Cambridge Lower Secondary CheckpointStage 8

Thinking and Working Scientifically: Scientific enquiry: purpose and planning

Science Stage 8 Chapter Notes

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

  1. 1Understanding and Identifying Variables
  2. 2Developing a Testable Hypothesis
  3. 3Planning a Detailed Method
  4. 4Risk Assessment and Safety
  5. 5Choosing Appropriate Apparatus

1. Understanding and Identifying Variables

In any scientific investigation, we change one thing to see its effect on another. These 'things' are called variables. The Independent Variable (IV) is the one you, the scientist, choose to change. The Dependent Variable (DV) is what you measure to see the effect of your change. To make sure it's a fair test, all other factors that could possibly affect the outcome must be kept the same. These are called Control Variables (or constants). For example, if investigating how light affects plant growth, the IV is light intensity, the DV is the plant's height increase, and control variables would be water amount, soil type, and temperature.

Key term

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

Examiner insight

Examiners award marks for correctly identifying the variables and often give extra credit for explaining *why* a specific variable must be controlled.

Common pitfall

Confusing the dependent variable (what is measured) with the independent variable (what is changed) is a very common mistake.

Fun fact

In clinical trials for new medicines, a 'placebo' (a sugar pill with no active ingredient) is given to a control group. This helps control for the 'placebo effect', where people feel better simply because they believe they are receiving treatment.

Worked example 14 marks

A student investigates how the temperature of water affects the time it takes for a 5g tablet to dissolve completely. Identify the independent, dependent, and two control variables in this investigation.

  1. 1

    Step 1: Identify what is being deliberately changed by the student. This is the independent variable. Here, it is the temperature of the water.

  2. 2

    Step 2: Identify what is being measured to see the result of the change. This is the dependent variable. Here, it is the time taken for the tablet to dissolve.

  3. 3

    Step 3: Identify other factors that could affect the dissolving time and must be kept the same for a fair test. These are the control variables. Examples include the volume of water, the mass of the tablet (which is stated as 5g), and whether the water is stirred or not.

Recap

  • The Independent Variable (IV) is the one you change.
  • The Dependent Variable (DV) is the one you measure.
  • Control Variables are all other factors you keep the same.
  • Controlling variables 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 see how exercise affects heart rate, what is the dependent variable?1 mark
  2. List two variables that should be controlled in an experiment investigating the effect of soil pH on seed germination.2 marks

2. Developing a Testable Hypothesis

A hypothesis is more than just a guess; it's a precise, testable prediction about the outcome of your investigation. It proposes a relationship between the independent and dependent variables. A good hypothesis is not a question, but a statement. A strong structure to follow is: 'If I change the [Independent Variable], then the [Dependent Variable] will [increase/decrease/change in a specific way], because [brief scientific reason]'. The scientific reason shows your understanding of the topic.

Key term

Hypothesis: A clear, predictive, and testable statement about the expected relationship between the independent and dependent variables in an investigation.

Examiner insight

A hypothesis that includes a prediction and a brief scientific justification can earn the top marks in planning questions.

Common pitfall

Writing a vague prediction like 'Temperature will affect the reaction' instead of a specific, directional hypothesis like 'As temperature increases, the rate of reaction will increase up to an optimum point'.

Worked example 13 marks

An investigation has the aim: 'To find out how the concentration of an acid affects the rate of reaction with magnesium ribbon.' Write a suitable hypothesis for this investigation.

  1. 1

    Step 1: Identify the IV (concentration of acid) and DV (rate of reaction).

  2. 2

    Step 2: Predict the relationship. Higher concentration means more acid particles in the same volume, leading to more frequent collisions with the magnesium.

  3. 3

    Step 3: Formulate the statement. 'If the concentration of the acid is increased, then the rate of reaction with magnesium will also increase.'

  4. 4

    Step 4: Add the scientific justification. 'This is because a higher concentration means there are more acid particles per unit volume, leading to a higher frequency of successful collisions with the magnesium surface.'

Recap

  • A hypothesis is a testable prediction, not a question.
  • It should clearly link the independent and dependent variables.
  • A good hypothesis predicts the direction of change (e.g., 'increase', 'decrease').
  • Including a brief scientific reason strengthens your hypothesis.
  • The hypothesis must be something that your planned experiment can actually test.

Quick check

  1. Write a hypothesis for an experiment investigating the effect of temperature on the activity of the enzyme amylase.2 marks

3. Planning a Detailed Method

A method is a clear, logical, step-by-step guide that another scientist could follow to repeat your exact experiment. It must include enough detail to ensure the results are reliable and valid. A good method specifies the apparatus used (e.g., '50 cm³ measuring cylinder'), the quantities of substances (e.g., '25 cm³ of 1.0 mol/dm³ hydrochloric acid'), how you will change the IV, how you will measure the DV, and crucially, how you will keep all control variables constant. It should also state that you will repeat the experiment at each value of the IV and calculate a mean to improve reliability.

Key term

Fair Test: An investigation where only the independent variable is changed, and all other conditions that might affect the outcome are kept constant.

Examiner insight

Marks for methods are often lost due to a lack of specific detail. State volumes, concentrations, and names of apparatus, not just general terms like 'some acid' or 'a beaker'.

Common pitfall

Forgetting to state how the control variables will be kept constant (e.g., not just saying 'control temperature', but explaining *how* - by using a thermostatically controlled water bath).

Worked example 16 marks

Plan an investigation to determine the effect of the surface area of marble chips on the rate of reaction with hydrochloric acid. (6 marks)

  1. 1
    1. State the IV (surface area of marble chips - e.g., large chips, small chips, powder) and DV (rate of reaction, measured by volume of CO₂ gas produced over time).
  2. 2
    1. List key control variables: Volume of acid, concentration of acid, temperature.
  3. 3
    1. Method Step 1: Place a conical flask on a top-pan balance. Add 50 cm³ of 1.0 mol/dm³ hydrochloric acid.
  4. 4
    1. Method Step 2: Add 5.0 g of large marble chips to the flask, quickly place cotton wool in the neck of the flask (to prevent acid spray escaping), and start a stopwatch immediately.
  5. 5
    1. Method Step 3: Record the mass shown on the balance every 30 seconds for 5 minutes.
  6. 6
    1. Method Step 4: Repeat the entire experiment using 5.0 g of small marble chips, and then again with 5.0 g of marble powder, keeping the acid volume, concentration and temperature the same.
  7. 7
    1. Method Step 5: To ensure reliability, repeat the experiment for each surface area size three times and calculate a mean rate of reaction.

Recap

  • A method must be a clear, logical sequence of steps.
  • Always state specific quantities, volumes, and concentrations.
  • Name the specific apparatus you will use for each measurement.
  • Clearly state how you will keep control variables constant.
  • Include repeats and the calculation of a mean to ensure reliability.

Quick check

  1. Why is it important to state the concentration of a solution used in a method?1 mark
  2. List three essential components of a good experimental method.3 marks

4. Risk Assessment and Safety

Planning an investigation must include thinking about safety. A risk assessment is the formal process for this. You need to identify the Hazards (anything with the potential to cause harm, like corrosive acid or a hot Bunsen burner), assess the associated Risk (the likelihood of that harm occurring, e.g., skin burns or setting fire to something), and then state the Control Measure or Precaution (what you will do to reduce or eliminate the risk, e.g., wear safety goggles or use a heatproof mat).

Key term

Hazard: A potential source of harm or adverse health effect, such as a toxic chemical, a hot object, or a sharp instrument.

Examiner insight

To score full marks on safety questions, you must clearly link the hazard to the specific risk and then provide a sensible, specific precaution.

Common pitfall

Giving generic safety precautions like 'be careful' or 'wear a lab coat' for every situation, instead of specific precautions relevant to the identified hazard (e.g., using a fume cupboard for a toxic gas).

Fun fact

The globally recognized hazard symbols (pictograms) were standardized by the UN to create a 'Globally Harmonized System' (GHS) so that a chemical's dangers are understood anywhere in the world, regardless of local language.

Worked example 13 marks

A student is heating a beaker of dilute acid with a Bunsen burner. Identify one hazard, the associated risk, and a suitable precaution.

  1. 1

    Step 1: Identify a hazard. The Bunsen burner flame is a hazard.

  2. 2

    Step 2: State the risk associated with that specific hazard. The risk is burning skin or setting fire to hair or clothing.

  3. 3

    Step 3: Give a specific, sensible precaution to manage that risk. A suitable precaution would be to tie back long hair and ensure no flammable materials are nearby.

  4. 4

    Alternative Answer: Hazard: Hot beaker/acid. Risk: Scalding or burning skin if spilled or touched. Precaution: Allow the beaker to cool before moving it, or use tongs to handle the hot beaker.

Recap

  • A hazard is the thing that can cause harm.
  • A risk is the chance of harm actually happening.
  • A precaution is an action taken to reduce the risk.
  • Always wear safety goggles when working with chemicals or heating substances.
  • Link your precaution directly to the specific hazard you have identified.

Quick check

  1. What is the primary hazard associated with using ethanol in a lab?1 mark
  2. State one safety precaution you should take when using a sharp scalpel to cut a piece of potato.1 mark

5. Choosing Appropriate Apparatus

Selecting the right tool for the job is critical in science. The choice of apparatus depends on the quantity you are measuring and the precision required. Precision refers to the smallest unit an instrument can measure. For example, a 50 cm³ burette has a precision of ±0.05 cm³ and is suitable for very accurate measurements in a titration, while a 50 cm³ measuring cylinder has a precision of ±0.5 cm³ and is fine for approximate volumes. Always choose apparatus that is precise enough for the task without being unnecessarily complicated.

Key term

Precision: The level of detail an instrument can measure to, often indicated by the number of decimal places or smallest division on its scale.

Examiner insight

Justifying the choice of apparatus with reference to its precision (e.g., 'use a gas syringe because it measures volume to the nearest 1 cm³') scores higher than just naming the equipment.

Common pitfall

Suggesting the use of a beaker for measuring accurate volumes. Beakers are for holding or mixing liquids; their volume markings are only approximate.

Worked example 13 marks

A student needs to measure out exactly 25.0 cm³ of sodium hydroxide solution for a titration. They have a 100 cm³ beaker, a 25 cm³ measuring cylinder, and a 25 cm³ pipette. Which should they use and why?

  1. 1

    Step 1: Identify the best piece of apparatus. The student should use the 25 cm³ pipette.

  2. 2

    Step 2: Justify the choice by comparing its precision to the alternatives. A pipette is designed to deliver a single, highly accurate volume (e.g., to ±0.06 cm³).

  3. 3

    Step 3: Explain why the others are unsuitable. A beaker is for holding liquids and has very inaccurate markings. A measuring cylinder is more accurate than a beaker but far less accurate than a pipette and would not be suitable for a titration where precision is key.

Recap

  • Choose apparatus with the appropriate range and precision for the measurement being made.
  • Pipettes and burettes are used for measuring variable or fixed volumes of liquid very accurately.
  • Measuring cylinders are suitable for approximate volume measurements.
  • A top-pan balance measures mass, typically to two decimal places.
  • A stopwatch is used to measure time intervals.

Quick check

  1. Which piece of equipment would you use to measure the mass of a chemical powder?1 mark
  2. To measure 75 cm³ of water for making a solution, would you use a 100 cm³ measuring cylinder or a 50 cm³ burette? Explain your choice.2 marks

End-of-chapter exercise

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

  1. A student investigated the effect of pH on the time taken for a protein-digesting enzyme to break down egg white. Identify the independent variable, dependent variable, and one control variable.3 marks
  2. Write a testable hypothesis for an investigation into the effect of temperature on the solubility of salt in water.2 marks
  3. When investigating the reaction between magnesium and acid, a student uses a thermometer. Explain why temperature is an important control variable in this experiment.2 marks
  4. Describe the safety precautions that should be taken when carrying out an experiment that involves heating copper sulfate solution in a beaker using a Bunsen burner. For each precaution, state the hazard it helps to prevent.4 marks
  5. A student needs to measure the volume of gas produced in a reaction every 15 seconds. Suggest a suitable piece of apparatus for this measurement and justify your choice.2 marks
  6. Explain why it is important to repeat measurements and calculate a mean when carrying out an investigation.3 marks
  7. Plan an investigation to determine how the length of a wire affects its electrical resistance. Your plan should include a list of apparatus, a step-by-step method, and details of your variables.6 marks
  8. A student wants to find out if brand A or brand B of fertiliser makes plants grow taller. They use two identical plants, give one brand A and the other brand B, and measure their heights after 4 weeks. Criticise this experimental plan and explain how it could be improved to produce more reliable and valid results.5 marks
  9. Distinguish between the 'range' and 'precision' of a measuring instrument, using a 30 cm ruler as an example.3 marks
  10. A method states 'Add some acid to a beaker'. Rewrite this instruction with the level of detail required for a good scientific method.2 marks

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