Cambridge Lower Secondary CheckpointStage 6

Thinking and Working Scientifically: Scientific enquiry (6TWSa)

Science Stage 6 Chapter Notes

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Thinking and Working Scientifically: Scientific enquiry — analysis, evaluation and conclusions
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Thinking and Working Scientifically: Scientific enquiry (6TWSa) notes

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In this chapter

  1. 1Asking Testable Scientific Questions
  2. 2Variables: The Keys to a Fair Test
  3. 3Planning a Reliable Method
  4. 4Collecting and Presenting Data
  5. 5Analysing Evidence and Drawing Conclusions
  6. 6Evaluating Your Investigation

1. Asking Testable Scientific Questions

Scientific enquiry begins with an observation or a moment of curiosity, which is then shaped into a specific, testable question. A good scientific question can be answered by carrying out an investigation. For example, observing that a plant in a sunny window grows taller than one in a dark corner might lead to the question: 'Does the amount of light affect how tall a plant grows?'. This question can be investigated. Once you have a question, you can form a hypothesis, which is a clever, scientific prediction about what you think the answer will be.

Key term

Hypothesis: A clear statement, based on scientific reasoning, that predicts the outcome of an investigation.

Examiner insight

Examiners reward hypotheses that clearly state the relationship between the two key factors (variables) being investigated, not just a vague guess.

Fun fact

The famous 'eureka' moment of Archimedes, where he figured out how to measure the volume of an irregular object by observing the water level rise in his bath, is a classic example of observation leading to a scientific principle.

Worked example 12 marks

Amna observes that stirring her tea seems to make the sugar dissolve faster. Formulate a testable question and a simple hypothesis for an investigation based on her observation.

  1. 1

    Step 1: Turn the observation into a question. The observation is about stirring and dissolving speed. A good question is: 'Does the amount of stirring affect the time it takes for sugar to dissolve?'

  2. 2

    Step 2: Formulate a hypothesis. A hypothesis is a prediction. A sensible prediction would be: 'The more the sugar is stirred, the faster it will dissolve.' or 'Increased stirring will decrease the time taken for sugar to dissolve.'

Recap

  • Scientific enquiry starts with observations and curiosity.
  • A testable question is one that can be answered through an experiment.
  • A hypothesis is a scientific prediction of the outcome.
  • Good questions are specific and clear.
  • Your hypothesis should be a statement, not a question.

Quick check

  1. An observation is made that ice melts faster on a metal tray than on a plastic tray. What is a suitable testable question?1 mark

2. Variables: The Keys to a Fair Test

To answer a scientific question fairly, you must conduct a 'fair test'. This means you only change one key thing and see what happens. These 'things' are called variables. There are three types: the Independent Variable (IV) is the one thing you deliberately change; the Dependent Variable (DV) is the thing you measure to see the effect of your change; and Control Variables (CVs) are all the other factors you must keep exactly the same to ensure your test is fair.

Key term

Control Variable: A factor in an investigation that is kept constant to ensure that the test is fair and the results are valid.

Common pitfall

Students often just write 'amount' for a control variable. You must be specific, for example, 'volume of water' or 'mass of sugar', not just 'amount of water' or 'amount of sugar'.

Worked example 14 marks

A student wants to investigate if the temperature of water affects how quickly a sugar cube dissolves. Identify the independent, dependent, and two control variables for this experiment.

  1. 1

    Step 1: Identify the independent variable (IV). This is what the student is deliberately changing. In this case, it is the 'temperature of the water'.

  2. 2

    Step 2: Identify the dependent variable (DV). This is what the student will measure to see the effect. Here, it is the 'time taken for the sugar cube to dissolve'.

  3. 3

    Step 3: Identify the control variables (CVs). These are factors that must be kept the same. Examples include: 'the volume of water', 'the mass/size of the sugar cube', 'the amount of stirring (or not stirring at all)', and 'the size/shape of the container'.

Recap

  • The independent variable is the one you change.
  • The dependent variable is the one you measure.
  • Control variables are everything you keep the same.
  • Changing only the independent variable makes a test fair.
  • Identifying all three types of variables is a key skill in planning.
  • A fair test allows you to be confident that the IV caused the change in the DV.

Quick check

  1. To investigate if the length of a wire affects the brightness of a bulb in a circuit, what is the dependent variable?1 mark
  2. List two control variables for the wire length investigation.2 marks

3. Planning a Reliable Method

A method is a step-by-step guide for your experiment. It should be so clear that another scientist could follow it exactly and get similar results. This is called making the experiment reliable. A good plan includes a list of all equipment (apparatus), a description of how you will change the independent variable and measure the dependent variable, and details on how you will manage the control variables. Repeating your measurements and calculating an average is a crucial part of a reliable method.

Key term

Reliability: The extent to which an investigation will produce similar results if repeated under the same conditions by the same or different investigators.

Examiner insight

Examiners award marks for methods that are logical and detailed enough to be practically carried out. Stating specific volumes, masses, or a range of values for your variables will gain you more credit than vague descriptions.

Worked example 16 marks

You are planning to investigate the effect of surface area on the rate of a reaction, using marble chips (calcium carbonate) and hydrochloric acid. Outline the key steps for a reliable method.

  1. 1
    1. Equipment: Select appropriate apparatus, such as a conical flask, gas syringe (or measuring cylinder in water), stopwatch, balance, and different sized marble chips (e.g., large chips, small chips, powder).
  2. 2
    1. Set up: Place a measured mass of large marble chips (e.g., 5g) in the conical flask.
  3. 3
    1. Add reactant: Add a measured volume of hydrochloric acid (e.g., 50 cm³) and immediately start the stopwatch and connect the gas syringe.
  4. 4
    1. Measure DV: Record the volume of gas produced every 30 seconds for 5 minutes.
  5. 5
    1. Control variables: Ensure the concentration of the acid, the temperature of the acid, and the total mass of marble chips are kept the same for each test.
  6. 6
    1. Repeat: Repeat the entire experiment using the same mass (5g) of small marble chips, and then again with 5g of marble powder.
  7. 7
    1. Reliability check: For each surface area, repeat the experiment three times and calculate the mean volume of gas produced at each time interval.

Recap

  • A method is a clear, step-by-step set of instructions.
  • List all the apparatus you need.
  • State the range of values you will use for your independent variable.
  • Explain how you will measure your dependent variable.
  • Describe how you will keep your control variables constant.
  • Always repeat measurements to check for reliability and spot errors.

Quick check

  1. Why is it important to repeat measurements in an experiment?1 mark

4. Collecting and Presenting Data

As you carry out your investigation, you must record your results accurately. The best way to do this is in a results table. A good table has clear column headings with the name of the variable and its units (e.g., 'Temperature (°C)' or 'Time (s)'). The independent variable usually goes in the first column, and the dependent variable goes in the subsequent columns. If you do repeats, you should have columns for each repeat and a final column for the average.

Key term

Results Table: An organised table with clear headings and units used to record experimental data systematically.

Common pitfall

Forgetting to include units in the column headings is a very common mistake that loses easy marks. The units should be in the heading, not next to every number in the table.

Worked example 13 marks

A student is investigating how the temperature of water affects the time taken for salt to dissolve. They will test temperatures of 20°C, 40°C, 60°C, and 80°C, and will repeat each measurement three times. Draw a suitable results table for this experiment, ready for the data to be filled in.

  1. 1

    Step 1: Create a table with enough rows for each temperature and enough columns for all measurements.

  2. 2

    Step 2: The first column should be for the independent variable: 'Temperature (°C)'.

  3. 3

    Step 3: The next columns are for the dependent variable (the repeats): 'Time to dissolve - Repeat 1 (s)', 'Time to dissolve - Repeat 2 (s)', and 'Time to dissolve - Repeat 3 (s)'.

  4. 4

    Step 4: Add a final column for the calculated average: 'Mean Time to dissolve (s)'.

  5. 5

    Step 5: Fill in the values for the independent variable in the first column: 20, 40, 60, 80. The table is now ready to be used.

  6. 6

    Final Table Structure:

    Temperature (°C)Time to dissolve - Repeat 1 (s)Time to dissolve - Repeat 2 (s)Time to dissolve - Repeat 3 (s)Mean Time to dissolve (s)
    20
    40
    60
    80

Recap

  • Record results immediately and accurately.
  • Use a results table to keep data organised.
  • Table columns must have headings with the quantity and the units.
  • The independent variable is usually in the first column.
  • Include columns for repeats and an average to show good scientific practice.

Quick check

  1. What two things must be included in the heading of a results table column?2 marks

5. Analysing Evidence and Drawing Conclusions

Once you have your results, you need to analyse them to find patterns. Does the dependent variable increase, decrease, or stay the same as you change the independent variable? Plotting a graph is often the best way to see a pattern. After identifying the pattern, you must write a conclusion. A conclusion is a short paragraph that answers the original scientific question. It must be based only on the evidence you have collected and should state the relationship between the independent and dependent variables.

Key term

Conclusion: A summary statement, based on experimental evidence, that explains what the results of an investigation show.

Examiner insight

Top marks for conclusions are given when students not only state the pattern but also use specific data from their results to justify their statement.

Fun fact

The discovery of penicillin came from Alexander Fleming noticing a pattern: no bacteria grew near a patch of mould on his culture plate. His conclusion, that the mould produced something that killed bacteria, changed medicine forever.

Worked example 13 marks

A student's results for the effect of temperature on dissolving time are: 20°C = 95s, 40°C = 52s, 60°C = 28s, 80°C = 15s. Describe the pattern in the results and write a conclusion.

  1. 1

    Step 1: Describe the pattern. Look at how the dependent variable (time) changes as the independent variable (temperature) increases. 'As the temperature of the water increases, the time taken for the salt to dissolve decreases.'

  2. 2

    Step 2: Write a conclusion that links back to the original question. 'My results show that higher water temperatures cause salt to dissolve faster. For example, at 80°C the salt dissolved in 15s, whereas at 20°C it took 95s. This supports the hypothesis that increasing the temperature increases the rate of dissolving.'

Recap

  • Look for patterns in your results table or graph.
  • Describe the relationship between the independent and dependent variables.
  • A conclusion must directly answer the initial question.
  • Your conclusion must be supported by your data.
  • Quote data from your results to support your conclusion.
  • Do not make claims that your evidence cannot support.

Quick check

  1. What is the primary purpose of drawing a graph from a set of results?1 mark

6. Evaluating Your Investigation

No experiment is perfect. A good scientist critically evaluates their work to identify weaknesses and suggest improvements. Evaluation involves looking for anomalies (results that don't fit the pattern), considering limitations in your method or equipment, and suggesting specific, practical improvements. For example, was it hard to judge the exact moment the solid dissolved? This is a limitation. A suggestion for improvement could be to dissolve a coloured solid in water to make the end-point clearer.

Key term

Anomaly: A result in a set of measurements that is clearly out of line with the other results and does not fit the overall pattern.

Common pitfall

A common but weak suggestion for improvement is 'use better equipment'. You must state what specific equipment would be better and why (e.g., 'use a digital thermometer instead of a glass one for a more precise temperature reading').

Worked example 13 marks

A student measures the volume of gas produced in a reaction over time. Their readings are: 0s=0cm³, 30s=12cm³, 60s=25cm³, 90s=35cm³, 120s=38cm³, 150s=49cm³. Identify any anomalous result and suggest a possible cause.

  1. 1

    Step 1: Look for the pattern. The volume of gas is increasing by about 10-13 cm³ every 30 seconds, but the reading at 120s breaks this pattern. The increase from 90s to 120s is only 3cm³, which is much smaller than the other intervals.

  2. 2

    Step 2: Identify the anomaly. The result at 120s (38cm³) appears to be anomalous. It should likely be higher, around 45-48cm³.

  3. 3

    Step 3: Suggest a cause. A possible cause could be a misreading of the gas syringe, the reaction slowing down temporarily for an unknown reason, or a brief leak in the apparatus at that moment.

Worked example 22 marks

In an experiment measuring the time it takes for a substance to dissolve, what is a common limitation and how could it be improved?

  1. 1

    Step 1: Identify a limitation. A common limitation is the difficulty in judging the exact moment when the last crystal has dissolved, which relies on human observation and reaction time.

  2. 2

    Step 2: Suggest a specific improvement. This could be improved by placing the beaker on a piece of paper with a large 'X' on it and stopping the timer only when the 'X' is clearly visible through the solution, providing a more consistent end-point.

Recap

  • Evaluation is about judging how good your investigation was.
  • Anomalous results are data points that do not fit the general trend.
  • Limitations are weaknesses in your method or equipment.
  • Always suggest specific and practical improvements.
  • Explaining why an improvement would make the results better is key.
  • Repeating the experiment helps identify anomalies and increases confidence in the conclusion.

Quick check

  1. What should you do if you identify an anomalous result in your repeats?1 mark

End-of-chapter exercise

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

  1. A student suggests the hypothesis: 'Bigger parachutes fall more slowly'. Describe how you would conduct an investigation to test this hypothesis. You should include: the equipment you would use, the variables you would need to control, and the measurements you would take.6 marks
  2. An experiment is carried out to find out how the concentration of an acid affects the rate of reaction with a metal. Identify the independent variable, the dependent variable, and two control variables.4 marks
  3. Look at the following set of results from an experiment measuring how far a toy car travels on different surfaces: Carpet = 45cm, Wood = 110cm, Sandpaper = 25cm, Bubble wrap = 48cm. What is the pattern in these results? Write a conclusion for the experiment.3 marks
  4. You are investigating the cooling of hot water in different containers. Why is it important to use the same starting temperature and the same volume of water in each container?2 marks
  5. A student records the time it takes for a chemical to change colour at different temperatures. Their results are: 10°C = 120s, 20°C = 62s, 30°C = 75s, 40°C = 20s. Identify the anomalous result and explain your choice.2 marks
  6. Design a results table to record data for an experiment investigating how the mass added to a spring affects its extension. You should plan to test masses from 100g to 500g in 100g intervals and do two repeats.4 marks
  7. What is the difference between a testable scientific question and a hypothesis?2 marks
  8. A student concludes from an experiment: 'My experiment proves that plants need light to live forever'. Criticise this conclusion based on good scientific practice.3 marks
  9. To improve the reliability of an experiment, a student suggests 'doing more repeats'. Explain why this is a good suggestion.2 marks
  10. You are investigating which material is the best electrical conductor. You have a battery, a bulb, wires, and samples of copper, plastic, wood, and steel. Describe a simple method to test the materials.5 marks

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