide is used each time and the catalyst is not used up and does not change the products, so the total volume of oxygen produced is unchanged — it is only produced faster.
  • Both curves must be labelled. An unlabelled pair of curves cannot score the comparison mark.
5.3 A pure solid substance is heated at a steady rate from below its melting point until it has completely turned into a gas. Sketch the heating curve. Label the two horizontal sections with the state change happening and mark the melting point and the boiling point. Label the state(s) present in each region. [4]
Temperature / °C Time / min two sloping sections, two flat sections, then a final slope
Model answer & marking points
Temperature / °C Time / min m.p. b.p. melting: solid → liquid (solid + liquid together) boiling: liquid → gas (liquid + gas together) solid liquid gas Temperature stays constant during a change of state because the energy supplied breaks forces of attraction between particles, not raising their kinetic energy
Marking points
  • 1 mark — curve rises, then a horizontal plateau, rises again, a second horizontal plateau, then rises — five sections in the right order.
  • 1 mark — first plateau labelled melting (solid → liquid) at the melting point; second labelled boiling (liquid → gas) at the boiling point.
  • 1 mark — states labelled: solid; solid + liquid; liquid; liquid + gas; gas.
  • 1 mark — the boiling plateau drawn longer than the melting plateau (more energy is needed to separate the particles completely) and both plateaus perfectly flat — a pure substance melts and boils at a fixed temperature.
  • An impure substance melts over a range of temperatures and at a lower temperature, so its “plateau” would slope.
5.4 A student titrated dilute hydrochloric acid from a burette into 25.0 cm³ of aqueous sodium hydroxide. Complete the results table below. Add the missing heading with its unit, complete the calculated row to the correct number of decimal places, tick the concordant results and work out the mean titre. [4]
Complete the table
Titration number rough123
Final burette reading / cm³ 24.5024.1048.2024.05
Initial burette reading / cm³ 0.000.1024.100.00
 
Concordant? (✓ or ✗)
Mean titre used in the calculation
Show your working. Which titres do you include, and which do you leave out? Why?
Model answer & marking points
Titration number rough123
Final burette reading / cm³ 24.5024.1048.2024.05
Initial burette reading / cm³ 0.000.1024.100.00
Volume of acid added / cm³ 24.5024.0024.1024.05
Concordant? ✗✓✓✓
  • Mean titre = (24.00 + 24.10 + 24.05) ÷ 3 = 72.15 ÷ 3 = 24.05 cm³
Marking points
  • 1 mark — heading written in full with its unit: Volume of acid added / cm³ (a heading without a unit loses the mark; the unit goes in the heading, never next to every number).
  • 1 mark — all four subtractions correct.
  • 1 mark — every value to 2 decimal places, matching the burette readings — write 24.00, not 24 or 24.0. Burettes are read to the nearest 0.05 cm³.
  • 1 mark — the rough titration is excluded; the mean is taken only from the concordant titres (those within 0.10 cm³ of each other) giving 24.05 cm³.
  • Other table rules examiners apply: independent variable in the first column, no units repeated inside the body of the table, and a ruled border round every cell drawn with a ruler.
5.5 The table shows the volume of carbon dioxide produced when marble chips react with excess dilute hydrochloric acid. Plot these results on the grid and draw a smooth curve. Then use your graph to state the time at which the reaction stopped, and explain how the shape of the curve shows that the rate decreases. [4]
Time / s020406080100120
Volume of CO₂ / cm³0223645505252
0 20 40 60 80 100 120 0 10 20 30 40 50 60 Volume of CO₂ / cm³ Time / s
Model answer & marking points
Volume of CO₂ / cm³ Time / s 0 20 40 60 80 100 120 0 20 40 60 curve becomes horizontal at about 100 s Gradient is steepest at t = 0 and gets less steep → the rate is fastest at the start and slows down
Marking points
  • 1 mark — all seven points plotted accurately (to within half a small square) using neat × crosses or dots in circles — not fat blobs.
  • 1 mark — a single smooth curve drawn through the points with a sharp pencil. Do not join dot-to-dot with a ruler and do not extend the line beyond the last point.
  • 1 mark — the reaction stops at about 100 s, read from where the curve becomes horizontal (the volume stays at 52 cm³).
  • 1 mark — the gradient of the curve decreases with time, which shows the rate is fastest at the start and slows down as the acid is used up and its concentration falls.

How examiners award drawing marks in Chemistry

Read this before every drawing question. Most lost marks are lost here, not in the chemistry.

The eight rules

  1. Sharp pencil and a ruler. Straight edges of apparatus, axes, table borders and level lines are drawn with a ruler. Curves are drawn freehand but smoothly, in one continuous stroke. Never use pen — you cannot correct it.
  2. Label every piece of apparatus and every substance. An unlabelled drawing rarely scores full marks. Use a straight leader line from the label to the item, and do not let labels cross each other or the diagram.
  3. No gaps in the apparatus. If a gas is being collected or measured, the vessel must be sealed with a bung and the tubing must join the apparatus with no breaks — a gap anywhere means the gas escapes and the diagram scores zero for that mark. Equally, apparatus that must be open (an evaporating basin, the receiver in a distillation) must be shown open.
  4. Draw apparatus in section. Show the vessel as if it has been cut down the middle, with a single line for the glass and a horizontal line for the liquid level. Do not shade with heavy scribble.
  5. Dot-and-cross: outer electrons only. Never draw the inner shells. Use dots for one atom and crosses for the other, put shared pairs in the overlap, and always show the lone pairs. Count the electrons around every atom before you stop.
  6. Ions need square brackets and a charge. Every ion in an ionic dot-and-cross diagram goes inside square brackets with the charge written outside the bracket at the top right — 2+ and 2−, in that order.
  7. Axes labelled with quantity and unit. “Energy” and “Progress of reaction” for reaction pathways; “Volume of gas / cm³” and “Time / s” for rate graphs. On an energy diagram, Ea is measured from the reactant level to the peak and ΔH from the reactant level to the product level, with the arrow pointing the right way.
  8. Let the mark allocation tell you how many features to include. A [4] diagram needs four separate creditworthy features. Before you move on, count the distinct things you have drawn or labelled — if you can only find three, something is missing.

Quick self-check before you turn the page

  1. Have I labelled everything, including what is inside each container?
  2. Is the apparatus sealed where it needs to be and open where it needs to be?
  3. For electrolysis: is the cathode marked negative and the anode positive, with electrons in the wires and ions in the electrolyte?
  4. For dot-and-cross: outer shells only, brackets and charges on ions, lone pairs shown, electron count correct?
  5. For graphs: axes labelled with units, points plotted accurately, one smooth curve, curves labelled?
  6. Does the number of features I have drawn match the number of marks?

Cambridge IGCSE Chemistry 0620 (Extended) · Drawing Skills Practice Sheet · 29 exercises · print, draw in pencil, then check against the model answers on screen.