Cambridge Lower Secondary CheckpointStage 9

Thinking and Working Scientifically: Scientific enquiry: purpose and planning

Science Stage 9 Chapter Notes

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

  1. 1Developing a Testable Hypothesis
  2. 2Variables: The Key to a Fair Test
  3. 3Writing a Clear and Detailed Method
  4. 4Recording and Presenting Your Results
  5. 5Analysing Data and Evaluating the Method

1. Developing a Testable Hypothesis

Scientific investigations begin with an idea or an observation about the world. This leads to a question that you want to answer. A hypothesis is a proposed answer to that question. It is not just a guess; it's a clear, predictive statement that can be tested through an experiment to see if it is supported or not. A strong hypothesis often follows an 'If... then...' structure, linking a cause to an effect. For example, starting with the idea that 'sunlight helps plants', you could form the hypothesis: 'If a plant is given more light, then it will grow taller.'

Key term

Hypothesis: A clear, predictive statement that can be scientifically tested to see if it is true or false.

Examiner insight

Examiners reward hypotheses that are specific and clearly state the relationship between what is being changed and what is being measured.

Common pitfall

Writing a vague idea instead of a testable prediction. For example, 'Light affects plants' is an idea, not a hypothesis because it doesn't predict the direction or nature of the effect.

Worked example 12 marks

An idea is that the temperature of a fizzy drink affects how quickly it loses its fizz. Turn this idea into a testable hypothesis.

  1. 1

    Step 1: Identify the cause (the thing you will change). This is the temperature of the drink.

  2. 2

    Step 2: Identify the effect (the thing you will measure). This is how quickly it loses fizz, which could be measured by the time taken to stop bubbling.

  3. 3

    Step 3: Write the hypothesis as an 'If... then...' statement. For example: 'If the temperature of a fizzy drink is increased, then it will lose its fizz more quickly.'

Recap

  • Scientific enquiries start with an idea or observation.
  • An idea is developed into a scientific question.
  • A hypothesis is a specific, testable prediction that proposes an answer to the question.
  • A good hypothesis clearly states the expected relationship between two factors.
  • Hypotheses are often written in an 'If... then...' format.

Quick check

  1. Is 'Plants need water' a good hypothesis? Explain your answer.2 marks

2. Variables: The Key to a Fair Test

To test a hypothesis fairly, you must only change one key factor at a time. These factors are called variables. There are three types:

  1. Independent Variable (IV): This is the one and only factor that you deliberately change to see what effect it has.
  2. Dependent Variable (DV): This is the factor that you measure to see the effect of changing the independent variable. Its value 'depends' on the independent variable.
  3. Control Variables: These are all the other factors that could possibly affect the outcome of the experiment. You must keep them constant (the same) throughout all trials to ensure that only the independent variable is affecting the dependent variable. This is what makes it a 'fair test'.

Key term

Fair Test: An experiment in which only the independent variable is changed, and all other conditions (control variables) are kept the same.

Examiner insight

Marks are often awarded for identifying specific control variables and briefly explaining why they must be controlled (e.g., 'keep the light intensity the same, because light affects the rate of photosynthesis').

Common pitfall

Confusing the independent and dependent variables. A good way to remember is: the Independent variable is what *I* change; the Dependent variable *depends* on that change.

Worked example 14 marks

A student investigates how the concentration of salt in water affects the boiling point of the water. Identify the independent, dependent, and two important control variables.

  1. 1

    Independent Variable: The concentration of salt in the water. This is what the student is purposefully changing.

  2. 2

    Dependent Variable: The boiling point of the water. This is what the student will measure to see the effect.

  3. 3

    Control Variable 1: The volume of water used. Different volumes would take different amounts of time to heat up and could affect the result.

  4. 4

    Control Variable 2: The atmospheric pressure (or simply, doing the experiment in the same location). Boiling point changes with pressure, so this must be kept constant.

Recap

  • The independent variable is the one factor you change.
  • The dependent variable is the factor you measure.
  • Control variables are all other factors you must keep the same.
  • Controlling variables correctly is essential for a fair test.
  • Only the independent variable should affect the dependent variable in a well-designed experiment.

Quick check

  1. In an experiment to see if a water plant produces more oxygen bubbles in brighter light, what is the dependent variable?1 mark
  2. Why must you only change one variable at a time in an experiment?1 mark

3. Writing a Clear and Detailed Method

A method is a recipe for your experiment. It must be a set of clear, step-by-step instructions that another scientist could follow to repeat your investigation exactly. A good method includes a list of all apparatus (with sizes, e.g., '250 cm³ beaker'), explains how you will vary the independent variable over a suitable range, describes precisely how you will measure the dependent variable, and states how you will keep all control variables constant. It should also include a risk assessment, where you identify potential hazards (e.g., 'hot water') and describe how to minimise the risk (e.g., 'wear safety goggles', 'stand up to avoid spills').

Key term

Method: A detailed, step-by-step description of how to carry out an experiment.

Fun fact

The concept of a detailed, repeatable method is so important that major scientific discoveries are only accepted after other labs have independently replicated the original experiment and got the same results!

Worked example 16 marks

Write a plan to investigate the effect of light intensity on the rate of photosynthesis of pondweed. The rate can be measured by counting the bubbles of oxygen produced per minute.

  1. 1

    Hypothesis: If the light intensity is increased, then the rate of photosynthesis will increase, producing more oxygen bubbles per minute.

  2. 2

    Variables: Independent = Light intensity (changed by moving a lamp closer). Dependent = Rate of photosynthesis (measured by counting bubbles per minute). Controls = Temperature of water, concentration of carbon dioxide in water, same piece of pondweed.

  3. 3

    Apparatus: A piece of pondweed, large beaker, lamp, stopwatch, ruler, thermometer, sodium hydrogen carbonate solution (to provide constant CO₂).

  4. 4

    Method:

  5. 5
    1. Place a 10 cm piece of pondweed into a beaker filled with sodium hydrogen carbonate solution.
  6. 6
    1. Place a lamp 50 cm away from the beaker. Measure this distance with a ruler.
  7. 7
    1. Allow the pondweed to acclimatise for 2 minutes.
  8. 8
    1. Count the number of bubbles produced in one minute. Record this in a table.
  9. 9
    1. Move the lamp so it is 40 cm away from the beaker.
  10. 10
    1. Repeat steps 3 and 4.
  11. 11
    1. Repeat the process for distances of 30 cm, 20 cm, and 10 cm.
  12. 12
    1. Monitor the water temperature with a thermometer to ensure it stays constant.
  13. 13

    Risk Assessment: Water spills are a slip hazard; wipe up immediately. The lamp will get hot; handle with care and avoid touching the bulb.

Recap

  • A method should be written as clear, numbered steps.
  • List all key apparatus, including sizes or capacities.
  • Describe how to change the independent variable and measure the dependent variable.
  • State how each control variable will be kept constant.
  • Always include a simple risk assessment for safety.

Quick check

  1. Why is it important to include quantities and equipment sizes in a method?1 mark

4. Recording and Presenting Your Results

Once you collect data, you need to record it systematically. The best way is in a results table. A good table has ruled lines, clear column headings, and includes the units for each measurement in the heading itself (e.g., 'Time (s)'). Do not put units next to every number in the table. To see patterns, you should present your data in a graph. Use a line graph for continuous data (when the independent variable can have any value, like temperature or time). Use a bar chart for categoric data (when the independent variable falls into distinct categories, like 'types of metal' or 'colours of light').

Key term

Anomalous Result: A result that does not fit the pattern of the other results in a set of data, often due to a measurement error.

Examiner insight

Marks for graphs are consistently awarded for: choosing the correct graph type, labelling both axes with quantity and units, using a sensible scale that covers at least half the graph paper, plotting points accurately (using 'x' or '⊙'), and drawing a suitable line of best fit.

Worked example 13 marks

A student measures the time taken for a chemical reaction at different temperatures. The results are: 10°C, 120s; 20°C, 60s; 30°C, 30s; 40°C, 15s. Draw a suitable results table for this data and state which type of graph should be used.

  1. 1

    Step 1: Draw a table with two columns and enough rows for the data.

  2. 2

    Step 2: Write the headings. The independent variable ('Temperature') goes in the first column, and the dependent variable ('Time taken') in the second.

  3. 3

    Step 3: Add the units to the headings in brackets. 'Temperature (°C)' and 'Time taken (s)'.

  4. 4

    Step 4: Fill in the pairs of data accurately into the table.

  5. 5
    Temperature (°C)Time taken (s)
    10120
    2060
    3030
    4015
  6. 6

    Step 5: Choose the graph type. The independent variable, temperature, is continuous. Therefore, a line graph should be used to show the relationship.

Recap

  • Record results in a neat table with ruled lines and borders.
  • Column headings must state the quantity and the unit (e.g., Mass (g)).
  • Use a bar chart for categoric independent variables.
  • Use a line graph for continuous independent variables.
  • All graphs need axes labelled with quantity and units, a sensible scale, and accurately plotted points.
  • Look for anomalous results that do not fit the trend.

Quick check

  1. A student measures the height of plants grown in red, blue, and green light. What type of graph should they draw?1 mark

5. Analysing Data and Evaluating the Method

After collecting and presenting data, you must analyse it. A conclusion is a summary of what your results show. It must:

  1. Describe the pattern or trend in the data.
  2. Quote specific data from your results to support this pattern.
  3. State whether your results support your original hypothesis.

Evaluation means critically reviewing your experiment. A key way to improve an experiment's reliability is to take repeat readings (e.g., do the experiment 3 times) and calculate a mean (average). This makes your results more trustworthy and helps you spot anomalous results. You should also think about accuracy: could you have used more precise measuring instruments? Were all control variables truly kept constant?

Mean = Sum of all values / Number of values

Key term

Reliability: The extent to which an experiment gives the same result if it is repeated; it can be improved by taking repeat readings and calculating a mean.

Examiner insight

To get full marks for an evaluation question, suggesting 'do repeats and calculate a mean' is a strong point. Explaining *why* this improves reliability (e.g., 'to spot anomalies' or 'to get a more representative value') secures further credit.

Common pitfall

Simply stating the conclusion (e.g., 'The reaction got faster') without using any numbers from the results table or graph to back it up. Evidence is essential.

Worked example 13 marks

A student's results for the effect of fertiliser on plant height are: 0g fertiliser, 10cm; 1g, 14cm; 2g, 18cm; 3g, 19cm; 4g, 12cm. The hypothesis was 'If more fertiliser is added, then the plant will grow taller.' Write a conclusion for this experiment.

  1. 1

    Step 1: Describe the main pattern. 'As the amount of fertiliser increased from 0g to 3g, the height of the plant increased.'

  2. 2

    Step 2: Quote data to support this. 'For example, the height increased from 10cm with 0g of fertiliser to a peak of 19cm with 3g.'

  3. 3

    Step 3: Describe the full pattern. 'However, at 4g of fertiliser, the plant height decreased to 12cm, suggesting that too much fertiliser is harmful.'

  4. 4

    Step 4: Link back to the hypothesis. 'The results partially support the hypothesis up to 3g, but show that at higher concentrations the hypothesis is incorrect.'

Worked example 22 marks

How could the student in the previous example improve the reliability of their results?

  1. 1

    Step 1: State the improvement. 'The student should repeat the experiment.'

  2. 2

    Step 2: Add detail. 'They should use several plants (e.g., 3 or 5) for each concentration of fertiliser.'

  3. 3

    Step 3: Explain the benefit. 'They could then calculate a mean height for each concentration. This would make the results more reliable and help to identify any anomalous results, for example if one plant was unhealthy.'

Recap

  • A conclusion must describe the pattern shown in your results.
  • Always use specific numbers from your data to justify your conclusion.
  • State whether your results support or contradict your hypothesis.
  • Evaluating an experiment means identifying weaknesses and suggesting improvements.
  • Repeating readings and calculating a mean is the most common way to improve reliability.

Quick check

  1. What are the three essential parts of a good scientific conclusion?3 marks

End-of-chapter exercise

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

  1. What is a hypothesis?2 marks
  2. A student wants to find out which type of soil (sandy, clay, or loam) is best for growing beans. What is the independent variable in this investigation?1 mark
  3. For the bean investigation in the previous question, state the dependent variable you would measure and two variables you must control.3 marks
  4. An idea is that woodlice prefer dark, damp conditions. Write a testable hypothesis for an investigation using a choice chamber.2 marks
  5. A student is investigating the effect of exercise on heart rate. They measure their pulse, run for 2 minutes, and then measure it again. Their results table is shown below. Identify two mistakes with this table. | Before | After | |---|---| | 72 | 130 |2 marks
  6. A student investigates how the length of a wire affects its resistance. Is a line graph or a bar chart more suitable for presenting the results? Explain your answer.2 marks
  7. Plan a simple experiment to find out if the surface area of a potato piece affects the rate at which it loses mass when placed in a concentrated salt solution. Your plan should identify the key variables and list the main steps in your method.6 marks
  8. In an experiment on enzyme activity, a student obtained the following results for the volume of gas produced at different temperatures: 10°C: 12 cm³ 20°C: 25 cm³ 30°C: 18 cm³ 40°C: 51 cm³ 50°C: 59 cm³ Identify the anomalous result and explain what the student should do about it.3 marks
  9. A conclusion from an experiment states: 'My results show that as the concentration of the acid increased, the reaction got faster. This supports my hypothesis.' Explain why this conclusion would not get full marks and how it could be improved.3 marks
  10. Explain the difference between the reliability and the accuracy of an experiment.2 marks

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