Cambridge Lower Secondary CheckpointStage 9

Thinking and Working Scientifically: Carrying out scientific enquiry

Science Stage 9 Chapter Notes

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

  1. 1Turning an Idea into a Hypothesis
  2. 2Planning a Fair Test: Variables
  3. 3Collecting and Recording Data
  4. 4Presenting Data in Graphs
  5. 5Drawing Conclusions and Evaluating
  6. 6Reliability, Accuracy and Precision

1. Turning an Idea into a Hypothesis

A scientific enquiry starts with an idea, but to test it, we need a hypothesis. A hypothesis is not just a guess; it's a clear, testable statement that predicts the relationship between two variables. It usually proposes a cause-and-effect relationship. A good way to structure a hypothesis is using an 'If... then...' format. For example, the idea 'Carbon dioxide might make plants photosynthesise faster' can be turned into the hypothesis: 'If the concentration of carbon dioxide is increased, then the rate of photosynthesis in a water plant will increase.' This statement is precise and can be tested through an experiment.

Key term

Hypothesis: A clear, precise, and testable statement that predicts the outcome of an investigation by linking an independent variable to a dependent variable.

Examiner insight

Examiners look for hypotheses that are specific and can be clearly tested. Vague statements like 'colour affects survival' are less likely to score full marks than a more detailed prediction.

Common pitfall

Stating an aim instead of a hypothesis. 'To investigate camouflage' is an aim, whereas 'If an animal is camouflaged, then it is less likely to be eaten' is a hypothesis.

Worked example 12 marks

An idea suggests that in a grassy area, green caterpillars are less likely to be eaten by birds than yellow caterpillars because they are better camouflaged. Turn this idea into a testable hypothesis.

  1. 1

    Step 1: Identify the proposed cause (the independent variable). This is the colour of the caterpillar (green or yellow).

  2. 2

    Step 2: Identify the proposed effect (the dependent variable). This is how likely the caterpillar is to be eaten by a predator.

  3. 3

    Step 3: Formulate a clear, testable 'If... then...' statement. A suitable hypothesis would be: 'If green and yellow caterpillars are placed in a grassy area, then fewer green caterpillars will be eaten by predators than yellow caterpillars.'

Recap

  • A hypothesis is a specific, testable prediction.
  • It links a cause (independent variable) with an effect (dependent variable).
  • The 'If... then...' structure is a good way to write a hypothesis.
  • A hypothesis must be a statement, not a question or an aim.
  • It turns a general idea into something that can be scientifically investigated.

Quick check

  1. What are the two key components that a scientific hypothesis should link together?1 mark

2. Planning a Fair Test: Variables

To test a hypothesis, you must plan a 'fair test'. This means you only change one thing at a time to see what effect it has. In science, these 'things' are called variables. There are three types:

  1. Independent Variable (IV): The one factor that you choose to change on purpose.
  2. Dependent Variable (DV): The factor that you measure to see the effect of changing the IV.
  3. Control Variables (CVs): All other factors that could possibly affect the outcome, which you must keep the same to ensure your test is fair. For example, to test if temperature affects how fast sugar dissolves, the IV is temperature, the DV is the time to dissolve, and CVs would include the volume of water and the mass of the sugar.

Key term

Control Variable: A factor that is kept constant throughout an experiment to ensure that only the independent variable is affecting the dependent variable.

Examiner insight

Marks are awarded for identifying specific and relevant control variables. Simply writing 'same equipment' is too vague; 'same volume of water measured with a 100 cm³ measuring cylinder' is much better.

Common pitfall

Confusing the independent and dependent variables. A good way to remember is that the dependent variable 'depends' on the independent variable.

Worked example 14 marks

A student wants to investigate if the surface area of a tablet affects the time it takes to dissolve in water. Identify the independent, dependent, and two important control variables.

  1. 1

    Independent Variable: The factor the student changes is the surface area of the tablet (e.g., using a whole tablet vs a crushed tablet).

  2. 2

    Dependent Variable: The factor the student measures is the time taken for the tablet to dissolve completely.

  3. 3

    Control Variable 1: The volume of water used must be the same for each test.

  4. 4

    Control Variable 2: The temperature of the water must be kept constant for each test.

Recap

  • The independent variable is the one you intentionally change.
  • The dependent variable is the one you measure to see the result.
  • Control variables are all other factors that must be kept the same for a fair test.
  • In a fair test, only the independent variable is changed.
  • Identifying relevant control variables is key to a valid experiment.

Quick check

  1. In an experiment testing how different types of soil affect plant height, what is the independent variable?1 mark

3. Collecting and Recording Data

Once you have a plan, you carry out the experiment and collect data. This involves making accurate measurements or careful observations. All raw data should be recorded neatly in a results table. A good table has a border drawn with a ruler, clear column headings, and units in the headings only (not next to every number). The independent variable goes in the first column. To make results more reliable, you should repeat each measurement at least three times and calculate an average (mean). This also helps you spot any anomalous results – readings that don't fit the pattern and should be ignored when calculating the mean.

Average = (Sum of repeat readings) / (Number of readings)

Key term

Anomaly: A result that does not fit the pattern of the other results and should be considered for exclusion when calculating an average.

Examiner insight

A clearly drawn and correctly labelled table is an easy way to secure marks. Examiners check for correct placement of variables and units.

Common pitfall

Calculating the mean using an anomalous result. This will make your average value inaccurate.

Worked example 13 marks

A student measures the height a ball bounces when dropped from different heights. For a drop height of 100 cm, they record bounce heights of 82 cm, 84 cm, and 61 cm. Identify the anomaly and calculate the mean bounce height.

  1. 1

    Step 1: Look at the three repeat readings: 82 cm, 84 cm, and 61 cm. The values 82 cm and 84 cm are close together, while 61 cm is much lower. Therefore, 61 cm is the anomalous result.

  2. 2

    Step 2: To calculate the mean, ignore the anomaly. Add the reliable results together: 82 + 84 = 166 cm.

  3. 3

    Step 3: Divide by the number of reliable results (which is 2): 166 / 2 = 83 cm.

  4. 4

    Step 4: State the final answer clearly. The mean bounce height is 83 cm.

Recap

  • Draw results tables with a ruler, with the independent variable in the first column.
  • Units should only be in the column headings.
  • Repeat measurements to improve reliability and allow you to calculate a mean.
  • Identify and circle any anomalous results.
  • Do not include anomalies in the calculation of the mean.

Quick check

  1. Where should the units be placed in a results table?1 mark

4. Presenting Data in Graphs

A graph is a powerful way to visualise the pattern in your results. The type of graph you choose depends on your data. Use a bar chart when your independent variable is categoric (in distinct groups, like 'types of metal' or 'caterpillar colour'). Use a line graph when your independent variable is continuous (can have any numerical value, like temperature or time). A good graph must have:

  • 'S'cale: Sensible, linear, and uses over half the grid.
  • 'P'lotting: Points marked accurately with a small 'x' or dot in a circle.
  • 'L'ine: A single, smooth line of best fit (straight or curved) that shows the trend, not a 'dot-to-dot' connection.
  • 'A'xes: Correct variables on the correct axes (IV on x-axis, DV on y-axis), labelled with quantity and units.

Key term

Line of Best Fit: A line drawn on a graph to show the general trend of the data, with roughly an equal number of points above and below the line.

Examiner insight

Examiners often use the acronym 'SPLA' (Scale, Plotting, Line, Axes) to award marks for graphs. Ensure you have all four elements correct to maximise your score.

Common pitfall

Choosing a scale that is difficult to read (e.g., going up in 3s or 7s) or that squashes the data into a small corner of the grid.

Worked example 12 marks

A student recorded the number of bubbles produced by a water plant per minute at different light intensities. Should they use a bar chart or a line graph to present their results? Explain your choice.

  1. 1

    Step 1: Identify the type of independent variable. 'Light intensity' is a continuous variable because it can have any value along a scale (e.g., 10 units, 10.5 units, 11 units).

  2. 2

    Step 2: State the correct graph type for this variable. A line graph is used to show the relationship between two continuous variables.

  3. 3

    Step 3: Justify the choice. Therefore, a line graph should be used because the independent variable (light intensity) is continuous.

Recap

  • Use a line graph for continuous independent variables (e.g., temperature, time, concentration).
  • Use a bar chart for categoric independent variables (e.g., species, colours, materials).
  • Label both axes with the quantity and its units.
  • Your scale should use at least half of the available graph paper.
  • For line graphs, draw a single, smooth line or curve of best fit; do not join the dots.
  • For bar charts, bars should be of equal width and have gaps between them.

Quick check

  1. On which axis should you plot the dependent variable?1 mark
  2. What is the key rule for drawing the line on a line graph?1 mark

5. Drawing Conclusions and Evaluating

The conclusion is where you answer the original question. It has two parts: first, describe the pattern or relationship shown in your results (e.g., 'As temperature increases, the reaction time decreases'). Second, you must support this statement by quoting data from your table or graph (e.g., 'At 20°C the time was 52s, but at 40°C the time was only 25s'). Evaluation involves critically looking back at your method. You should identify specific weaknesses or sources of error (e.g., 'It was difficult to judge the exact moment the reaction stopped') and suggest practical, specific improvements (e.g., 'Use a colorimeter to measure the colour change more objectively').

Key term

Conclusion: A summary of what the results of an investigation show, explaining the relationship between the variables and supported by specific data.

Examiner insight

To get full marks for a conclusion, you must quote data. For evaluation, marks are given for identifying a weakness AND suggesting a corresponding, workable improvement.

Common pitfall

Suggesting unrealistic or vague improvements. 'Use better equipment' is not a good suggestion; 'Use a digital thermometer with a precision of 0.1°C instead of a glass one' is excellent.

Worked example 12 marks

The results of an experiment show that as the concentration of an acid increases, the time taken for magnesium to react decreases. At 1 mol/dm³, the time was 90s. At 2 mol/dm³, the time was 45s. Write a conclusion based on these results.

  1. 1

    Step 1: State the relationship between the variables. 'As the concentration of the acid increases, the time taken for the reaction decreases, meaning the rate of reaction increases.'

  2. 2

    Step 2: Support the statement with data. 'For example, when the concentration was doubled from 1 mol/dm³ to 2 mol/dm³, the reaction time halved from 90s to 45s.'

Worked example 21 mark

In an experiment measuring reaction time with a stopwatch, a student identifies that human reaction time in starting and stopping the watch is a source of error. Suggest an improvement.

  1. 1

    Step 1: Identify the core problem. The error is caused by manual timing.

  2. 2

    Step 2: Suggest a specific, technological solution. 'The experiment could be improved by using a light gate connected to a data logger to start and stop the timer automatically when the reaction begins (e.g., gas is produced) and ends (e.g., a colour change is detected). This would remove the error from human reaction time.'

Recap

  • A conclusion must state the pattern and be supported with data.
  • Use figures from your table or graph to justify your conclusion.
  • Evaluation means identifying specific limitations in your method.
  • Suggesting improvements requires practical and specific ideas, not vague ones like 'be more careful'.
  • Consider errors in measurement, control of variables, and the range of data collected.

Quick check

  1. What two things must a good scientific conclusion contain?2 marks

6. Reliability, Accuracy and Precision

These three terms describe the quality of your data, and they mean different things.

  • Reliability: This is about consistency. If you repeat the experiment, will you get similar results? Reliability is improved by taking at least three repeat readings for each condition and calculating a mean.
  • Accuracy: This is about how close your measured value is to the true, accepted value. Accuracy is improved by using correctly calibrated instruments and minimising errors (e.g., reading a burette at eye level to avoid parallax error).
  • Precision: This refers to how close your repeat measurements are to each other. It is also related to the resolution of your measuring instrument. A reading of 25.12 s is more precise than 25 s. Using a ruler marked in millimetres allows for more precise measurements than one marked only in centimetres.

Key term

Accuracy: How close a measured value is to the true or accepted value of the quantity being measured.

Fun fact

The concepts of accuracy and precision are vital in fields like GPS. A GPS that is precise but not accurate might tell you your location to within a few centimetres, but that location could be 50 metres away from where you actually are! Both are needed for it to work properly.

Worked example 13 marks

A student measures the boiling point of pure water three times and gets readings of 96°C, 96.5°C, and 96.2°C. The true boiling point at that pressure is 100°C. Describe these results in terms of precision and accuracy.

  1. 1

    Step 1: Assess precision. The readings (96°C, 96.5°C, 96.2°C) are all very close to each other. This means the results are precise.

  2. 2

    Step 2: Assess accuracy. The readings are all significantly different from the true value of 100°C. This means the results are not accurate.

  3. 3

    Step 3: Summarise the findings. The student's measurements were precise but not accurate. This might suggest a systematic error, such as a faulty thermometer.

Recap

  • Reliability means your results are repeatable and consistent.
  • Accuracy means your results are close to the true value.
  • Precision means your repeat results are close to each other.
  • Improve reliability by repeating and averaging.
  • Improve accuracy by using calibrated equipment and good technique.
  • An instrument with a finer scale gives more precise readings.

Quick check

  1. How can you make the results of an experiment more reliable?1 mark
  2. A clock that is 10 minutes fast is used for timing. Are the timings accurate? Are they precise?2 marks

End-of-chapter exercise

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

  1. Define 'dependent variable' and give an example from an experiment investigating how light intensity affects the rate of photosynthesis.2 marks
  2. A student records the time for a chemical reaction as 25.6 s, 25.9 s, and 31.2 s. Identify the anomalous result and calculate the mean time for the reaction.2 marks
  3. A researcher is investigating the idea that 'larger seeds germinate faster'. Turn this idea into a testable hypothesis. Then, list the independent variable, the dependent variable, and two control variables for an experiment to test it.4 marks
  4. Describe the difference between a categoric variable and a continuous variable, and state which type of graph should be used to display data for each.3 marks
  5. A student concludes from an experiment: 'The results show that the enzyme works better at higher temperatures.' Criticise this conclusion and explain how it could be improved to make it valid.3 marks
  6. The table shows the time taken for a 1g sample of calcium carbonate to react with acid at different temperatures. Plot a suitable graph of this data. Temperature (°C): 10, 20, 30, 40, 50. Time (s): 150, 80, 45, 25, 15.5 marks
  7. Plan an experiment to investigate the effect of the concentration of a salt solution on the change in mass of a potato chip. Your plan should include a list of apparatus, a step-by-step method, and details of the key measurements you will take.6 marks
  8. Explain the difference between reliability and accuracy in a scientific investigation. For each term, describe a specific action a scientist could take to improve it.4 marks
  9. A student investigates how the length of a pendulum affects the time it takes to complete one swing (the period). Why is it better to measure the time for 20 swings and divide by 20, rather than just measuring the time for one swing?2 marks
  10. In an experiment modelling camouflage, a student scattered 50 green and 50 red pasta pieces on a green lawn. A friend then had 30 seconds to collect as many as they could. Describe two limitations of this method and suggest a specific improvement for each.4 marks

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