Study Guide

Experimental design and procedure

IB Chemistry SLΒ· Unit 7: Practical and Investigative ChemistryΒ· 20 min read

1. Variables and Experimental Validityβ˜…β˜…β˜†β˜†β˜†β± 5 min

All valid experiments test the causal relationship between two key variables, with all other factors kept constant to isolate the effect being tested. IB examiners and IA moderators require clear, correct identification of all variable types in any design question.

πŸ“˜ Definition

Types of Experimental Variables

Three core variable types are required for a valid experiment: independent (manipulated by the experimenter), dependent (measured to test the relationship), and controlled (held constant to avoid confounding results).

Example:

When testing how temperature affects reaction rate, temperature is independent, rate is dependent, and reactant concentration/volume are controlled.

πŸ“ Worked Example

A student plans to investigate how acid concentration affects the volume of COβ‚‚ produced when calcium carbonate reacts with excess hydrochloric acid. Identify all three types of variables for this experiment.

  1. 1

    Step 1: Identify the variable intentionally changed by the experimenter:

  2. 2

    Independent variable: Concentration of hydrochloric acid

  3. 3

    Step 2: Identify the variable measured to observe the effect of the change:

  4. 4

    Dependent variable: Total volume of COβ‚‚ gas produced

  5. 5

    Step 3: List all variables that could affect the result, that must be kept constant:

  6. 6

    Controlled variables: Mass/surface area of calcium carbonate, reaction temperature, total volume of acid solution

2. Planning a Valid, Repeatable Procedureβ˜…β˜…β˜…β˜†β˜†β± 6 min

When designing an experimental procedure for IB, you need to outline clear, step-by-step instructions that can be replicated exactly by another chemist. Procedures must include repeats for reliability, appropriate measurement tools, and explicit controls for all constant variables.

πŸ“ Worked Example

Outline a procedure to test the effect of HCl concentration on the rate of reaction with magnesium, using gas collection.

  1. 1

    Step 1: Prepare 5 different concentrations of HCl, ranging from to , keeping total solution volume constant at for each trial.

  2. 2

    Step 2: Cut 15 identical pieces of magnesium ribbon (0.05 g each, equal surface area) to control this variable.

  3. 3

    Step 3: Add one magnesium piece to the first acid solution, start a stopwatch immediately, and collect Hβ‚‚ gas in an inverted burette over water.

  4. 4

    Step 4: Record the time taken to collect of Hβ‚‚ gas, then repeat 2 more times for this concentration.

  5. 5

    Step 5: Repeat the full process for all 5 concentrations, then calculate mean rate () for each concentration.

3. Identifying and Addressing Experimental Errorβ˜…β˜…β˜…β˜†β˜†β± 5 min

All experiments have error, and IB expects you to distinguish between random and systematic error, then suggest targeted improvements to reduce their impact. This is a common Paper 3 question and a core requirement for your IA evaluation.

πŸ“˜ Definition

Random vs Systematic Error

Random error causes unpredictable variation in measurements around the true value, reducing precision. Systematic error is a consistent offset in all measurements, shifting results in one direction and reducing accuracy.

Example:

Random error: Uncertainty in reading gas volume from a burette. Systematic error: An incorrectly zeroed balance that reads 0.02 g too high for all measurements.

πŸ“ Worked Example

A student measures the enthalpy of neutralisation using an uncovered polystyrene cup and obtains a value lower than the literature value. Identify the error type and suggest an improvement.

  1. 1

    Step 1: Identify the cause of the consistent error: Heat is lost to the surroundings during the reaction, so all temperature measurements are lower than the true value.

  2. 2

    Step 2: Classify the error: This is a systematic error, because it shifts all results in the same direction (lower temperature change = lower enthalpy magnitude).

  3. 3

    Step 3: Suggest a targeted improvement: Add an insulating lid to the cup to reduce heat loss to air, or add extra foam insulation around the cup to reduce heat transfer.

4. Interpreting IB Exam Design Questionsβ˜…β˜…β˜…β˜…β˜†β± 4 min

βœ“ Quick check

Check your understanding of command term expectations:

  1. What is required when you are asked to 'design an experiment'?

    • Only state a testable hypothesis

    • Full procedure, variables and measurement methods

    • Only list sources of error

    Reveal answer
    1 β€”

    Correct! The command term 'design' requires a full outline of all core components of the experiment. Vague partial answers do not earn full marks.

5. Common Pitfalls

Wrong move:

Forgetting to list relevant controlled variables

Why:

Examiners expect you to identify variables that actually impact the result. Missing key controls costs easy marks.

Correct move:

List 2-3 specific controlled variables that directly affect your dependent variable for the experiment being tested.

Wrong move:

Confusing random and systematic error

Why:

Random error affects precision, while systematic error affects accuracy. Mixing these up loses marks in error analysis.

Correct move:

Remember: if all results are shifted in one direction, it is systematic. If results are scattered around the true value, it is random.

Wrong move:

Not including repeats in a designed procedure

Why:

Repeats are required to assess reliability and identify outliers. This is a common easy mark that many students miss.

Correct move:

Always explicitly state you will repeat each measurement at least 3 times, calculate a mean, and remove anomalies.

Wrong move:

Suggesting 'use more accurate equipment' as a vague improvement

Why:

Examiners require specific improvements linked to the specific error. Vague answers do not earn marks.

Correct move:

Name the specific change, e.g. 'use a burette instead of a measuring cylinder to measure volume' instead of 'use better equipment'.

6. Quick Reference Cheatsheet

Component

Key IB Requirements

Variables

  1. State independent (manipulated)
  2. State dependent (measured)
  3. List 2-3 specific controlled variables

Procedure

  1. Clear, repeatable step-by-step
  2. 3+ repeats per condition
  3. Specify measurement tools

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Improvements

Link to specific error, name the exact change to equipment or procedure

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2025 Β· 3

    Design enthalpy change experiment

  • 2024 Β· 3

    Identify variables in rate experiment

  • 2023 Β· IA

    Core of personal investigation

Going deeper

What's Next

Experimental design is the foundation of all practical work in IB Chemistry SL, and this skill is assessed across both your written Paper 3 exam and your weighted Internal Assessment investigation. Mastering the principles here will help you design a valid, high-mark IA, as well as answer all practical-based questions on the final exam. After understanding experimental design, the next step is to learn how to process and analyse your experimental data, including calculating uncertainty and drawing evidence-based conclusions from your results. You will build on these design skills when planning your own personal investigation for IA, where you will apply all of these principles independently to answer your own research question.