Biology 9700/33 — May/June 2020

Background Concept

A serial dilution is a stepwise reduction in the concentration of a stock solution, made by adding a fixed volume of the stock to a fixed volume of diluent (here distilled water, W). Because every dilution has the same final volume (10 cm³), the volume of the 20% stock (G) needed to make a target concentration $C$ is:

and the diluent volume is $V_W = 10 - V_G$.

Understanding the Question

You are told to prepare 10 cm³ of each glucose concentration using G (20% stock) and W (distilled water), with 12% as the lowest concentration. The 20% and 12% rows are already filled in. You must decide which intermediate concentrations to use and complete the G and W volumes for each row.

Approach

Choose at least three intermediate concentrations between 20% and 12% at a regular interval, then use the dilution formula to find the volume of G required for each. The simplest interval is 2% (20, 18, 16, 14, 12). Five evenly spaced rows will make any later graph smooth and any interpolation between standards more accurate.

Step-by-Step Reasoning

• 20%: $V_G = 20/2 = 10\ \text{cm}^3$, $V_W = 0\ \text{cm}^3$ (already in the table).
• 18%: $V_G = 18/2 = 9\ \text{cm}^3$, $V_W = 10 - 9 = 1\ \text{cm}^3$.
• 16%: $V_G = 16/2 = 8\ \text{cm}^3$, $V_W = 10 - 8 = 2\ \text{cm}^3$.
• 14%: $V_G = 14/2 = 7\ \text{cm}^3$, $V_W = 10 - 7 = 3\ \text{cm}^3$.
• 12%: $V_G = 12/2 = 6\ \text{cm}^3$, $V_W = 10 - 6 = 4\ \text{cm}^3$ (already in the table).

Each row sums to 10 cm³, satisfying the requirement.

Key Takeaways

• A dilution of a stock of concentration $C_0$ to a target $C$ in a fixed final volume $V$ needs $V_G = (C/C_0) \times V$ of stock and $V - V_G$ of diluent.
• Choosing regular intervals between concentrations (e.g. every 2%) gives a smoother calibration curve and easier interpolation later.

Common Mistakes

• Choosing a non-integer number of cm³ (e.g. trying to make 13% would give 6.5 cm³ of G, which is hard to measure accurately with a 10 cm³ measuring cylinder).
• Forgetting that the volumes of G and W must add up to 10 cm³.
• Listing concentrations that don't change regularly from 20% down to 12%.

Things to Be Careful About

• The mark scheme requires at least three concentrations between 20% and 12%; four or five is fine and usually safer.
• Leave the two given rows (20% and 12%) exactly as they appear - only the intermediate rows are completed.

Background Concept

In Paper 3, a results table is the formal record of raw data. It must show:

• the independent variable (what you deliberately changed) in the left-hand column;
• the dependent variable (what you measured) in the right-hand column, with its unit in the heading;
• one row per experimental condition;
• a clear trend or pattern that matches the prediction.

In this experiment the independent variable is the percentage concentration of glucose and the dependent variable is the time taken for potassium manganate(VII) (P) to be decolourised from purple to colourless. Because higher concentrations of glucose provide more reducing sugar, they decolourise the permanganate faster, so the time decreases as the concentration rises.

Understanding the Question

You carry out the procedure for each of the five concentrations in Table 1.2, starting all the reactions at the same moment and recording the time at which each test-tube becomes completely colourless. You then write these times in a results table.

Approach

Before you start, decide the layout of your results table - it should be ready before you mix any reagents because the timer is running continuously. Use a left-hand column for the independent variable and a right-hand column for the dependent variable (with units in the heading). Record each time as a whole second (the mark scheme will not accept decimals here).

Step-by-Step Reasoning

• The independent variable is the percentage concentration of glucose, so this heading goes in the left column.
• The dependent variable is the time taken for the end-point, so this heading goes in the right column. The unit is seconds, so write 'time / s' (or 'time / seconds').
• As each tube becomes colourless, glance at the timer and record the whole-second reading in the corresponding row.
• Because higher concentrations react faster, the 20% tube should clear first, the 18% tube next, and so on down to 12% last. The expected trend is therefore monotonically decreasing time with increasing concentration.
• A representative set of times (illustrative only) might be: 20% = 32 s, 18% = 45 s, 16% = 60 s, 14% = 90 s, 12% = 150 s. The exact values will differ for every student but the order must be the same.

Key Takeaways

• Always pre-draw the results table before the timed run so you can record immediately.
• The independent variable is on the left, the dependent variable on the right, and the unit is part of the dependent-variable heading.
• Record times as whole seconds; the mark scheme will not credit decimal places here.

Common Mistakes

• Putting the headings in the wrong order (independent variable on the right).
• Forgetting the unit in the dependent-variable heading.
• Recording decimal seconds (e.g. 32.4 s) - the mark scheme requires whole seconds.
• Recording fewer than four concentrations when five were prepared.
• Misreading the timer because the reactions are started simultaneously and the eye is drawn to the first tube to clear.

Things to Be Careful About

• The timer is NOT to be stopped between tubes; each end-point is recorded as the elapsed time on a continuously running clock.
• If a tube has not cleared by 600 s, record 'more than 600' rather than leaving the cell blank.
• The order in which tubes clear must match the order of decreasing concentration - if not, the data are likely to be wrong and should be repeated.

Background Concept

The unknown energy drink U contains glucose (and possibly other reducing sugars). The same decolourisation reaction with potassium manganate(VII) will therefore take a time that depends on the glucose concentration. By comparing the time for U with the times for the standards, the concentration of glucose in U can be estimated.

Understanding the Question

You have just run a new test-tube containing 1 cm³ of U instead of a standard glucose solution. The timer has been running continuously since the standards were started. You read the time at which the U tube becomes completely colourless and record it.

Approach

Carry out steps 5-7 of the procedure on the U tube exactly as for the standards. As the purple colour disappears, read the timer to the nearest whole second and write this in the answer line.

Step-by-Step Reasoning

• Add 1 cm³ of A (dilute sulfuric acid) to the U tube and shake.
• Add 1 cm³ of P (potassium manganate(VII)) and start observing.
• When the purple colour has completely disappeared, glance at the continuously running timer and read the time to the nearest whole second.
• Record this time. The mark scheme requires the time for U to be closest to that of the 20% standard, so for representative data where the 20% standard took 32 s, U might give a reading of about 35 s.

Key Takeaways

• The same procedural steps and timing precision must be applied to the unknown as to the standards, otherwise the comparison is invalid.
• The time for U will be read against the same continuously running timer that was used for the standards.

Common Mistakes

• Stopping and restarting the timer for U (the timer must keep running).
• Using a different volume of U from that of the standards (must be 1 cm³).
• Recording a time that is wildly different from the 20% standard, which would suggest either a procedural slip or that U is much more or much less concentrated than expected.

Things to Be Careful About

• The mark scheme only credits the reading if it is close to the 20% standard - if it is not, the procedure has probably been done incorrectly and should be repeated before (a)(iv) is attempted.

Background Concept

The reaction time decreases as the glucose concentration increases. By placing the time for U on the same scale as the standards, the glucose concentration in U can be estimated. If the time lies between two standard times, the concentration is estimated by linear interpolation between those two standards.

Understanding the Question

You have a time for U from (a)(iii) and a table of times for the standards from (a)(ii). You need to read across the table to find which standard time is closest to U's time and quote the corresponding concentration (or interpolate if U falls between two standards).

Approach

Locate the time for U in the results table. If it is closest to one particular standard, quote that standard's concentration. If it falls between two standards, estimate the concentration by linear interpolation between them.

Step-by-Step Reasoning

• Representative standards: 20% = 32 s, 18% = 45 s, 16% = 60 s, 14% = 90 s, 12% = 150 s.
• Representative time for U = 35 s, which lies between 20% (32 s) and 18% (45 s) and is much closer to 20%.
• Linear interpolation: U is $\frac{35 - 32}{45 - 32} = \frac{3}{13} \approx 0.23$ of the way from 20% to 18%, i.e. about 0.5% below 20%, giving ≈ 19.5%, which rounds to 19%.
• The mark scheme accepts a reasonable estimate from the student's own data, so anything between about 18% and 20% would be credited here.

Key Takeaways

• The closer the standards bracket the unknown, the more accurate the estimate - this is why (a)(v) suggests a narrower range of standards around U's estimate.
• Linear interpolation between two adjacent standards is the standard way to estimate an intermediate value.

Common Mistakes

• Picking the standard whose time is the same as U's, when U's time lies between two standards (interpolate instead).
• Quoting a value outside the range of the standards (e.g. saying 25% when 20% is the highest standard).
• Quoting a value without any reference to the recorded data (must come from the student's own table).

Things to Be Careful About

• The estimate should be expressed to a sensible precision - usually to the nearest whole percent or to one decimal place, not to several significant figures that are not justified by the data.

Background Concept

Improving the accuracy of an estimate usually means reducing random error (by replication and means), reducing systematic error (by more precise end-point detection), and improving the resolution of the calibration (by using a narrower range of standards centred on the unknown, and by reading off a graph rather than a single nearest-standard match).

Understanding the Question

You have estimated the glucose concentration in U using a coarse dilution series (20 to 12% in 2% steps) and a single read-off against the nearest standard. The examiner is asking what changes to the procedure would make this estimate more accurate.

Approach

The most effective set of improvements targets the two main weaknesses of the current procedure: the coarse concentration steps, and the fact that the time is taken only once. The first is fixed by using a narrower, denser set of standards and reading off a graph; the second is fixed by replication and means.

Step-by-Step Reasoning

The mark scheme offers six creditworthy improvements (any three earn full marks):

1. Use a narrower range of concentrations centred on the estimate of U: a calibration curve is most accurate where the standards are densest, so the standards should cluster around U's estimated value rather than span a wide range.
2. Use at least five more concentrations within this narrower range: a denser set of standards gives more points on the calibration curve and a more accurate read-off.
3. Plot a graph of time against concentration for the standards and read off U: graphical interpolation is more accurate than a single nearest-standard match.
4. Repeat measurements for all standards and for U, and calculate means: repetition reduces the effect of random error in the end-point judgement.
5. Use a colorimeter (or record a video of the reaction with a visible timer, or use several independent observers): an instrumental or multi-observer end-point is more objective than a single human judgement.
6. Run the reactions one at a time (or stagger the start times): avoids the difficulty of recording end-points for several simultaneously running reactions.

Key Takeaways

• Accuracy is improved by reducing both random error (replication, better timing) and systematic error (calibration resolution, objective end-point detection).
• A calibration graph plus a narrow standard range is the most accurate way to estimate an unknown from a reaction-time assay.

Common Mistakes

• Vague answers such as 'be more careful' or 'do it again' - these are not creditworthy because they are not specific to the procedure.
• Suggesting changes that are not feasible in a school lab (e.g. mass spectrometry).
• Listing changes that would not actually improve accuracy (e.g. using a larger volume of P, which would simply change the time scale without improving the precision of the read-off).

Things to Be Careful About

• The mark scheme caps this question at 3 marks, so the examiner will only credit the first three creditworthy points you list - make sure each is a distinct, specific improvement.

Background Concept

A 'significant source of error' is one that has a material effect on the accuracy or precision of the measurement - not a generic 'human error'. For Paper 3, the source must be tied to a specific step in the procedure and a specific way in which that step is unreliable.

Understanding the Question

Step 7 of the procedure is the recording of the end-point times. You are asked for two significant sources of error that occur during this step.

Approach

Think about exactly what could go wrong while the timer is running. The two clearest problems are (i) the practical difficulty of watching several tubes at once and reading the timer at the right moment, and (ii) the fact that the end-point is a gradual colour change rather than a sharp transition.

Step-by-Step Reasoning

• Source 1: All five (or more) reactions are started at the same time, but each one takes a different length of time to clear. The experimenter has to watch several tubes simultaneously, decide which is the next to clear, and read the timer at the precise moment of clearing. This is very difficult to do accurately, and small delays in noticing or reading the timer will cause appreciable error in the recorded time.
• Source 2: The end-point is defined as 'P is completely colourless', but the colour change is gradual. Different observers will judge the end-point at slightly different stages, and the same observer may judge it differently on different occasions. The end-point is therefore subjective.

Key Takeaways

• A 'source of error' must be specific to the procedure, not a vague statement.
• The two main categories of error here are practical (timing multiple simultaneous reactions) and observational (judging a gradual colour change).

Common Mistakes

• Vague answers such as 'human error' or 'the experimenter might make a mistake' - these are not creditworthy.
• Naming errors that are not actually part of step 7 (e.g. errors in pipetting or in making up the dilutions).
• Naming a source without explaining why it affects the result.

Things to Be Careful About

• The question is about step 7 specifically, not about the procedure as a whole. Errors earlier in the procedure (e.g. dilutions) are not what is being asked for here.

Background Concept

Once a source of error has been identified, the next step is to suggest a specific, practical change to the procedure that would reduce it. The improvement must be directly linked to the source - not a generic 'be more careful'.

Understanding the Question

You have just listed two sources of error in step 7. The question asks for one modification to the procedure that would reduce one of those sources of error.

Approach

Choose one of the two errors and propose a concrete change. The most straightforward change is to deal with the simultaneous-reactions problem by timing each one separately. Alternatively, the subjective end-point can be removed by using a colorimeter, video, or several observers.

Step-by-Step Reasoning

• If the chosen error is the difficulty of timing several simultaneous reactions: time each concentration separately. Set up one tube, start the reaction, record the time to the end-point, then move on to the next tube.
• If the chosen error is the subjectivity of the end-point: use a colorimeter set to the wavelength of permanganate absorption; the colorimeter will register the moment the absorbance falls below a defined threshold. Alternatively, record the reactions on video with the timer visible and review the video frame-by-frame later, or have several observers each call the end-point and take the mean of their times.

Any one of these modifications is worth the mark.

Key Takeaways

• An 'improvement' must be specific to the procedure, not a generic call for more care.
• A modification should clearly reduce the named error, not just alter the experiment in some unrelated way.

Common Mistakes

• Vague answers such as 'be more accurate' or 'take more readings' - these do not target the specific error.
• Suggesting a change that would not actually reduce the named error (e.g. using a different concentration of P would change the time scale but not the difficulty of timing several reactions).
• Suggesting a change that is not feasible in a school lab (e.g. spectrophotometer with a flow cell).

Things to Be Careful About

• The mark scheme only credits one modification here, so there is no need to list several - but they should be clearly tied to one of the errors in (a)(vi).