Cambridge Lower Secondary CheckpointStage 6

Thinking and Working Scientifically: Carrying out scientific enquiry

Science Stage 6 Chapter Notes

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Thinking and Working Scientifically: Carrying out scientific enquiry
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In this chapter

  1. 1Identifying and Controlling Variables
  2. 2Developing a Clear Experimental Method
  3. 3Risk Assessment and Safety
  4. 4Recording Data Accurately
  5. 5Selecting Appropriate Apparatus
  6. 6Evaluating and Improving Experiments

1. Identifying and Controlling Variables

In any scientific investigation, you change one thing to see its effect on another. These 'things' are called variables. The Independent Variable (IV) is the one you deliberately change. The Dependent Variable (DV) is the one you measure to see the effect of changing the IV. To ensure a 'fair test', all other relevant factors, called Control Variables (CVs), must be kept constant. If a control variable is not kept constant, you cannot be sure that the change in the IV is the only reason for the changes you observe in the DV.

Key term

Fair Test: An experiment in which only the independent variable has been changed to affect the dependent variable, while all other control variables are kept constant.

Examiner insight

Examiners award marks for identifying specific, measurable control variables. For example, 'volume of water' is better than just 'amount of water'.

Common pitfall

Confusing control variables with a 'control experiment'. A control experiment is a separate setup, often at a baseline or zero level of the independent variable, used for comparison.

Worked example 14 marks

A student investigates how the temperature of water affects the time it takes for a sugar cube to dissolve. They use the same volume of water and the same size of sugar cube for each test. Identify the independent, dependent, and two control variables in this investigation.

  1. 1

    Independent Variable (IV): This is what the student changes. Here, it is the temperature of the water.

  2. 2

    Dependent Variable (DV): This is what the student measures. Here, it is the time taken for the sugar cube to dissolve.

  3. 3

    Control Variable 1 (CV1): A factor kept the same. The question states the volume of water is the same for each test.

  4. 4

    Control Variable 2 (CV2): Another factor kept the same. The question states the size of the sugar cube is the same. Other valid controls would be 'stirring' (or lack of it) and the 'type of sugar'.

Recap

  • The Independent Variable (IV) is the factor you change.
  • The Dependent Variable (DV) is the factor you measure.
  • Control Variables (CVs) are all other factors you keep the same.
  • Keeping control variables constant is essential for a fair test.
  • A fair test ensures that only the IV is affecting the DV.

Quick check

  1. In an experiment to find the effect of light intensity on the rate of photosynthesis, what is the dependent variable?1 mark

2. Developing a Clear Experimental Method

A scientific method is a recipe for your experiment. It must be a clear, logical, and detailed set of instructions that another person could follow to get the same results. A good method includes: what apparatus to use, the exact quantities of substances, a step-by-step procedure, how to measure the dependent variable, the range and intervals of the independent variable you will test, and how many times you will repeat the experiment to ensure reliability.

Key term

Method: A detailed, step-by-step description of how an experiment will be carried out, including apparatus, quantities, and procedure.

Examiner insight

Marks are often allocated for specifying a suitable range and number of values for the independent variable, typically at least 5 different values are expected.

Common pitfall

Writing vague steps like 'add some acid' instead of 'use a 25 cm³ measuring cylinder to add 20 cm³ of 1.0 mol/dm³ hydrochloric acid'.

Worked example 16 marks

Plan an experiment to investigate how the concentration of hydrochloric acid affects the rate of reaction with marble chips (calcium carbonate). Your plan should include a list of apparatus and a step-by-step method.

  1. 1

    Apparatus: Conical flask, gas syringe (or measuring cylinder in a trough of water), measuring cylinders (e.g., 50 cm³), stop clock, electronic balance, marble chips (medium size), hydrochloric acid in a range of concentrations (e.g., 0.5M, 1.0M, 1.5M, 2.0M, 2.5M), safety goggles.

  2. 2

    Step 1: Put on safety goggles. Use a measuring cylinder to measure 40 cm³ of 0.5M hydrochloric acid and pour it into the conical flask.

  3. 3

    Step 2: Use an electronic balance to measure 5.0 g of medium-sized marble chips.

  4. 4

    Step 3: Add the marble chips to the acid in the flask, immediately connect the gas syringe and start the stop clock.

  5. 5

    Step 4: Record the volume of gas collected in the gas syringe every 20 seconds for 180 seconds (3 minutes).

  6. 6

    Step 5: Repeat steps 1-4 two more times to get a total of three results for this concentration.

  7. 7

    Step 6: Repeat the entire experiment (steps 1-5) using the other concentrations of hydrochloric acid (1.0M, 1.5M, 2.0M, 2.5M), keeping the volume of acid and mass of marble chips the same each time.

Recap

  • A method must be a clear, step-by-step guide.
  • Always state the specific quantities and concentrations of chemicals.
  • Name the specific apparatus you will use for each measurement.
  • Include a suitable range and number of values for your independent variable.
  • State that you will repeat the experiment and calculate a mean for reliability.
  • Always begin your plan by considering safety.

Quick check

  1. Why is it important to state the range of values you will test for the independent variable?1 mark
  2. What is the purpose of repeating measurements?1 mark

3. Risk Assessment and Safety

Safety is the top priority in any lab. Before starting an experiment, you must conduct a risk assessment. This involves three stages: 1. Identify the hazards (e.g., a corrosive acid, a hot Bunsen burner, fragile glassware). 2. Assess the risk associated with each hazard (e.g., the acid could splash into your eyes causing damage). 3. State the control measures or precautions you will take to minimise the risk (e.g., wear safety goggles).

Key term

Hazard: A substance, piece of equipment, or procedure that has the potential to cause harm.

Common pitfall

Stating 'wear safety goggles' as a precaution for every single experiment without linking it to a specific risk, like splashing chemicals or liquids boiling over.

Fun fact

The globally recognized hazard symbols (pictograms) were standardized by the UN in a system called the GHS (Globally Harmonized System) to ensure safety information is consistent across countries.

Worked example 13 marks

In an experiment involving heating a solution of copper sulfate in a beaker with a Bunsen burner, identify one hazard, describe the risk, and suggest a suitable precaution.

  1. 1

    Hazard: The hot beaker and tripod stand (or the Bunsen burner flame).

  2. 2

    Risk: Touching the hot apparatus could cause a serious burn to the skin.

  3. 3

    Precaution: Allow the apparatus to cool down completely before moving it, or use heat-proof tongs to move the beaker.

Worked example 22 marks

A student is handling a corrosive acid. What is the single most important precaution they should take and why?

  1. 1

    Precaution: Wear safety goggles.

  2. 2

    Justification: To prevent the corrosive acid from splashing into the eyes, which could cause permanent eye damage or blindness.

Recap

  • A hazard is something with the potential to cause harm.
  • A risk is the likelihood of that harm occurring.
  • A precaution is a step taken to reduce the risk.
  • Always wear safety goggles when working with chemicals, heating, or glassware under pressure.
  • Tie back long hair and ensure the workspace is clear of clutter.
  • Know the location of safety equipment like fire extinguishers and eye wash stations.

Quick check

  1. What is the difference between a hazard and a risk?2 marks

4. Recording Data Accurately

Collecting good data is pointless if you don't record it clearly. The standard way is to use a results table. A good table has ruled lines, a title, and clear column headings. The first column should be for your independent variable. Subsequent columns are for the dependent variable, including columns for repeated readings and a final column for the mean. Crucially, every column heading must include both the quantity being measured and its units, separated by a forward slash (e.g., 'Time / s'). Record data as you collect it, to the precision of your measuring instrument.

Key term

Precision: The closeness of repeated measurements to each other, often determined by the resolution of the measuring instrument.

Examiner insight

Examiners check that the mean is calculated to an appropriate number of significant figures, which is usually the same as, or one more than, the raw data.

Common pitfall

Forgetting to include units in table headings, or writing units next to every data point in the table body.

Worked example 15 marks

A student measures the time taken for a reaction at different temperatures. The results are: At 20°C: 85s, 87s, 86s. At 30°C: 44s, 46s, 45s. At 40°C: 22s, 29s, 23s. Draw a suitable table to record these results and calculate the mean time for each temperature. Comment on the result at 40°C.

  1. 1

    Step 1: Draw a table with columns for the IV (Temperature), repeats (Time 1, Time 2, Time 3) and the calculated mean time. Ensure headings have units.

  2. 2

    Step 2: Fill in the data provided into the table.

  3. 3

    | Temperature / °C | Time 1 / s | Time 2 / s | Time 3 / s | Mean Time / s |

  4. 4
  5. 5

    | 20 | 85 | 87 | 86 | 86.0 |

  6. 6

    | 30 | 44 | 46 | 45 | 45.0 |

  7. 7

    | 40 | 22 | 29 | 23 | 22.5 |

  8. 8

    Step 3: Calculate the mean for 20°C: (85 + 87 + 86) / 3 = 86.0 s.

  9. 9

    Step 4: Calculate the mean for 30°C: (44 + 46 + 45) / 3 = 45.0 s.

  10. 10

    Step 5: Identify the anomaly at 40°C. The result '29s' is not close to the other two readings (22s and 23s). This is an anomalous result.

  11. 11

    Step 6: Recalculate the mean for 40°C, ignoring the anomaly: (22 + 23) / 2 = 22.5 s.

Recap

  • Draw tables with a ruler, enclosing all data.
  • The first column should contain the independent variable.
  • All column headings must state the quantity and its units (e.g., 'Volume / cm³').
  • Record raw data directly into the table; don't do calculations in your head.
  • Include columns for repeats and a calculated mean.
  • Identify and exclude anomalous results from the calculation of the mean.

Quick check

  1. A student measures a length as 15.2 cm. What is the quantity and what is the unit?1 mark

5. Selecting Appropriate Apparatus

The quality of your results depends heavily on choosing the right tools for the job. The choice of apparatus affects the accuracy and precision of your measurements. For example, to measure approximately 100 cm³ of water, a beaker is fine. To measure 100 cm³ for a rough experiment, a measuring cylinder is better. To measure exactly 100.0 cm³ for making a standard solution, you must use a volumetric flask. Consider the resolution of the instrument – the smallest change it can detect. A digital stop clock with a resolution of 0.01 s is more precise than an analogue watch.

Key term

Resolution: The smallest change in the quantity being measured that can be detected by an instrument.

Common pitfall

Suggesting a beaker for measuring any volume that needs to be even remotely accurate. Beakers are for holding or mixing, not measuring.

Fun fact

To avoid parallax error when reading an analogue meter (like a voltmeter), some high-quality meters have a small mirror behind the needle. You know your eye is directly above the needle when the needle covers its own reflection.

Worked example 13 marks

A student needs to measure 25.0 cm³ of an alkali solution to be used in a titration. Suggest the most appropriate piece of apparatus for this measurement and justify your choice.

  1. 1

    Apparatus: A 25 cm³ pipette.

  2. 2

    Justification 1: A pipette is designed to deliver a very accurate and fixed volume of liquid (in this case, 25.0 cm³).

  3. 3

    Justification 2: It has a much higher precision and lower uncertainty (typically ±0.06 cm³) than a measuring cylinder, which is essential for an accurate titration.

Worked example 22 marks

You need to measure the change in temperature during a reaction. Why is a digital thermometer with a resolution of 0.1 °C better than a liquid-in-glass thermometer with a resolution of 1 °C?

  1. 1

    Reason 1: The digital thermometer has a higher resolution (0.1 °C vs 1 °C), meaning it can detect smaller changes in temperature. This makes the measurement more precise.

  2. 2

    Reason 2: It is easier to read a value from a digital display, which reduces the chance of human error (like parallax error) associated with reading a scale.

Recap

  • Choose apparatus with a resolution appropriate for the measurement you are making.
  • For measuring variable volumes, use a burette; for fixed, accurate volumes, use a pipette.
  • Use a measuring cylinder for approximate volumes, not for accurate work.
  • Use a digital balance for measuring mass accurately.
  • Avoid parallax error by reading scales at eye level.
  • A water bath is used to control the temperature of a reaction mixture accurately.

Quick check

  1. Which is more precise for measuring volume: a 50 cm³ burette or a 50 cm³ beaker?1 mark

6. Evaluating and Improving Experiments

No experiment is perfect. A key scientific skill is to evaluate your own method and results. This involves identifying limitations and sources of error. An anomalous result is one that doesn't fit the pattern – you should identify it and, if possible, explain its cause (e.g., a misreading or a change in conditions). Suggesting improvements is vital. These must be specific and practical. Instead of 'be more careful', suggest 'use a burette instead of a measuring cylinder to measure the volume of acid more accurately'. Good evaluations explain how the weakness affected the results and how the improvement would increase accuracy or reliability.

Key term

Anomalous Result: A result that does not fit the pattern of the other results in a set of data and should be ignored when calculating a mean.

Examiner insight

High-level responses not only identify a weakness but also explain its effect on the results and why the suggested improvement is better. For example, 'The reaction is very fast, so there is a large percentage error in timing with a stopwatch. Using a data logger with a gas pressure sensor would record the initial rate more accurately.'

Common pitfall

Suggesting vague improvements like 'be more careful' or 'use better equipment'. Improvements must be specific, such as 'use a digital colorimeter to judge the endpoint instead of relying on the human eye'.

Worked example 12 marks

In an experiment measuring heat released, a student records the temperature change in a polystyrene cup. A major source of error is heat loss to the surroundings. Suggest two improvements to the method that would reduce this heat loss.

  1. 1

    Improvement 1: Place a lid on the polystyrene cup. This will reduce heat loss by convection and evaporation from the surface of the liquid.

  2. 2

    Improvement 2: Place the polystyrene cup inside a larger beaker and fill the gap with cotton wool. This will provide insulation and reduce heat loss through the sides of the cup by conduction and convection.

Worked example 23 marks

A student's results for an experiment are plotted on a graph. Most points form a smooth curve, but one point is far from the curve. What is the scientific term for this point, and what are two possible reasons for it?

  1. 1

    Term: This point is an anomalous result (or an anomaly).

  2. 2

    Reason 1: A measurement was read incorrectly. For example, the student may have misread the scale on a thermometer or stop clock.

  3. 3

    Reason 2: The experimental conditions may have changed for that one reading. For example, the temperature may have dropped, or a different amount of catalyst was accidentally added.

Recap

  • Evaluate your method by identifying specific limitations or sources of error.
  • Anomalous results are data points that do not fit the overall trend.
  • Suggest concrete improvements, not vague ideas like 'work more carefully'.
  • Explain how a weakness affects the results (e.g., 'heat loss causes the measured temperature to be lower').
  • Explain how your suggested improvement will lead to more accurate or reliable data.
  • Reliability is improved by repeating readings and checking for consistency.

Quick check

  1. What is the difference between accuracy and precision?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. A student investigates the effect of surface area on the rate of a reaction. They react a single large marble chip with acid and then the same mass of powdered marble with the same acid. Identify the independent, dependent, and two control variables for this investigation.4 marks
  2. You are asked to plan an investigation into how the temperature of a water bath affects the activity of the enzyme amylase, which breaks down starch. Amylase activity can be measured by timing how long it takes for a starch solution to no longer give a blue-black colour with iodine. Describe the method you would use.6 marks
  3. In an experiment to measure the energy released when a fuel burns, a student heats water in a metal can. Identify two significant sources of error in this experiment and suggest a practical improvement for each.4 marks
  4. A student obtains the following results for the volume of gas produced in a reaction: Time/s: 0, 20, 40, 60, 80, 100. Volume/cm³: 0, 34, 58, 72, 80, 80. Draw a suitable results table and plot a graph of Volume (y-axis) against Time (x-axis).5 marks
  5. When handling concentrated sulfuric acid, state one hazard, the associated risk, and two essential safety precautions.4 marks
  6. Explain why a 50 cm³ burette is a better choice than a 50 cm³ measuring cylinder for a titration, referring to the precision of the apparatus.3 marks
  7. A student repeats a measurement three times and gets the values 25.4 s, 25.6 s, and 29.2 s. Explain how they should process these results.3 marks
  8. Describe how you would make sure an investigation into the effect of temperature on rate of reaction was a 'fair test'.3 marks
  9. A student is investigating osmosis in potato cylinders. They need to measure the length of the cylinders before and after soaking them in sugar solutions. Suggest a piece of apparatus to measure the length and describe how to use it to get an accurate reading.3 marks
  10. Critique the following step from a student's method: 'Add some magnesium to the acid and see how fast it bubbles.' Suggest two ways to improve this part of the method to make it scientific and quantitative.4 marks

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