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13+ · Science

CE.S-W3 Science: Working Scientifically and Practical Skills Set 3

ISEB CE ISEB Common Entrance 13+ Science · Calculator allowed · about 100 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Jack is investigating how the intensity of light affects the rate of growth of mustard cress seedlings. He sets up five trays of seedlings at different distances from a lamp and measures the height of the seedlings after seven days.

(a)State the independent variable in this investigation.(1)

(b)State the dependent variable in this investigation.(1)

(c)Give two variables that Jack should keep the same for each tray, so that the comparison is a fair test.(2)

(d)Jack only grows one tray of seedlings at each distance from the lamp. Explain why growing several trays at each distance, rather than just one, would make his conclusion more reliable.(2)

(Total for Question 1 is 6 marks)

A student sets up a circuit with a battery of cells, a lamp and an ammeter connected in series, to investigate how the current through the lamp changes as more cells are added to the battery. Before connecting the circuit, she notices that the ammeter's pointer does not rest exactly on 0 A when no current is flowing.

(a)State the name given to an instrument reading that is not zero when it should be, before any measurement is taken.(1)

(b)Explain how this zero error would affect every current reading taken during the investigation, and describe how the student should correct her results.(2)

(c)Name a circuit component that could be added in series with the ammeter to allow the current to be changed without altering the number of cells, and explain how it works.(2)

(d)The table shows the current through the lamp, after the zero error has been corrected, for different numbers of cells connected in series.
Number of cells: 1, 2, 3, 4
Current (A): 0.20, 0.40, 0.60, 0.72
(i) Describe the relationship between the number of cells and the current shown by the readings for 1, 2 and 3 cells.
(ii) Suggest why the reading for 4 cells does not fit this pattern.(3)

(Total for Question 2 is 8 marks)

A student electrolyses copper sulfate solution using two copper electrodes and a fixed current of 0.5 A. She removes, dries carefully and reweighs the cathode (negative electrode) every 5 minutes to find the mass of copper deposited on it.
Time (min): 0, 5, 10, 15, 20
Mass of copper deposited (g): 0.00, 0.16, 0.31, 0.47, 0.62

(a)Calculate the mean rate of copper deposition between 0 and 10 minutes, in g/min.(2)

(b)Describe the relationship between time and the mass of copper deposited shown by this data.(2)

(c)Explain why the cathode must be carefully dried before it is reweighed each time.(1)

(d)State one variable, other than the identity of the electrodes, that should be kept constant throughout this investigation to ensure the results are valid.(1)

(Total for Question 3 is 6 marks)

Choosing the correct type of graph or chart depends on the type of data being displayed.

(a)A student counts the number of leaves on trees growing at five different distances from a busy road. State the type of graph that should be used to display this data, and explain your choice.(2)

(b)A different student records each student's favourite school subject in her class, and the number of students who chose each subject. State the type of graph that should be used to display this data, and explain your choice.(2)

(Total for Question 4 is 4 marks)

Amara measures her pulse rate immediately after a fixed period of star jumps, and repeats the measurement four times.
Repeat 1: 78 bpm
Repeat 2: 80 bpm
Repeat 3: 122 bpm
Repeat 4: 79 bpm

(a)Identify the anomalous result, and suggest one possible cause for it.(2)

(b)Calculate the mean pulse rate, excluding the anomalous result.(2)

(Total for Question 5 is 4 marks)

A cyclist's motion is recorded on a distance-time graph. Between 0 s and 10 s, the graph is a straight line rising from 0 m to 20 m. Between 10 s and 20 s, the graph is a horizontal (flat) line at 20 m.

(a)Calculate the speed of the cyclist during the first 10 seconds.(2)

(b)Describe the motion of the cyclist between 10 s and 20 s, and explain how the graph shows this.(2)

(Total for Question 6 is 4 marks)

Elif carries out a titration to find the volume of dilute hydrochloric acid needed to exactly neutralise a fixed volume of sodium hydroxide solution, using phenolphthalein indicator. She carries out the titration once and records a result of 24.60 cm^3.

(a)Explain one problem with using only a single titration result to determine the volume of acid needed.(1)

(b)Explain why obtaining concordant results (titres within 0.10 cm³ of each other) improves the accuracy of the final result.(2)

(c)State the piece of glassware Elif should use to add the acid, and the precision it can typically be read to.(1)

(Total for Question 7 is 4 marks)

A student measures her breathing rate (breaths per minute) at rest and then while cycling on an exercise bike at three different resistance settings.
Exercise intensity: Rest, Low, Medium, High
Breathing rate (breaths/min): 14, 22, 31, 40

(a)Describe the trend shown by the data as exercise intensity increases from rest to high intensity.(2)

(b)Explain, using ideas about respiration, why breathing rate increases in this way during exercise.(3)

(Total for Question 8 is 5 marks)

Tariq is investigating how the length of a piece of resistance wire affects its resistance. He uses an ammeter, a voltmeter, a power supply and a metre rule.

(a)Identify the independent variable and the dependent variable in this investigation.(2)

(b)State two variables that Tariq should keep the same for each length tested, so that the comparison is a fair test.(2)

(c)Describe two key steps Tariq should take to measure each length of wire accurately and consistently.(2)

(Total for Question 9 is 6 marks)

A student is preparing to carry out a titration using dilute hydrochloric acid, heating some of the solution gently with a Bunsen burner beforehand.

(a)Name the piece of apparatus that should be used to measure out exactly 25.0 cm³ of the acid.(1)

(b)State two safety precautions the student should take during this experiment.(2)

(Total for Question 10 is 3 marks)

Kwame investigated the effect of exercise duration on heart rate. He carried out star jumps for 1, 2, 3, 4 and 5 minutes (on separate occasions during one lesson), and immediately after each duration he counted his pulse for 15 seconds and multiplied this by 4 to estimate his heart rate in beats per minute. He took only one reading for each duration, and moved straight on to the next duration after a short rest.

Evaluate Kwame's method, including its reliability, validity and possible improvements.

(Total for Question 11 is 6 marks)

A student investigates the terminal velocity of small ball bearings falling through a tall tube of oil. She marks two points, exactly 0.50 m apart, on the tube and uses a hand-operated stopwatch to time how long a ball bearing takes to fall between them, once it has reached terminal velocity.

(a)The stopwatch reading was 2.8 s. Calculate the terminal velocity of the ball bearing, in m/s, to 2 significant figures.(2)

(b)Explain why using light gates connected to a computer/timer, rather than a hand-operated stopwatch, would give a more accurate value for the terminal velocity.(2)

(Total for Question 12 is 4 marks)

A student mixes two chemicals that react exothermically, and records the temperature of the mixture every minute.
Time (min): 0, 1, 2, 3, 4, 5
Temperature (degC): 20, 26, 31, 35, 28, 42

(a)Identify the anomalous temperature reading, and the time at which it occurs.(1)

(b)Calculate the overall rise in temperature over the full 5 minutes, ignoring the anomalous reading.(2)

(c)Suggest one possible cause of the anomalous reading at 4 minutes.(1)

(Total for Question 13 is 4 marks)

A student investigates the effect of sucrose solution concentration on the mass of potato chips, using concentrations of 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm^3.

(a)State three column headings, with units where appropriate, that should be included in the results table (other than the concentration of sucrose solution itself).(3)

(b)One potato chip had an initial mass of 4.20 g and a final mass of 3.78 g after being placed in a sucrose solution. Calculate the percentage change in mass.(2)

(Total for Question 14 is 5 marks)

Tariq carries out the resistance investigation described in Question 9. The table shows his results.
Length (cm): 10, 20, 30, 40
Resistance (ohms): 2.0, 4.0, 6.0, 8.0

(a)Describe the relationship between the length of the wire and its resistance shown by this data.(2)

(b)Use the data to predict the resistance of a 50 cm length of the same wire. Show your working.(3)

(Total for Question 15 is 5 marks)

Scientists use precise language to describe the quality of their measurements.

(a)Define what is meant by the accuracy of a measurement.(1)

(b)Define what is meant by the precision of a set of measurements.(1)

(c)Define what is meant by the reproducibility of a result.(1)

(d)A student weighs a metal block three times using an electronic balance and obtains readings of 102.3 g, 102.4 g and 102.2 g. The true (accepted) mass of the block is actually 105.0 g, because the balance had not been zeroed correctly before use. Are these readings precise, accurate, both or neither? Explain your answer.(2)

(Total for Question 16 is 5 marks)

A student investigates the effect of sodium thiosulfate concentration on the rate of its reaction with dilute hydrochloric acid, which produces a cloudy sulfur precipitate. She places the flask over a cross drawn on paper and times how long it takes for the cross to be obscured by the cloudiness.
Concentration of sodium thiosulfate (g/dm³): 10, 20, 30, 40
Time for cross to disappear (s): 200, 100, 67, 50

(a)Explain why 1/time is used as a measure of the rate of this reaction, rather than time itself.(2)

(b)Calculate the rate of reaction, in s^-1, at a concentration of 20 g/dm^3.(2)

(c)Describe the relationship between concentration and rate shown in the table, and explain this in terms of particle collisions.(2)

(Total for Question 17 is 6 marks)

Aisha counts the number of bubbles of oxygen released by a piece of pondweed in one minute, at different distances from a lamp, as a measure of the rate of photosynthesis.

(a)Identify one problem with using the number of bubbles as a measure of the volume of oxygen produced.(2)

(b)Describe an improved method, using a gas syringe, that would give a more accurate measure of the volume of oxygen produced in a set time.(2)

(Total for Question 18 is 4 marks)

A student uses a gas syringe with graduations (scale markings) every 1 cm³ to measure the volume of gas produced in a reaction.

(a)State the smallest change in volume that this gas syringe can reliably be read to.(1)

(b)Explain why recording volumes to the nearest 0.5 cm³ (e.g. 24.5 cm³) is more useful than rounding to the nearest whole cm³ (e.g. 25 cm³) when comparing results from two experiments with only a small difference between them.(1)

(c)A results table shows volumes recorded as: 24 cm³, 25.5 cm³, 26 cm³, 24.0 cm^3. State one error in how this data has been recorded, and explain why it is a problem.(1)

(Total for Question 19 is 3 marks)

A student is planning an investigation into the effect of the surface area of zinc (comparing zinc granules with zinc powder) on the rate of its reaction with dilute sulfuric acid, which produces hydrogen gas.

(a)Identify the independent variable and the dependent variable in this investigation.(2)

(b)State two variables that must be controlled to make this a fair test.(2)

(c)Describe two key steps that should be included in the method to make sure the investigation is carried out safely and gives valid results.(2)

(d)Predict, with a reason, how increasing the surface area of the zinc would affect the rate of reaction.(2)

(Total for Question 20 is 8 marks)

A group of students wants to plan a full investigation into how the concentration of sucrose solution affects the change in mass of potato chips by osmosis, building on the investigation described in Question 14.

Plan a method they could use, including how they would make it a fair test, ensure their results are reliable, and work safely.

(Total for Question 21 is 6 marks)

Mark scheme · CE.S-W3 Science: Working Scientifically and Practical Skills Set 3

Question 1

(a) B1 light intensity (or distance of the tray from the lamp) oe
(a) Answer: Light intensity (the distance of the seedlings from the lamp).
(b) B1 height of the seedlings (growth) oe
(b) Answer: The height of the mustard cress seedlings.
(c) B1 any one suitable controlled variable, e.g. type/number of seeds, volume of water given, temperature, type/amount of compost
(c) B1 a second different suitable controlled variable oe
(c) Answer: Any two of: the type and number of seeds used in each tray; the volume of water given to each tray; the temperature the trays are kept at; the type and amount of compost/growing medium used.
(d) M1 growing several trays allows a mean height to be calculated for each distance oe
(d) A1 this reduces the effect of any unusual/anomalous individual seedling or tray on the overall result, making the pattern easier to trust oe
(d) Answer: With several trays at each distance, a mean can be calculated, which reduces the effect of any one unusually fast- or slow-growing tray, making the results more reliable.

Question 2

(a) B1 zero error oe
(a) Answer: A zero error.
(b) M1 every reading will be wrong by the same fixed amount (a systematic error), either too high or too low oe
(b) A1 the student should note the size of the zero error and add or subtract it from every reading taken, or reset/zero the ammeter before starting oe
(b) Answer: Every current reading will be out by the same fixed amount, since it is a systematic (zero) error. The student should measure the size of this error and add or subtract it from each reading taken, or zero the ammeter before starting.
(c) B1 variable resistor (rheostat) oe
(c) B1 sliding its contact changes the length of resistance wire included in the circuit, changing its resistance and therefore the current oe
(c) Answer: A variable resistor (rheostat). Sliding its contact changes the length of resistance wire included in the circuit, which changes its resistance and therefore the current flowing.
(d) B1 for 1-3 cells, current is directly proportional to the number of cells (increases by 0.20 A for each extra cell) oe
(d) B1 this pattern predicts a current of 0.80 A at 4 cells, but only 0.72 A is recorded oe
(d) B1 ft as the current increases the lamp filament gets hotter, which increases its resistance, so the current increases by less than the proportional pattern predicts (the lamp is non-ohmic) oe
(d) Answer: For 1-3 cells, the current is directly proportional to the number of cells (rising by 0.20 A each time). This pattern predicts 0.80 A at 4 cells, but only 0.72 A is recorded, because the filament heats up more at higher currents, increasing its resistance and reducing the current below the proportional value.

Question 3

(a) M1 rate = mass / time = 0.31 / 10 oe
(a) A1 = 0.031 g/min cao
(a) Answer: 0.031 g/min
(b) B1 mass deposited increases steadily (at an approximately constant rate) as time increases oe
(b) B1 the relationship is directly proportional (the mass increases by roughly the same amount, about 0.15-0.16 g, every 5 minutes) oe
(b) Answer: The mass of copper deposited increases steadily with time, at an approximately constant rate. The relationship is directly proportional: roughly equal amounts (about 0.15-0.16 g) are deposited every 5 minutes.
(c) B1 any solution left on the electrode would add to its mass, giving a reading that is too high and not a true measure of the copper deposited oe
(c) Answer: If any solution is left on the electrode, it would add extra mass to the reading, making the mass of copper deposited appear larger than it really is.
(d) B1 any one of: the current, the concentration/volume of copper sulfate solution, the temperature oe
(d) Answer: Any one of: the current (kept fixed at 0.5 A); the concentration and volume of copper sulfate solution; the temperature.

Question 4

(a) M1 line graph oe
(a) A1 because both distance from the road and number of leaves are continuous (numerical) data oe
(a) Answer: A line graph, because both the distance from the road and the number of leaves are continuous numerical data.
(b) M1 bar chart oe
(b) A1 because favourite subject is categoric (non-numerical/discrete) data oe
(b) Answer: A bar chart, because favourite subject is categoric data, not numerical/continuous data.

Question 5

(a) B1 122 bpm
(a) B1 any sensible cause, e.g. Amara miscounted her pulse, misread the stopwatch, or had not fully recovered from the previous repeat before taking this reading oe
(a) Answer: 122 bpm is the anomalous result. A possible cause is that Amara miscounted her pulse beats, misread the stopwatch, or had not fully recovered from the previous repeat before taking this reading.
(b) M1 (78 + 80 + 79) / 3 oe
(b) A1 = 79 bpm cao
(b) Answer: 79 bpm

Question 6

(a) M1 speed = distance / time = 20 / 10 oe
(a) A1 = 2 m/s cao
(a) Answer: 2 m/s
(b) B1 the cyclist is stationary (not moving) oe
(b) B1 because the line on the graph is horizontal/flat, showing the distance is not changing over time oe
(b) Answer: The cyclist is stationary. The graph shows this because the line is horizontal (flat), meaning the distance travelled is not changing as time passes.

Question 7

(a) B1 there is no way to check whether the result is reliable/repeatable, or to spot an anomalous reading, from a single titration oe
(a) Answer: With only one result, Elif cannot check whether it is reliable, or identify whether it might be anomalous.
(b) M1 concordant results show the titration can be repeated to give a similar value, so any anomalous readings can be identified and excluded oe
(b) A1 a mean can then be calculated from the concordant results, which is more accurate/reliable than any single reading oe
(b) Answer: Getting concordant results shows the titration is repeatable, so any anomalous readings can be spotted and excluded. A mean of the concordant results can then be calculated, which is more accurate than relying on a single titre.
(c) B1 burette, read to the nearest 0.05 cm³ oe
(c) Answer: A burette, which can typically be read to the nearest 0.05 cm^3.

Question 8

(a) B1 breathing rate increases steadily/continuously as exercise intensity increases oe
(a) B1 supported by data, e.g. breathing rate almost tripling from 14 breaths/min at rest to 40 breaths/min at high intensity oe
(a) Answer: Breathing rate increases steadily as exercise intensity increases, almost tripling from 14 breaths/min at rest to 40 breaths/min at the highest intensity.
(b) B1 during exercise, muscles work harder and respire more, releasing more energy from (aerobic) respiration oe
(b) B1 this produces more carbon dioxide, increasing its concentration in the blood oe
(b) B1 this is detected, causing the breathing (control) centre in the brain to increase the rate and depth of breathing, to remove the excess carbon dioxide and take in more oxygen oe
(b) Answer: During exercise, muscles respire more to release the extra energy needed, producing more carbon dioxide, which raises its concentration in the blood. This is detected by the brain's breathing control centre, which increases the rate and depth of breathing to remove the excess carbon dioxide and take in more oxygen.

Question 9

(a) B1 independent variable: length of the wire
(a) B1 dependent variable: resistance of the wire (calculated from the voltmeter and ammeter readings) oe
(a) Answer: Independent variable: the length of the wire. Dependent variable: the resistance of the wire.
(b) B1 any one suitable controlled variable, e.g. thickness/diameter of the wire, type of metal the wire is made from, temperature of the wire, current/voltage applied
(b) B1 a second different suitable controlled variable oe
(b) Answer: Any two of: the thickness (diameter) of the wire; the type of metal the wire is made from; the temperature of the wire; the current or voltage applied.
(c) B1 lay the wire alongside a metre rule and measure from a fixed end point (e.g. the 0 cm mark) each time oe
(c) B1 keep the wire straight (but not stretched) when measuring, connecting the circuit at exactly the measured length each time oe
(c) Answer: Lay the wire flat alongside a metre rule, measuring from the same fixed starting point (e.g. the 0 cm mark) each time; and keep the wire straight but not stretched, connecting the circuit at exactly the measured length.

Question 10

(a) B1 (volumetric/graduated) pipette
(a) Answer: A (volumetric/graduated) pipette.
(b) B1 wear eye protection/safety goggles oe
(b) B1 any second sensible precaution, e.g. use the yellow safety flame on the Bunsen burner when not actively heating, keep the burner on a heatproof mat, tie back long hair, stand up while heating oe
(b) Answer: Any two of: wear eye protection/safety goggles; use the yellow safety flame when the Bunsen burner is not actively heating; keep the burner on a heatproof mat; tie back long hair; stand up while heating.

Question 11

Level 0 (0): No relevant content, or the answer does not evaluate the method.
Level 1 (1-2): Basic, general comments about the method, with little or no reference to specific detail from the scenario. May simply state that the results might not be reliable or accurate, without development.
Level 2 (3-4): A clear evaluation identifying at least one relevant strength or weakness of the method, with some linked explanation, e.g. that taking only one reading per duration limits reliability, or that manual pulse counting is prone to error.
Level 3 (5-6): A detailed, logically structured evaluation that considers reliability, validity/fair testing and accuracy, with specific, workable improvements linked to each issue identified.
Indicative content:
- Only one reading was taken at each exercise duration, so there is no way to check reliability or identify an anomalous result.
- Repeating each duration at least three times and calculating a mean would improve reliability.
- Counting the pulse manually for 15 seconds and multiplying by 4 introduces human counting error, and any small counting mistake is magnified by four.
- Using a heart rate monitor or pulse oximeter would give a faster, more accurate and more precise reading.
- Kwame's resting heart rate before exercise was not recorded or controlled, so a raised starting heart rate could affect the comparison between durations.
- Only a short rest was taken between the different durations, so his heart rate may not have returned to a resting level before the next test, making the comparison between durations less valid/fair.
- Other variables, such as the intensity/effort of the star jumps and the room temperature, were not controlled between the different durations.

Question 12

(a) M1 speed = distance / time = 0.50 / 2.8 oe
(a) A1 = 0.18 m/s (2 s.f.) awrt
(a) Answer: 0.18 m/s
(b) M1 light gates start and stop the timing automatically as the ball bearing passes each point, removing human reaction-time error oe
(b) A1 this reduces the uncertainty in the measured time, giving a more accurate value for the calculated velocity oe
(b) Answer: Light gates start and stop timing automatically as the ball bearing passes each point, removing the human reaction-time error involved in starting and stopping a stopwatch by hand, which gives a more accurate value for the velocity.

Question 13

(a) B1 28 degC at 4 minutes
(a) Answer: 28 degC at 4 minutes.
(b) M1 42 - 20 oe
(b) A1 = 22 degC cao
(b) Answer: 22 degC
(c) B1 any sensible cause, e.g. the thermometer was briefly removed from/not fully immersed in the mixture, a draught cooled the thermometer bulb, or the scale was misread oe
(c) Answer: For example, the thermometer may have been briefly lifted out of (or not fully immersed in) the reacting mixture, or a draught may have cooled the thermometer bulb at that moment.

Question 14

(a) B1 initial mass / g
(a) B1 final mass / g
(a) B1 change in mass / g (or percentage change in mass / %) oe
(a) Answer: Initial mass / g; final mass / g; change in mass / g (or percentage change in mass / %).
(b) M1 (3.78 - 4.20) / 4.20 x 100 oe
(b) A1 = -10% cao (accept -10.0%)
(b) Answer: -10% (a decrease of 10%)

Question 15

(a) B1 resistance increases as length increases oe
(a) B1 resistance is directly proportional to length (e.g. doubling the length doubles the resistance) oe
(a) Answer: Resistance increases as the length of the wire increases. The relationship is directly proportional: doubling the length doubles the resistance.
(b) M1 finds resistance per cm using the data, e.g. 2.0 / 10 oe
(b) dM1 multiplies their resistance per cm by 50 (dependent on M1)
(b) A1 = 10.0 ohms cao
(b) Answer: 10.0 ohms

Question 16

(a) B1 how close a measured value is to the true (actual) value oe
(a) Answer: How close a measured value is to the true (actual) value.
(b) B1 how close repeated measurements are to each other / how little spread there is between repeat readings oe
(b) Answer: How close a set of repeated measurements are to each other (how little they are spread out).
(c) B1 the result can be obtained again, e.g. by a different person, using different equipment or a different method oe
(c) Answer: The result can be obtained again, for example by a different person, using different equipment or a slightly different method.
(d) M1 the readings are precise, because they are all very close to each other oe
(d) A1 but they are not accurate, because they are not close to the true value of 105.0 g oe
(d) Answer: The readings are precise, because they are very close together, but they are not accurate, because they are not close to the true mass of 105.0 g.

Question 17

(a) M1 time and rate are inversely related, i.e. a shorter time means a faster reaction oe
(a) A1 1/time gives a value that increases as the reaction gets faster, so it can be used and plotted directly as a measure of rate oe
(a) Answer: A shorter time means a faster reaction, so time and rate are inversely related. Using 1/time gives a value that increases as the reaction speeds up, so it can be used directly as a measure of rate.
(b) M1 rate = 1 / time = 1 / 100 oe
(b) A1 = 0.01 s^-1 cao (awrt 0.01)
(b) Answer: 0.01 s^-1
(c) B1 rate of reaction increases as concentration increases oe
(c) B1 at a higher concentration there are more thiosulfate particles in the same volume of solution, so successful collisions between reacting particles happen more frequently oe
(c) Answer: The rate of reaction increases as the concentration of sodium thiosulfate increases. This is because a higher concentration means more thiosulfate particles in the same volume, so collisions between reacting particles happen more often.

Question 18

(a) M1 the bubbles produced are not all the same size oe
(a) A1 so counting bubbles does not give an accurate or reliable measure of the actual volume of gas produced oe
(a) Answer: The bubbles released are not all the same size, so simply counting them does not give an accurate or reliable measure of the actual volume of oxygen produced.
(b) M1 connect the pondweed to a gas syringe (or collect the gas by downward displacement of water in an inverted measuring cylinder) so the gas produced is collected oe
(b) A1 read the volume of gas collected directly from the syringe/cylinder, in cm³, after a set time, rather than counting bubbles oe
(b) Answer: Connect the pondweed to a gas syringe (or collect the gas in an inverted, water-filled measuring cylinder) so that the oxygen produced is collected, and read off the actual volume of gas, in cm³, after a set time, instead of counting bubbles.

Question 19

(a) B1 0.5 cm³ (half of the smallest graduation) oe
(a) Answer: 0.5 cm^3.
(b) B1 smaller (more precise) differences between the two sets of results can be detected/compared oe
(b) Answer: Recording to the nearest 0.5 cm³ allows smaller differences between the two experiments to be detected, which would be lost if volumes were rounded to the nearest whole cm^3.
(c) B1 the readings are not recorded to a consistent number of decimal places/degree of precision, so it is unclear exactly how precisely each reading was actually taken oe
(c) Answer: The volumes are not all recorded to the same number of decimal places, so it is unclear how precisely each individual reading was actually taken.

Question 20

(a) B1 independent variable: surface area (form/size) of the zinc
(a) B1 dependent variable: rate of reaction (e.g. volume of hydrogen gas produced per unit time) oe
(a) Answer: Independent variable: the surface area (form/size) of the zinc. Dependent variable: the rate of reaction, for example the volume of hydrogen gas produced per unit time.
(b) B1 any one suitable controlled variable, e.g. total mass of zinc used, concentration/volume of sulfuric acid, temperature
(b) B1 a second different suitable controlled variable oe
(b) Answer: Any two of: the total mass of zinc used; the concentration and volume of sulfuric acid used; the temperature of the acid.
(c) B1 weigh out the same mass of zinc for each surface area using a balance oe
(c) B1 wear eye protection and start the stopwatch at the same moment the acid is added each time, e.g. taking gas volume readings at the same fixed time intervals (hydrogen gas produced is flammable, so keep away from naked flames) oe
(c) Answer: Weigh out the same mass of zinc each time using a balance; and wear eye protection, starting the stopwatch at the exact moment the acid is added, taking gas volume readings at the same fixed time intervals each time, keeping the flammable hydrogen gas away from naked flames.
(d) M1 increasing the surface area increases the rate of reaction oe
(d) A1 because more particles of zinc are exposed and available to collide with acid particles, increasing the frequency of successful collisions per second oe
(d) Answer: Increasing the surface area of the zinc increases the rate of reaction, because more zinc particles are exposed on the surface, so more collisions with acid particles happen each second.

Question 21

Level 0 (0): No relevant content, or the answer does not address planning an investigation.
Level 1 (1-2): A basic method is given. The independent and/or dependent variable may be named, but there is little or no reference to controlling other variables, apparatus, or improving reliability.
Level 2 (3-4): A clear method is given describing suitable apparatus and key steps, e.g. cutting potato chips and preparing solutions. Some relevant variables to control are stated and/or repeats are mentioned, but detail is incomplete or not fully linked together.
Level 3 (5-6): A detailed, logically ordered method is given. It states a suitable range of concentrations, clearly controlled variables, an appropriate method for preparing and measuring the potato chips (including blotting and using percentage change in mass), and explains how repeats and means will be used to improve reliability. Relevant safety precautions are included.
Indicative content:
- Cut potato chips of a consistent size (e.g. the same length and diameter, using a cork borer and ruler or scalpel), all from the same potato.
- Blot each chip dry with a paper towel before weighing on a balance, to remove surface moisture, and record its initial mass.
- Prepare a range of sucrose concentrations by diluting a stock solution, e.g. 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm^3.
- Use the same volume of solution in each boiling tube/beaker.
- Leave the chips in the solutions for the same fixed length of time, e.g. 24 hours.
- Blot each chip dry again before reweighing it at the end, and record its final mass.
- Calculate the percentage change in mass for each chip, rather than just the mass change, so results can be fairly compared even if starting masses varied slightly.
- Repeat at each concentration (e.g. three chips per concentration) and calculate a mean percentage change in mass.
- Keep the temperature constant throughout, e.g. by leaving all tubes at room temperature or in a water bath.
- Wear eye protection when preparing the sucrose solutions.
- Plot a graph of mean percentage change in mass against concentration, and identify the concentration at which the line crosses zero change in mass.

Science: Working Scientifically and Practical Skills Set 3 - Worksheets, Questions and Revision