Cambridge Lower Secondary CheckpointStage 6

Thinking and Working Scientifically: Scientific enquiry (6TWSp)

Science Stage 6 Chapter Notes

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Thinking and Working Scientifically: Scientific enquiry — purpose and planning
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In this chapter

  1. 1From Idea to Testable Question
  2. 2Identifying and Controlling Variables
  3. 3Designing a Reliable Method
  4. 4Recording and Analysing Results
  5. 5Drawing Conclusions and Evaluation

1. From Idea to Testable Question

Scientific enquiry begins with an observation or an idea. To investigate it scientifically, you must turn this idea into a specific, testable question. A good scientific question is not a matter of opinion and can be answered by collecting evidence through an experiment. It usually explores the relationship between two factors, asking how changing one thing affects another.

Key term

Testable Question: A question that can be answered by designing and conducting an experiment to collect data.

Examiner insight

Examiners look for questions that clearly state both the factor being changed and the factor being measured.

Common pitfall

Formulating a question that is too broad or impossible to answer in a lab, such as 'How did life on Earth begin?'. A good question is focused and practical.

Worked example 14 marks

A student observes that their potted plant on the windowsill grows towards the light. Their initial idea is 'Plants need light to grow'. How can they refine this into a testable question for a school laboratory experiment?

  1. 1

    Step 1: Identify the cause and effect in the idea. The 'cause' is light, and the 'effect' is plant growth.

  2. 2

    Step 2: Think about what specific aspect of the 'cause' can be changed. Instead of just 'light', we can change the 'direction of light' or the 'duration of light'.

  3. 3

    Step 3: Think about what specific aspect of the 'effect' can be measured. Instead of just 'growth', we can measure 'change in height', 'number of leaves', or 'mass of the plant'.

  4. 4

    Step 4: Combine these specific factors into a question. A good testable question would be: 'How does the direction of the light source affect the direction of stem growth in cress seedlings?' or 'How does the number of hours of light per day affect the final height of a bean plant after 3 weeks?'

Recap

  • A scientific investigation starts with a testable question.
  • A testable question links a cause (something you change) with an effect (something you measure).
  • It must be possible to answer the question by collecting data.
  • Avoid questions based on opinion, like 'Are roses prettier than tulips?'.

Quick check

  1. Turn the idea 'Fizzy drinks are bad for your teeth' into a testable question.2 marks

2. Identifying and Controlling Variables

To conduct a fair test, you must understand variables. A variable is any factor that can change or be changed in an investigation. There are three types: the Independent Variable (IV) is the one factor you deliberately change to see what effect it has. The Dependent Variable (DV) is the factor you measure to see if it has been affected by the change in the IV. Control Variables (CVs) are all the other factors that could possibly affect the outcome, which you must keep constant to ensure your results are only due to the change in the IV.

Key term

Fair Test: An investigation where only the independent variable is changed, and all other variables that could affect the dependent variable are controlled (kept the same).

Examiner insight

Examiners award marks for identifying specific, relevant control variables. Simply stating 'control all other variables' is not enough.

Common pitfall

Confusing the dependent variable (the effect) with the independent variable (the cause). Remember, the dependent variable 'depends' on the independent variable.

Worked example 14 marks

A student investigates the question: 'How does the temperature of water affect the time it takes for a sugar cube to dissolve?'. Identify the independent, dependent, and two key control variables.

  1. 1

    Step 1: Identify the factor being deliberately changed by the investigator. This is the temperature of the water. Therefore, the Independent Variable is the temperature of the water.

  2. 2

    Step 2: Identify the factor being measured to see the result of the change. This is how long the sugar takes to dissolve. Therefore, the Dependent Variable is the time taken for the sugar cube to dissolve.

  3. 3

    Step 3: Identify other factors that could also affect the time taken to dissolve and must be kept the same for a fair test. Examples include: the volume of water, the mass/size of the sugar cube, and whether the water is stirred or not. These are the Control Variables.

Recap

  • The Independent Variable (IV) is what you change.
  • The Dependent Variable (DV) is what you measure.
  • Control Variables are everything else you keep the same.
  • Controlling variables is essential for a fair test.

Quick check

  1. For an experiment investigating how light intensity affects the rate of photosynthesis, state the independent and dependent variables.2 marks

3. Designing a Reliable Method

A good method is a clear, step-by-step set of instructions that another scientist could follow to repeat your experiment exactly. It must include a list of all apparatus, how you will change the independent variable, how you will measure the dependent variable, and how you will control other key variables. To ensure your results are reliable (repeatable), you should plan to repeat your measurements at least three times for each value of the independent variable and then calculate a mean.

Key term

Reliability: The extent to which an experiment's results can be reproduced when the investigation is repeated, indicating that the results are not just due to chance.

Examiner insight

Marks are awarded for specifying quantities and concentrations (e.g., '50 cm³ of 1.0 mol/dm³ acid' is better than 'some acid'). Justifying the choice of apparatus also gains credit.

Fun fact

The standard for the kilogram was, until 2019, a physical cylinder of platinum-iridium kept in a vault in France. Repeating measurements against this 'perfect' kilogram was key to global trade and science.

Worked example 16 marks

Outline the key steps in a method to investigate the effect of surface area on the rate of reaction between marble chips (calcium carbonate) and hydrochloric acid. The rate can be measured by the volume of gas produced.

  1. 1

    Step 1: Set up the apparatus. Place a conical flask on a balance. Add a set volume and concentration of hydrochloric acid (e.g., 50 cm³ of 1 M HCl).

  2. 2

    Step 2: Change the IV. Use the same mass (e.g., 5g) of marble chips for each test, but vary the surface area: large chips, small chips, and powder.

  3. 3

    Step 3: Measure the DV. For one type of chip (e.g., large), add them to the acid, place cotton wool in the flask's neck (to stop acid spray but let gas escape), and immediately start a stopwatch. Record the mass on the balance every 30 seconds for 5 minutes.

  4. 4

    Step 4: Control variables. Key variables to control are the mass of marble chips, the volume and concentration of the acid, and the temperature of the reaction.

  5. 5

    Step 5: Ensure reliability. Repeat the entire experiment (steps 1-3) two more times for each surface area (large, small, powder) and calculate a mean volume of gas produced at each time point.

Recap

  • A method should be a logical sequence of steps.
  • List all necessary apparatus with appropriate precision (e.g., '50 cm³ measuring cylinder').
  • State the range and intervals for your independent variable.
  • Always include repeats to check reliability and calculate a mean.
  • Clearly state how control variables will be kept constant.

Quick check

  1. Why is it important to repeat measurements in an investigation?1 mark
  2. What piece of apparatus would you use to measure 25.0 cm³ of a solution accurately?1 mark

4. Recording and Analysing Results

Data must be recorded systematically in a table. The table should have clear headings for each column, with the independent variable in the first column. Crucially, units should be in the column headings, not next to every number in the table. After collecting data, especially with repeats, you should calculate the mean (average). When doing this, you must first identify and exclude any anomalous results – readings that don't fit the pattern of the others. The processed data is then usually plotted on a graph to visualise the relationship between the variables. A graph must have: labelled axes with units, a sensible scale, accurately plotted points, and a line (or curve) of best fit.

Mean = Sum of readings / Number of readings

Key term

Anomalous Result: A result that deviates significantly from the others in a set of repeated measurements and is likely due to an error.

Examiner insight

Examiners award separate marks for each component of a good graph: suitable scale, correct labels and units for both axes, all points plotted accurately (to within half a small square), and a well-judged line of best fit.

Common pitfall

Drawing a 'dot-to-dot' line on a graph instead of a smooth line or curve of best fit that shows the overall trend.

Worked example 13 marks

A student investigates how temperature affects enzyme activity. They record the time taken for the enzyme to break down a substrate. Their results for 20°C are: Trial 1: 110 s, Trial 2: 112 s, Trial 3: 150 s, Trial 4: 108 s. How should they process this data?

  1. 1

    Step 1: Present the raw data clearly. The four readings are 110, 112, 150, and 108 seconds.

  2. 2

    Step 2: Identify any anomalous results. The readings 110, 112, and 108 are all close together. The reading of 150 s is much higher and does not fit the pattern. This is an anomalous result.

  3. 3

    Step 3: Exclude the anomaly from the calculation of the mean. The student should state that 150 s is an anomaly and will be ignored.

  4. 4

    Step 4: Calculate the mean from the remaining, reliable results. Mean = (110 + 112 + 108) / 3.

  5. 5

    Step 5: Complete the calculation. Mean = 330 / 3 = 110 s. This is the value that should be recorded in the final results table and used for plotting the graph.

Recap

  • Record raw data in a table with units in the headings.
  • Identify and circle or highlight anomalous results.
  • Calculate the mean from the repeated, non-anomalous results.
  • Graphs must have labelled axes, units, a good scale, and a line of best fit.
  • Do not join the dots on a graph; draw a smooth line or curve of best fit.

Quick check

  1. A table column is headed 'Time / s'. A student writes '30 s' in a cell in that column. What is their mistake?1 mark

5. Drawing Conclusions and Evaluation

A conclusion is a short summary that answers the original question. It must be based only on the evidence from your results. You should state the relationship between the independent and dependent variables, quoting data from your graph or table to support your statement. For example, 'As temperature increased from 20°C to 40°C, the rate of reaction increased from X to Y'. Evaluation involves critically reflecting on your investigation. You should identify specific limitations or sources of error in your method (e.g., 'It was difficult to judge the endpoint by eye') and suggest specific, practical improvements (e.g., 'Use a colorimeter to measure the colour change more objectively').

Key term

Validity: The extent to which an investigation is a fair test and measures what it intends to measure, ensuring the conclusion is sound.

Examiner insight

Top marks for conclusions are given to students who not only state the trend but also use the words 'independent variable' and 'dependent variable' correctly and manipulate data (e.g., calculate a gradient or a percentage change).

Common pitfall

In evaluation, suggesting vague improvements like 'be more careful' or 'use better equipment'. Improvements must be specific, such as 'use a digital thermometer for more precise temperature readings' instead of 'measure temperature better'.

Worked example 15 marks

A student's graph shows that as the concentration of an acid increases, the time taken for a reaction decreases.(a) Write a conclusion for this investigation.(b) The student measured the reaction time by watching for a magnesium ribbon to disappear. Identify one limitation and suggest an improvement.

  1. 1

    Part(a) Conclusion: Start by stating the relationship. 'As the concentration of the acid increases, the rate of reaction increases.' Then, support this with data. 'This is because the time taken for the reaction to finish decreases. For example, at a concentration of 0.5 mol/dm³, the time taken was 120 s, but at 2.0 mol/dm³, the time taken was only 30 s.'

  2. 2

    Part(b) Evaluation: Identify a specific limitation. 'A limitation is that judging the exact moment the magnesium ribbon completely disappears is subjective and can vary between observers, leading to inaccurate time measurements.'

  3. 3

    Part(b) Improvement: Suggest a specific and practical improvement. 'To improve this, the experiment could be repeated by measuring the volume of hydrogen gas produced over time using a gas syringe. The initial rate of reaction could then be calculated from the gradient of the graph of volume against time, which is a more accurate and objective measurement.'

Recap

  • Your conclusion must be supported by your data.
  • Quote data to back up the trend you describe.
  • State the relationship between the independent and dependent variables.
  • Evaluation is about identifying specific weaknesses in your method.
  • Suggested improvements must be practical and explain how they increase accuracy or reliability.

Quick check

  1. What is the difference between a conclusion and simply describing your results?2 marks

End-of-chapter exercise

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

  1. A student states: 'The bigger the parachute, the slower a toy soldier falls.' Rewrite this as a testable question and identify the independent, dependent, and one control variable.4 marks
  2. A student records the following times for a reaction: 45 s, 48 s, 61 s, 46 s. Calculate the mean time that should be used for analysis. Show your working.3 marks
  3. Describe two key features of a good results table, before any data is entered.2 marks
  4. Explain why repeating an experiment and calculating a mean improves the reliability of the results.3 marks
  5. A student is investigating how the length of a wire affects its resistance. They are using a voltmeter and an ammeter. Describe two safety precautions they should take.2 marks
  6. A plan for an experiment says: 'Add some acid to some alkali and see if the temperature changes.' Criticise this plan and suggest two specific improvements to make it a valid scientific method.4 marks
  7. Outline a method to investigate how the concentration of a salt solution affects the mass of a potato chip after 24 hours.6 marks
  8. A graph shows a straight line passing through the origin, with 'Force (N)' on the y-axis and 'Extension (m)' on the x-axis. Write a conclusion that describes the relationship shown in the graph.3 marks
  9. An investigation measured plant growth in different colours of light. The student forgot to control the temperature, and the lamp for red light was much hotter than the others. Explain how this affects the validity of the conclusion about which colour of light is best for growth.4 marks
  10. Explain the difference between accuracy and precision using the analogy of shooting arrows at a target.4 marks

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