Biology 9700/35 — May/June 2013

Background Concept

A dilution keeps the total amount of solute constant while increasing the total volume. The dilution equation

relates the stock concentration $C_1$, the volume of stock $V_1$, the desired final concentration $C_2$ and the final total volume $V_2$. Solving for $V_1$ gives the volume of stock to use; the remainder of the final volume is made up with the diluent (here, distilled water W).

Understanding the Question

Table 1.1 specifies that $20\text{ cm}^3$ of each of four glucose solutions (6%, 8%, 10% and 12%) must be prepared from the 20% stock G and distilled water W. The 12% row is already filled in (12 cm³ glucose + 8 cm³ water). The candidate must complete the three missing rows for 6%, 8% and 10%.

Approach

Apply the dilution equation with $C_1 = 20$ (the stock percentage), $C_2 =$ the target percentage, and $V_2 = 20\text{ cm}^3$. Calculate $V_1$ (volume of 20% glucose) for each target, then subtract $V_1$ from $20\text{ cm}^3$ to get the volume of distilled water.

Step-by-Step Reasoning

For 6% glucose:

• $20 \times V_1 = 6 \times 20$, so $V_1 = 6\text{ cm}^3$ of 20% glucose.
• Water $= 20 - 6 = 14\text{ cm}^3$.

For 8% glucose:

• $20 \times V_1 = 8 \times 20$, so $V_1 = 8\text{ cm}^3$ of 20% glucose.
• Water $= 20 - 8 = 12\text{ cm}^3$.

For 10% glucose:

• $20 \times V_1 = 10 \times 20$, so $V_1 = 10\text{ cm}^3$ of 20% glucose.
• Water $= 20 - 10 = 10\text{ cm}^3$.

The 12% row is already given: $12\text{ cm}^3$ of stock + $8\text{ cm}^3$ of water = $20\text{ cm}^3$ of 12%.

Key Takeaways

• The dilution equation $C_1 V_1 = C_2 V_2$ is the standard way to prepare a solution of known concentration from a more concentrated stock.
• The volume of diluent (water) = total volume − volume of stock.
• The candidate must give BOTH the stock volume AND the water volume for each row.

Common Mistakes

• Only giving the stock volume and forgetting the water volume (the mark scheme credits both).
• Substituting the wrong $C_1$ (e.g. using 12% instead of 20% as the stock).
• Writing the target concentration (6, 8, 10) in the water column instead of the actual cm³ of water.

Things to Be Careful About

• Verify that stock + water = $20\text{ cm}^3$ for every row.
• Keep the unit ($\text{cm}^3$) consistent throughout.

Background Concept

Whenever a quantity is recorded, the unit and the precision must be stated explicitly. For time, the unit is usually seconds when the reaction is on the order of tens of seconds to minutes. The smallest value the timer can show is its display resolution; the smallest unit recorded is the precision the candidate chooses to round to.

Understanding the Question

Before the experiment starts, the candidate must state two things about the timer: (1) the smallest value it can display and (2) the smallest unit to which the candidate will round all readings. For a typical laboratory stopwatch or digital clock, both answers are 1 and seconds.

Approach

Look at the timer or clock available. The smallest division on its display is the smallest value it can show. Then decide what precision to record to. For this experiment, whole seconds is appropriate.

Step-by-Step Reasoning

A standard digital stopwatch or wall clock shows whole seconds (with smaller divisions that are not meaningful for a reaction lasting tens of seconds). The candidate will record all readings to the nearest whole second.

Key Takeaways

• Always state the unit of any measured quantity.
• The chosen precision should be sensible for the timescale of the experiment.

Common Mistakes

• Stating minutes instead of seconds (the reaction is fast enough that seconds is the appropriate unit).
• Stating "time" alone, without naming the unit.
• Confusing "smallest value shown" with "smallest value recorded".

Things to Be Careful About

• The unit must be appropriate to the timescale (seconds, not minutes, for this reaction).

Background Concept

A fair test requires that the only thing that changes between trials is the independent variable. In this experiment the independent variable is the glucose concentration, and the dependent variable is the time for PM to turn colourless. Any other variable that could affect the time must be kept the same (standardised) for every test-tube.

Understanding the Question

The question asks for ONE variable to standardise when setting up the test-tubes for U1 and U2. The mark scheme accepts any of: the volume of S, the volume of PM, or the volume of glucose/U1/U2 solution added to each tube. The cleanest single answer is the volume of PM because, if the volume of oxidising agent differs between the U1/U2 and the standard tests, the end-point time would change independently of the glucose concentration.

Approach

Think of the variables that go into each test-tube (volumes of glucose, S, PM) and choose one that is both controllable AND able to alter the time if it were not standardised.

Step-by-Step Reasoning

• $10\text{cm}^3$ of glucose solution goes into each test-tube.
• $5\text{cm}^3$ of S is added to each.
• $2\text{cm}^3$ of PM is added to each.

The volume of PM is the cleanest single answer because it directly determines how much oxidising agent is available to be reduced, and any change in it would change the time independently of the glucose concentration.

Key Takeaways

• A standardised variable is one kept the same across all trials so that any difference in the dependent variable is due only to the independent variable.
• The cleanest variables to standardise here are the volumes of the reagents added to each test-tube.

Common Mistakes

• Stating a variable that does not actually change between trials (so does not need standardising).
• Stating a variable that is not controllable in a school lab (e.g. ambient temperature is hard to standardise reliably).
• Naming a variable that is not relevant to this experiment.

Things to Be Careful About

• The variable must be one that could plausibly differ between the U1/U2 tests and the standard tests, and that would affect the time if it differed.

Background Concept

A standardised variable only stays the same if it is measured and delivered in the same way each time. For small liquid volumes in a Cambridge A-level practical, the syringe is the standard apparatus because it can be read to $0.1\text{cm}^3$ or better. To use it accurately, draw up slightly more than needed and expel down to the required volume with the meniscus on the line.

Understanding the Question

The candidate has identified a variable to standardise in (a)(iii) and is now asked to describe how that standardisation will be enforced for the U1 and U2 tests. Four specific points earn the four marks: (1) use a syringe; (2) record the start and end-point times for U1 and U2; (3) start the timer for U1 before U2; (4) record all times to the same precision.

Approach

For each marking point, think about what the candidate is actually doing in the lab, not in the abstract. The order of operations matters: set up U1, add PM, record start; then set up U2, add PM, record start; then watch both and record end-point times.

Step-by-Step Reasoning

• A syringe is the right apparatus for measuring $2\text{cm}^3$ of PM (or $5\text{cm}^3$ of S, or $10\text{cm}^3$ of glucose solution) accurately. A measuring cylinder would be too coarse for these small volumes.
• Two readings are needed for each tube: the start time (when PM is added) and the end-point time (when the pink colour disappears). Both are written on Fig. 1.4.
• Because the procedure is the same as for the standards, U1 is set up first so that its reaction is underway before U2 is started. This is consistent with the way the standards are tested (6% first, then 8%, 10%, 20%).
• All four times (U1 start, U1 end, U2 start, U2 end) are read to the same precision — for example, whole seconds — so that they are directly comparable.

Key Takeaways

• Standardisation requires both the right apparatus (a syringe for small volumes) and the right procedure (consistent order and precision).
• A clear, ordered description earns the four marks: apparatus, recording, order, precision.

Common Mistakes

• Saying "measuring cylinder" instead of "syringe".
• Forgetting one of the four points (especially precision or order).
• Describing a different procedure from the one used for the standards (e.g. timing each tube individually — that is a modification, not a description of how the variable is standardised here).

Things to Be Careful About

• The description should match the actual lab procedure, not a generic answer.
• The order of the four points does not matter, but all four must be present for full marks.

Background Concept

The time taken to reach the end-point is the difference between the end-point time and the start time. To subtract two clock readings, they must first be converted to the same unit (here, seconds).

Understanding the Question

The question asks the candidate to complete Table 1.2 by showing the calculation of the time taken for the 6% glucose solution. The actual numbers come from Fig. 1.3 and so are student-dependent; the format of the answer is what is being tested.

Approach

Read the start time and the end-point time for the 6% tube from Fig. 1.3, convert both to seconds, and subtract.

Step-by-Step Reasoning

Using the mark-scheme example values:

• Start time: $1$ minute $24$ seconds $= 60 + 24 = 84$ s.
• End-point time: $2$ minutes $55$ seconds $= 2 \times 60 + 55 = 175$ s.
• Time taken $= 175 - 84 = 91$ s.

The candidate should write the start time, the end-point time, and the difference (in seconds) into Table 1.2.

Key Takeaways

• Convert both readings to the same unit before subtracting.
• The answer is reported in the chosen unit (here, seconds).

Common Mistakes

• Subtracting minutes from seconds without converting.
• Recording the end-point time itself as the answer instead of the difference.
• Not writing the unit with the final answer.

Things to Be Careful About

• The example given in the question (91 s) uses minutes:seconds; if a digital stopwatch was used, the values are already in seconds and no conversion is needed.

Background Concept

A results table is the standard way to present processed data. The conventions the CIE mark scheme rewards are: cell boundaries drawn, a heading that names the quantity, units in the heading (not in the data cells), and data presented in a sensible order. The trend (how one variable changes with the other) should be visible from the table.

Understanding the Question

The question asks the candidate to prepare the space below the procedure to record the time taken for all six solutions (6%, 8%, 10%, 20%, U1, U2). Four marks are available: (1) a table with cells drawn and the heading percentage concentration of glucose; (2) a time / s (or time / seconds) heading; (3) whole-number processed results for all six solutions; (4) the trend that 20% (the highest concentration standard) has the shortest time compared with 6%, 8% and 10%.

Approach

Draw a two-column table. Head the first column with the independent variable (concentration) and the second with the dependent variable (time, in seconds). List the six concentrations in the first column and the processed times (in whole seconds) in the second. Make sure the 20% has the shortest time among the four standards.

Step-by-Step Reasoning

• A heading is required for the column that lists the concentrations, and that heading must name the quantity (percentage concentration of glucose). Units are not required here because the entries are percentages, but the percentage sign is fine.
• A heading is required for the time column; the unit (seconds) goes in the heading, not in the data cells.
• The candidate's actual times are student-dependent but should be whole numbers. The 20% row must have the smallest value among the four standards (because higher concentration = faster reduction of PM = shorter time).
• The trend: as concentration increases, time decreases.

Key Takeaways

• Headings carry the quantity AND the unit; data cells contain only the number.
• Processed data should be reported to a sensible precision (here, whole seconds).
• The table should make the trend visible.

Common Mistakes

• Putting units in the data cells (e.g. writing "120 s") instead of in the heading.
• Using the wrong units in the heading (e.g. $\text{mol dm}^{-3}$).
• Not drawing all cell boundaries.
• Including non-whole-number times (decimals are not credited).
• Missing one of the six concentrations.
• Reporting times that are inconsistent with the trend (e.g. 6% shorter than 20%).

Things to Be Careful About

• The mark scheme does NOT credit units in the data cells.
• The trend must be visible in the numbers: 20% < 10% < 8% < 6%.

Background Concept

A source of error is anything that introduces variability or bias into the measurements. The CIE mark scheme distinguishes between errors in the dependent variable (e.g. subjective end-point), errors in the standardised variables (e.g. hard-to-read syringe), errors in the procedure (e.g. inconsistent mixing), and inherent errors of the reaction (e.g. being too quick to time).

Understanding the Question

The question asks for ONE significant source of error in this investigation. The mark scheme accepts any of: a subjective end-point; difficulty reading the dark PM solution in the syringe; inconsistent mixing; or the reaction being too quick to time accurately. The cleanest single answer is the subjective end-point.

Approach

Identify the step in the procedure that is most dependent on human judgement. The end-point of the reaction is the moment the pink colour just disappears, and this transition is gradual rather than instantaneous, so different observers (or the same observer on different occasions) may judge it at slightly different times.

Step-by-Step Reasoning

• The end-point is when the pink colour of PM has fully disappeared and the solution is colourless.
• This change happens over a short interval rather than instantaneously, so the moment of "colourless" is not precisely defined.
• Consequently the recorded end-point time can vary by a few seconds between trials, even when the actual reaction rate is the same.

Key Takeaways

• Subjective end-points are a common source of error in colour-change reactions.
• The error is the variability in the recorded end-point time, not a single wrong number.

Common Mistakes

• Stating "human error" without specifying the aspect of the procedure that is affected.
• Stating an error that is not actually significant (e.g. "the room was too cold") without explaining how it affects the result.
• Naming a problem that is not specific to this experiment (e.g. "the equipment was old").

Things to Be Careful About

• The error should be specific to this experiment and have a clear cause.
• The answer must name the variable affected (e.g. the end-point time) AND give the reason (e.g. the colour change is subjective) to earn the mark.

Background Concept

An improvement is any change to the procedure that reduces error (random or systematic) and so increases confidence in the results. CIE mark schemes usually group improvements into: better range of the independent variable, more replicates, better control of standardised variables, and more objective measurement of the dependent variable.

Understanding the Question

The question asks for TWO modifications that would improve confidence in the results. The mark scheme accepts any two of: a wider or different range of concentrations; repeating more than once to obtain three readings; or using a colorimeter (or timing each test-tube individually) to detect the end-point.

Approach

Choose two modifications that target different aspects of the experiment. The most impactful pair is usually: more replicates (improves reliability) + more objective end-point detection (improves accuracy). The mark scheme allows any two of the three credits, so any clean pair is fine.

Step-by-Step Reasoning

• Replicates. With only one reading per concentration, a single anomalous timing cannot be distinguished from a real effect. Repeating each concentration at least three times and taking a mean smooths out the random error in judging the end-point.
• Objective end-point. A colorimeter measures the absorbance of the solution and can be set to a specific transmittance threshold that defines "colourless" unambiguously. Alternatively, timing each test-tube with its own stopwatch removes the rush of trying to add PM to four tubes in quick succession and judge their end-points simultaneously.

Key Takeaways

• Replicates improve reliability (consistency) by averaging out random errors.
• Objective measurement improves accuracy (closeness to the true value) by removing human judgement.
• The two modifications should address different aspects of the experiment.

Common Mistakes

• Stating two modifications that are essentially the same (e.g. "repeat twice" and "repeat three times").
• Stating a modification that is too vague to act on (e.g. "be more careful").
• Stating an improvement that is not specific to this experiment (e.g. "use a better stopwatch" without saying why).

Things to Be Careful About

• Each modification should be specific and should clearly explain how it improves the result.
• The mark scheme awards a maximum of 2 marks for 2 modifications, so no more than two are needed.