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GCSE · Chemistry

C-RP Chemistry Required Practicals and Data Analysis

AQA 8464 (Combined Science: Trilogy); required-practical content shared with GCSE Chemistry 8462 · Calculator allowed · about 100 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Working scientifically across the chemistry required practicals

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This pack is synoptic: rather than testing one topic in depth, it revisits the apparatus, method, safety and data-analysis skills that come up across six required practicals - preparing a soluble salt, titration, electrolysis, temperature changes, rate of reaction, and paper chromatography - plus the analysis and purification of water. Marks are frequently awarded for precise practical detail (the correct piece of apparatus, the right variable, a reason linked to the data given) rather than a single word, so answer in full sentences, always check any table given before calculating, and show your working for every calculation so that method marks can be awarded even if the final answer is wrong.

A student prepares pure, dry crystals of zinc sulfate by reacting zinc oxide (an insoluble base) with dilute sulfuric acid: ZnO(s) + H2SO4(aq) -> ZnSO4(aq) + H2O(l)

(a)Name the type of reaction taking place between zinc oxide and dilute sulfuric acid.(1)

(b)The equation above is already balanced. Rewrite it including state symbols for every substance.(2)

(c)The five steps below describe the rest of the method, but are in the wrong order. Write down the correct order using the letters A to E.
A. Filter the mixture to remove the excess, unreacted zinc oxide.
B. Warm the dilute sulfuric acid gently in a beaker.
C. Heat the zinc sulfate solution gently to evaporate some of the water, then leave it to cool undisturbed so that crystals form.
D. Add zinc oxide powder to the warm acid a little at a time, stirring, until no more reacts and some solid remains (in excess).
E. Filter off the zinc sulfate crystals and dry them between sheets of filter paper.(3)

(Total for Question 1 is 6 marks)

Continuing the zinc sulfate preparation from Question 1.

(a)Explain why the acid is stirred continuously while the zinc oxide is being added, and why the zinc oxide is added gradually, a little at a time, rather than all at once.(2)

(b)State an observation that would show the reaction between the zinc oxide and the acid is complete.(1)

(c)State one safety precaution the student should take when handling the dilute sulfuric acid, and explain the hazard it addresses.(2)

(Total for Question 2 is 5 marks)

A student carries out a titration to find the concentration of a solution of hydrochloric acid. They pipette 25.0 cm3 of 0.100 mol/dm3 sodium hydroxide solution into a conical flask, add a few drops of phenolphthalein indicator, and add the hydrochloric acid from a burette until the indicator just turns colourless. The table shows the burette readings from four titrations.
Titration: Rough, 1, 2, 3
Initial reading (cm3): 0.00, 0.20, 0.00, 24.50
Final reading (cm3): 24.60, 23.40, 23.15, 47.70

(a)Name the piece of apparatus that should be used to measure the 25.0 cm3 of sodium hydroxide solution accurately into the conical flask.(1)

(b)State why a white tile is usually placed under the conical flask during this titration.(1)

(c)Complete the table by calculating the titre (final reading minus initial reading) for each of the four titrations.(2)

(d)Titres that agree within 0.10 cm3 of each other are described as concordant. Identify which titres are concordant, and use only those values to calculate the mean titre.(2)

(Total for Question 3 is 6 marks)

Using the mean titre of 23.18 cm3 from Question 3 (0.100 mol/dm3 sodium hydroxide reacting with hydrochloric acid of unknown concentration): HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)

(a)Calculate the number of moles of sodium hydroxide used in the titration.(2)

(b)The equation shows a 1:1 mole ratio between hydrochloric acid and sodium hydroxide. Calculate the concentration, in mol/dm3, of the hydrochloric acid. Give your answer to 3 significant figures.(3)

(c)Higher tier only. Calculate the concentration of the hydrochloric acid in g/dm3. (Mr of HCl = 36.5)(2)

(Total for Question 4 is 7 marks)

Further questions on the titration described in Questions 3 and 4.

(a)Suggest why the rough titration result should not be included when calculating the mean titre.(1)

(b)Higher tier only. The burette has a resolution of 0.05 cm3, so each individual reading has an uncertainty of plus or minus 0.05 cm3, and every titre is found from two burette readings (an initial and a final reading). Calculate the percentage uncertainty in the mean titre of 23.18 cm3. Give your answer to 3 significant figures.(3)

(Total for Question 5 is 4 marks)

A technician electrolyses dilute sulfuric acid using inert (platinum) electrodes, collecting the gas produced at each electrode in an inverted, water-filled test tube.

(a)Describe a chemical test the technician could use to confirm that the gas collected at the cathode is hydrogen, including the positive result.(2)

(b)Describe a chemical test the technician could use to confirm that the gas collected at the anode is oxygen, including the positive result.(2)

(c)The technician notices that, in any given time, roughly twice as much gas is collected at the cathode as at the anode. Explain this observation in terms of the formula of water.(2)

(Total for Question 6 is 6 marks)

A different student investigates how the current affects the mass of copper deposited on the cathode during the electrolysis of copper sulfate solution, using copper electrodes. Table of results (10 minutes of electrolysis at each current):
Current (A): 0.50, 1.00, 1.50, 2.00
Mass of copper deposited (g): 0.10, 0.20, 0.30, 0.40

(a)State the independent variable, the dependent variable, and one variable that should be controlled in this investigation.(3)

(b)Describe the relationship shown by the table between the current and the mass of copper deposited.(2)

(c)Suggest one safety precaution that should be taken during this investigation.(1)

(Total for Question 7 is 6 marks)

A student investigates the temperature change when zinc powder reacts with copper sulfate solution in an insulated cup: Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)

(a)State the independent variable in this investigation, and suggest one variable, other than the mass of zinc used, that should be controlled to make it a fair test.(2)

(b)Explain why a polystyrene cup with a lid is used, rather than a glass beaker.(2)

(Total for Question 8 is 4 marks)

In the investigation from Question 8, a student reacts excess zinc powder with 25.0 cm3 of 1.00 mol/dm3 copper sulfate solution. The initial temperature was 19.5 degC and the maximum temperature reached was 60.0 degC. Assume the density of the solution is 1.00 g/cm3 and its specific heat capacity is 4.2 J/(g degC).

(a)Calculate the temperature change, delta T, for this reaction.(1)

(b)Calculate the energy transferred, Q, during this reaction, using Q = mass x specific heat capacity x delta T. Give your answer in joules.(3)

(c)Calculate the number of moles of copper sulfate that reacted.(2)

(d)Using your answers to parts (b) and (c), calculate the energy change per mole of copper sulfate, in kJ/mol, and state whether the reaction is exothermic or endothermic.(3)

(Total for Question 9 is 9 marks)

Higher tier only, continuing Question 9.

(a)Explain, in terms of bond breaking and bond making, why the reaction between zinc and copper sulfate solution is exothermic.(2)

(Total for Question 10 is 2 marks)

A student investigates the effect of hydrochloric acid concentration on the rate of its reaction with excess marble chips (calcium carbonate): CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)

(a)Describe how the apparatus should be set up to collect and measure the volume of gas produced, naming two pieces of apparatus other than the conical flask.(2)

(b)State the independent variable, the dependent variable, and one variable that should be controlled in this investigation.(3)

(Total for Question 11 is 5 marks)

The table shows the volume of gas collected over time for one concentration of acid in the investigation from Question 11.
Time (s): 0, 10, 20, 30, 40, 50, 60
Volume of gas (cm3): 0, 22, 38, 48, 54, 56, 56

(a)Calculate the mean rate of reaction between 0 and 30 seconds, in cm3/s.(2)

(b)Explain, in terms of particle collisions, why the rate of reaction decreases as the reaction proceeds, and why the graph becomes flat after 50 seconds.(3)

(c)A repeat experiment uses the same total mass of marble chips, but with a smaller chip size (a greater surface area), and the same volume and concentration of acid. State and explain two differences you would expect to see between this new graph and the one described above.(3)

(Total for Question 12 is 8 marks)

A student investigates the effect of sodium thiosulfate concentration on the rate of its reaction with dilute hydrochloric acid: Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + S(s) + SO2(g) + H2O(l). A conical flask containing the reaction mixture is placed over a black cross drawn on paper, and the time taken for the cross to become no longer visible from above is measured.

(a)State two variables, other than the concentration of sodium thiosulfate, that must be controlled for a valid comparison between different concentrations.(2)

(b)The time taken for the cross to disappear was 40 s. Calculate the rate of this reaction, in s^-1, using rate = 1 / time.(2)

(c)Suggest one reason why judging when the cross disappears could make the results less precise, and suggest an improvement.(2)

(Total for Question 13 is 6 marks)

A technician is deciding which method to use to investigate how the concentration of an acid affects the rate of a reaction: measuring the volume of gas produced over time with a gas syringe (as in Questions 11 and 12), or timing how long a cross takes to disappear using the turbidity method (as in Question 13).

(a)Evaluate these two methods for investigating rate of reaction, and recommend which gives more objective and informative data. In your answer, you should consider objectivity, precision, and the type of data each method produces.(6)

(b)State two variables, other than the volume of acid, that must be kept the same when comparing the gas syringe method with the turbidity method for the same acid concentration.(2)

(Total for Question 14 is 8 marks)

A student uses paper chromatography to analyse the food colourings in a sample of orange squash, comparing it with four pure reference dyes, A, B, C and D.

(a)Describe how the chromatography should be set up, including how the sample should be applied to the paper.(3)

(b)The solvent front travelled 8.8 cm from the baseline. One spot from the orange squash sample travelled 6.6 cm. Calculate the Rf value of this spot.(2)

(Total for Question 15 is 5 marks)

Continuing Question 15: the orange squash sample produced three separate spots on the chromatogram, at Rf values matching pure dyes A, C and D, but not dye B.

(a)State what this shows about the composition of the orange squash sample, and explain how paper chromatography can be used to tell whether a substance is pure or a mixture.(3)

(b)State one reason why the baseline is drawn in pencil rather than in ink.(1)

(c)Higher tier only. A different, more polar solvent is used to repeat the chromatography of the orange squash sample. Explain why the Rf value of a given dye might be different with this new solvent.(2)

(Total for Question 16 is 6 marks)

A technician tests the purity of three water samples, P, Q and R, by measuring their electrical conductivity using a simple circuit with a battery, an ammeter and two graphite electrodes dipped into each sample in turn. Results: Sample P (tap water): current = 0.35 A. Sample Q (deionised water): current = 0.02 A. Sample R (sea water): current = 1.20 A.

(a)Identify which sample is most likely to be pure water, and explain your reasoning based on the results.(2)

(b)Explain why sample R (sea water) produces the largest current.(2)

(c)Suggest one variable that should be controlled for this to be a valid comparison between the three samples.(1)

(d)Higher tier only. Suggest how a calibration curve could be produced to estimate the concentration of dissolved ions in an unknown water sample from its current reading.(2)

(Total for Question 17 is 7 marks)

Two ideas used across many of the required practicals in this pack are repeatability and reproducibility.

(a)Define the term repeatability, and explain how the titration results in Question 3 show that the method was repeatable.(2)

(b)Define the term reproducibility, explaining how it differs from repeatability.(1)

(c)In the rate of reaction investigation from Question 12, one repeat gave an anomalous result that did not fit the trend of the other results. Suggest what the student should do with this anomalous result, and justify your answer.(2)

(Total for Question 18 is 5 marks)

Mark scheme · C-RP Chemistry Required Practicals and Data Analysis

Question 1

(a) B1 neutralisation
(a) Answer: Neutralisation
(b) M1 ZnO(s) and H2SO4(aq) correct on the left
(b) A1 ZnSO4(aq) and H2O(l) correct on the right, all four state symbols correct overall
(b) Answer: ZnO(s) + H2SO4(aq) -> ZnSO4(aq) + H2O(l)
(c) M1 B placed before D
(c) M1 D placed before A
(c) A1 A, C and E placed in that order after D, giving the fully correct sequence B, D, A, C, E
(c) Answer: B, D, A, C, E

Question 2

(a) M1 stirring mixes the reactants evenly and speeds up the reaction (increases the frequency of collisions) oe
(a) M1 adding the zinc oxide gradually allows each addition to react before the next is added, preventing the mixture from fizzing over/overflowing or excess solid clumping together unreacted oe
(a) Answer: Stirring mixes the reactants evenly and speeds up the reaction; adding the zinc oxide gradually lets each portion react before the next is added, preventing the mixture from overflowing or excess solid clumping unreacted.
(b) B1 some white solid (zinc oxide) remains undissolved/visible and stops disappearing, even when more is added and stirred oe
(b) Answer: Some white zinc oxide powder remains undissolved and no more of it reacts, even when the mixture is stirred.
(c) B1 wear eye protection (safety goggles) throughout the practical
(c) B1 because dilute sulfuric acid is an irritant and could splash into the eyes, especially while being warmed or stirred, causing harm oe
(c) Answer: Wear eye protection throughout, because dilute sulfuric acid is an irritant and could splash into the eyes while being warmed or stirred.

Question 3

(a) B1 pipette (with a pipette filler)
(a) Answer: A pipette (used with a pipette filler).
(b) B1 it makes it easier to see the colour change of the indicator at the end point oe
(b) Answer: It makes the colour change of the indicator easier to see.
(c) M1 method shown: titre = final reading - initial reading, for at least two titrations
(c) A1 all four titres correct: Rough = 24.60 cm3, Titration 1 = 23.20 cm3, Titration 2 = 23.15 cm3, Titration 3 = 23.20 cm3
(c) Answer: Rough = 24.60 cm3; Titration 1 = 23.20 cm3; Titration 2 = 23.15 cm3; Titration 3 = 23.20 cm3
(d) M1 identifies titrations 1, 2 and 3 (23.20, 23.15, 23.20 cm3) as concordant, excluding the rough titration
(d) A1 mean = 23.18 cm3 (accept 23.2 cm3), cao
(d) Answer: 23.18 cm3 (accept 23.2 cm3 to 3 sf)

Question 4

(a) M1 moles = concentration x volume (dm3) = 0.100 x (25.0/1000) shown
(a) A1 2.50 x 10^-3 mol (0.00250 mol) cao
(a) Answer: 2.50 x 10^-3 mol
(b) M1 moles HCl = moles NaOH = 2.50 x 10^-3 mol (ft from a, 1:1 ratio)
(b) M1 concentration = moles / volume (dm3) = 2.50 x 10^-3 / (23.18/1000) shown
(b) A1 0.108 mol/dm3, awrt 3 sf, ft
(b) Answer: 0.108 mol/dm3
(c) M1 concentration (g/dm3) = concentration (mol/dm3) x Mr = 0.1079 x 36.5 shown (ft from b)
(c) A1 3.94 g/dm3, awrt 3 sf, ft
(c) Answer: 3.94 g/dm3

Question 5

(a) B1 the rough titration is only used to find an approximate volume, so the acid was added quickly rather than dropwise near the end point, making it less precise than the accurate titrations oe
(a) Answer: It was only used to find an approximate end point, so the acid was added quickly rather than dropwise; it is therefore less precise than the accurate (concordant) titrations.
(b) M1 total uncertainty in a titre = 0.05 + 0.05 = 0.10 cm3
(b) M1 percentage uncertainty = (0.10 / 23.18) x 100 shown (ft mean titre)
(b) A1 0.431%, awrt 3 sf, ft
(b) Answer: 0.431%

Question 6

(a) B1 hold a lit (burning) splint at the mouth of the test tube
(a) B1 hydrogen burns rapidly with a squeaky pop
(a) Answer: Hold a lit splint at the mouth of the tube; hydrogen ignites with a squeaky pop.
(b) B1 insert a glowing splint into the mouth of the test tube
(b) B1 oxygen relights the glowing splint
(b) Answer: Insert a glowing splint into the tube; oxygen relights the glowing splint.
(c) M1 water, H2O, contains twice as many hydrogen atoms as oxygen atoms oe
(c) A1 so hydrogen and oxygen gas are produced in a 2:1 ratio by volume, matching the 2:1 ratio of hydrogen to oxygen atoms in water oe
(c) Answer: Water is H2O, containing twice as many hydrogen atoms as oxygen atoms, so hydrogen and oxygen gas are produced in the same 2:1 ratio by volume.

Question 7

(a) B1 independent variable: current
(a) B1 dependent variable: mass of copper deposited (on the cathode)
(a) B1 control: any one of time of electrolysis, concentration of copper sulfate solution, distance between electrodes
(a) Answer: Independent variable: current. Dependent variable: mass of copper deposited. Control: e.g. time of electrolysis, kept the same for every current tested.
(b) M1 as the current increases, the mass of copper deposited increases oe
(b) A1 the mass deposited is directly proportional to the current (e.g. doubling the current doubles the mass deposited), supported by the data given
(b) Answer: The mass of copper deposited is directly proportional to the current; for example, doubling the current from 1.00 A to 2.00 A doubles the mass deposited from 0.20 g to 0.40 g.
(c) B1 any sensible precaution, e.g. do not touch the electrodes or bare wires with wet hands, to avoid an electric shock oe
(c) Answer: Do not touch the electrodes or bare wires with wet hands, to avoid the risk of an electric shock.

Question 8

(a) B1 independent variable: volume or concentration of copper sulfate solution used
(a) B1 control: e.g. starting temperature of the solution, or the type/size of insulated container used
(a) Answer: Independent variable: volume or concentration of copper sulfate solution. Control: e.g. the starting temperature of the solution, kept the same each time.
(b) M1 polystyrene is a poor conductor of heat (good insulator), reducing energy transfer to the surroundings, unlike glass oe
(b) A1 the lid further reduces heat loss (and evaporation), giving a more accurate reading of the maximum temperature reached oe
(b) Answer: Polystyrene is a poor conductor of heat, reducing energy loss to the surroundings compared with glass; the lid reduces heat loss and evaporation further, giving a more accurate maximum temperature.

Question 9

(a) B1 40.5 degC cao
(a) Answer: 40.5 degC
(b) M1 mass of solution = 25.0 x 1.00 = 25.0 g
(b) M1 correct substitution: Q = 25.0 x 4.2 x 40.5 shown (ft delta T from a)
(b) A1 4250 J, accept 4252.5 J or 4.25 kJ, awrt 3 sf, ft
(b) Answer: 4250 J (4252.5 J, or 4.25 kJ)
(c) M1 moles = concentration x volume (dm3) = 1.00 x (25.0/1000) shown
(c) A1 0.0250 mol cao
(c) Answer: 0.0250 mol
(d) M1 energy per mole = 4252.5 / 0.0250 shown (ft from b and c)
(d) A1 170 kJ/mol, awrt 3 sf, ft
(d) A1 exothermic, because the temperature increased (energy was released to the solution)
(d) Answer: 170 kJ/mol (170.1 kJ/mol); exothermic

Question 10

(a) M1 less energy is needed to break the bonds/separate the ions in the reactants than is released when new bonds/ions form in the products oe
(a) A1 so overall energy is transferred to the surroundings, causing the temperature to rise oe
(a) Answer: Less energy is needed to break the bonds in the reactants than is released forming the bonds in the products, so overall energy is transferred to the surroundings, causing the temperature to rise.

Question 11

(a) B1 the flask is fitted with a bung and delivery tube
(a) B1 the delivery tube leads into a gas syringe, which measures the volume of gas collected
(a) Answer: The flask is sealed with a bung connected by a delivery tube to a gas syringe, which measures the volume of gas collected.
(b) B1 independent variable: concentration of hydrochloric acid
(b) B1 dependent variable: volume of gas (carbon dioxide) produced
(b) B1 any one control variable named, e.g. mass/surface area of marble chips, volume of acid, or temperature
(b) Answer: Independent variable: concentration of hydrochloric acid. Dependent variable: volume of carbon dioxide gas produced. E.g. the mass of marble chips should be controlled.

Question 12

(a) M1 rate = volume / time = 48 / 30 shown
(a) A1 1.6 cm3/s cao
(a) Answer: 1.6 cm3/s
(b) M1 the acid is used up as the reaction proceeds, so its concentration decreases oe
(b) A1 fewer acid particles are present in a given volume, so successful collisions with the marble chip surface happen less often, decreasing the rate oe
(b) A1 the graph becomes flat once one of the reactants (here, the acid) has been completely used up, so no more gas can be produced
(b) Answer: As the acid is used up its concentration falls, so successful collisions between acid particles and the marble surface happen less often, decreasing the rate; the graph becomes flat once the acid is completely used up, so no more gas can form.
(c) M1 the graph would be steeper at the start (a faster initial rate)
(c) A1 because the smaller chips have a greater surface area to volume ratio, increasing the frequency of collisions between acid particles and the marble surface oe
(c) A1 the graph would level off at the same final (maximum) volume of gas, since the same total mass (moles) of calcium carbonate reacts
(c) Answer: The graph would be steeper at the start (faster initial rate), because the greater surface area increases the frequency of collisions between acid and marble; it would level off at the same final volume, since the same mass of marble reacts overall.

Question 13

(a) B1 any one of: volume of hydrochloric acid, total volume of the reaction mixture, temperature
(a) B1 any second, different one of the above, or: the same person judging when the cross disappears
(a) Answer: Any two of: the volume of hydrochloric acid used; the total volume of the mixture; the temperature; the same person judging the end point each time.
(b) M1 1 / 40 shown
(b) A1 0.025 s^-1 cao
(b) Answer: 0.025 s^-1
(c) B1 judging by eye when the cross disappears is subjective, so different people (or the same person on different attempts) may judge the moment slightly differently oe
(c) B1 improvement: e.g. use the same person to judge every reading, or use a light sensor/data logger to detect the change objectively oe
(c) Answer: Judging the end point by eye is subjective and can vary between observers; using the same person for every reading, or a light sensor/data logger, would give more objective, precise results.

Question 14

(a) L1 (1-2): Simple, isolated statement(s) are made about one or both methods, for example that one is 'more accurate', with little or no reference to the specific features of either method and no clear recommendation.
(a) L2 (3-4): Some relevant points of comparison are made (for example, that judging when the cross disappears is subjective, or that a gas syringe gives numerical readings), with some justification, but the evaluation may be one-sided or the recommendation may be missing or only partly supported.
(a) L3 (5-6): A detailed, balanced evaluation compares both methods across objectivity, precision and the type of data produced (a continuous graph against a single time reading), leading to a clearly justified recommendation.
(a) Answer: Indicative full-mark response: identifies that the gas syringe method gives objective, continuous quantitative data (allowing a full rate-time graph) while the turbidity method relies on a subjective single judgement by eye, but also recognises that the choice of method depends on which is practical for the reaction studied, ending in a clearly justified recommendation.
(b) B1 any one of: temperature, mass/surface area of solid reactant (or concentration of the second reactant), total volume of the mixture
(b) B1 any second, different one of the above
(b) Answer: Any two of: temperature; the mass or surface area of the solid reactant (or concentration of the second reactant); the total volume of the reaction mixture.

Question 15

(a) B1 a pencil baseline (origin line) is drawn near the bottom of the chromatography paper
(a) B1 a small, concentrated spot of the sample (and each reference dye) is applied to the baseline and allowed to dry
(a) B1 the paper is placed in a container with a shallow layer of solvent, with the level below the baseline, and covered with a lid
(a) Answer: Draw a pencil baseline near the bottom of the paper; apply small, concentrated spots of the sample and reference dyes on it and let them dry; stand the paper in a covered container with a shallow layer of solvent, kept below the baseline.
(b) M1 Rf = distance travelled by spot / distance travelled by solvent = 6.6 / 8.8 shown
(b) A1 0.75 cao
(b) Answer: 0.75

Question 16

(a) B1 the orange squash contains dyes A, C and D, but not dye B
(a) B1 a pure substance produces only one spot on a chromatogram in any solvent
(a) A1 a substance that produces more than one spot (as here) must be an impure substance (a mixture)
(a) Answer: The orange squash contains dyes A, C and D, but not B. A pure substance always gives only one spot, so producing more than one spot shows a substance is a mixture, not pure.
(b) B1 pencil is insoluble in the solvent, so it will not dissolve/move up the paper and interfere with the results, unlike most inks oe
(b) Answer: Pencil is insoluble in the solvent, so the baseline does not move or interfere with the results, unlike most inks.
(c) M1 the Rf value depends on the relative attraction (solubility) of the dye for the solvent (mobile phase) compared with the paper (stationary phase) oe
(c) A1 a more polar solvent may dissolve the dye more or less readily than before, changing the balance of time the dye spends in each phase, so changing how far it travels relative to the solvent front oe
(c) Answer: Rf depends on how strongly the dye is attracted to the solvent compared with the paper; a more polar solvent changes this balance, so the dye may travel a different distance relative to the solvent front, changing its Rf value.

Question 17

(a) B1 sample Q
(a) B1 pure water contains (almost) no free/dissolved ions to carry charge, so it conducts electricity very poorly, giving the smallest current oe
(a) Answer: Sample Q, because pure water contains very few free-moving ions to carry charge, so it conducts electricity poorly, giving the smallest current.
(b) M1 sea water contains a high concentration of dissolved ionic compounds (e.g. sodium chloride) oe
(b) A1 this provides many free-moving ions to carry the charge, so it conducts electricity well, giving the largest current oe
(b) Answer: Sea water contains a high concentration of dissolved ionic compounds, providing many free ions to carry the charge, so it conducts well and gives the largest current.
(c) B1 any one of: the voltage (p.d.) applied, the distance between the electrodes, the volume of sample tested, the temperature of the samples
(c) Answer: E.g. the voltage applied to the circuit should be the same for every sample tested.
(d) M1 measure the current for a series of solutions of known concentration (e.g. standard sodium chloride solutions) using the same apparatus oe
(d) A1 plot a graph of current against known concentration, then read the current of the unknown sample against this graph to estimate its concentration oe
(d) Answer: Measure the current for several standard solutions of known ion concentration, plot current against concentration to give a calibration graph, then use the unknown sample's current to read off its estimated concentration from the graph.

Question 18

(a) B1 repeatability: getting very similar results when the same person repeats a measurement using the same method and equipment oe
(a) B1 the titres of 23.20, 23.15 and 23.20 cm3 are all within 0.10 cm3 of each other (concordant), showing the method was repeatable
(a) Answer: Repeatability means getting very similar results when the same person repeats a measurement with the same method and equipment; the three titres (23.20, 23.15 and 23.20 cm3) all agree within 0.10 cm3, showing repeatability.
(b) B1 reproducibility: getting very similar results when a different person, using different equipment or a different method, repeats the same investigation oe
(b) Answer: Reproducibility means getting very similar results when a different person, or different equipment or method, repeats the investigation; unlike repeatability, it involves a change in person, equipment or technique.
(c) B1 repeat that reading; if it is confirmed as anomalous, exclude it from any mean calculated oe
(c) B1 because including it would make the mean less accurate, as it does not represent the true value shown by the pattern of the other consistent results oe
(c) Answer: Repeat the anomalous reading; if it is confirmed as anomalous, it should be excluded from any mean calculated, because including it would make the mean less representative of the true value shown by the other results.

Chemistry Required Practicals and Data Analysis - Worksheets, Questions and Revision