# Experimental design and procedure

> IB Chemistry SL · IB Chemistry SL
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u7-experimental-design-and-procedure/

This sub-topic covers core principles for designing valid, reliable experiments for IB Chemistry SL. You will learn to define variables, plan procedures, and address error, critical for both Paper 3 and your Internal Assessment.

**Prerequisites:** [Basic measurement and uncertainty in Chemistry](https://www.owlsprep.com/study/ib-chemistry-sl-u1-measurement-and-uncertainty/)

## Learning objectives

- Identify key components of a valid experimental design
- Distinguish between independent, dependent and controlled variables
- Outline appropriate repeatable procedures for common IB Chemistry practicals
- Recognise and classify sources of error in experimental procedures

## Variables and Experimental Validity

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.

**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.

**Worked example:** A student plans to investigate how acid concentration affects the volume of CO₂ produced when calcium carbonate reacts with excess hydrochloric acid. Identify all three types of variables for this experiment.

1. Step 1: Identify the variable intentionally changed by the experimenter:
2. Independent variable: Concentration of hydrochloric acid
3. Step 2: Identify the variable measured to observe the effect of the change:
4. Dependent variable: Total volume of CO₂ gas produced
5. Step 3: List all variables that could affect the result, that must be kept constant:
6. Controlled variables: Mass/surface area of calcium carbonate, reaction temperature, total volume of acid solution

## Planning a Valid, Repeatable Procedure

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.

> **tip**
>
> Always include at least 3-5 repeats for each level of your independent variable. This lets you calculate a reliable mean and identify anomalous results.

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

1. Step 1: Prepare 5 different concentrations of HCl, ranging from $0.5\terxt{mol dm}^{-3}$ to $2.5\terxt{mol dm}^{-3}$, keeping total solution volume constant at $50\terxt{cm}^3$ for each trial.
2. Step 2: Cut 15 identical pieces of magnesium ribbon (0.05 g each, equal surface area) to control this variable.
3. Step 3: Add one magnesium piece to the first acid solution, start a stopwatch immediately, and collect H₂ gas in an inverted burette over water.
4. Step 4: Record the time taken to collect $20\terxt{cm}^3$ of H₂ gas, then repeat 2 more times for this concentration.
5. Step 5: Repeat the full process for all 5 concentrations, then calculate mean rate ($\text{rate} = 20\terxt{cm}^3 / \text{mean time}$) for each concentration.

## Identifying and Addressing Experimental Error

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.

**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.

**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. 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. 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. 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.

## Interpreting IB Exam Design Questions

**Exam command terms**

IB Chemistry uses common command terms for design questions with specific mark expectations:

- **Design an experiment** — Outline full procedure, all variables, and measurement methods *(Worth 4-6 marks on Paper 3)*

- **Identify variables** — Name independent, dependent and 2-3 relevant controlled variables *(1 mark per correct variable)*

- **Suggest an improvement** — Propose a specific change linked to the stated error *(Vague answers do not earn marks)*

**Check your understanding**

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

   *Answer:* Full procedure, variables and measurement methods

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

## Common pitfalls

- **Wrong:** Forgetting to list relevant controlled variables
  - Why it fails: Examiners expect you to identify variables that actually impact the result. Missing key controls costs easy marks.
  - Correct: List 2-3 specific controlled variables that directly affect your dependent variable for the experiment being tested.
- **Wrong:** Confusing random and systematic error
  - Why it fails: Random error affects precision, while systematic error affects accuracy. Mixing these up loses marks in error analysis.
  - Correct: Remember: if all results are shifted in one direction, it is systematic. If results are scattered around the true value, it is random.
- **Wrong:** Not including repeats in a designed procedure
  - Why it fails: Repeats are required to assess reliability and identify outliers. This is a common easy mark that many students miss.
  - Correct: Always explicitly state you will repeat each measurement at least 3 times, calculate a mean, and remove anomalies.
- **Wrong:** Suggesting 'use more accurate equipment' as a vague improvement
  - Why it fails: Examiners require specific improvements linked to the specific error. Vague answers do not earn marks.
  - Correct: Name the specific change, e.g. 'use a burette instead of a measuring cylinder to measure volume' instead of 'use better equipment'.

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

## 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.

- [Data collection and processing](https://www.owlsprep.com/study/ib-chemistry-sl-u7-data-collection-and-processing/)
- [Results analysis and evaluation](https://www.owlsprep.com/study/ib-chemistry-sl-u7-results-analysis-and-evaluation/)
- [Collaborative scientific inquiry](https://www.owlsprep.com/study/ib-chemistry-sl-u7-collaborative-scientific-inquiry/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-sl-u7-experimental-design-and-procedure/