Introduction to Planning Experiments
Welcome to one of the most important parts of your Chemistry course! In Units 3 and 6, you aren't just asked to remember facts; you are asked to think like a scientist in a lab. Planning an experiment is like writing a recipe. If your instructions are vague or you forget to control the temperature, your "cake" (the results) won't turn out right. In your exam, you will often be asked to devise an experimental procedure, which means you need to explain how to do it, what to use, and how to stay safe.
Note: This chapter focuses on the "how-to" of planning. For details on specific equipment, see the chapter on "Practical Techniques and Apparatus." For the math behind the results, check "Uncertainty, Errors and Data Analysis."
1. Understanding Variables
To make an experiment a "fair test," we must manage three types of variables. Think of this as the "Who, What, and Why" of your method.
The Independent Variable
This is the factor you change to see what happens.
Example: If you are investigating how concentration affects the rate of reaction, the concentration of your reactant (e.g., \( \text{HCl} \)) is the independent variable.
The Dependent Variable
This is what you measure as a result.
Example: In the same rate experiment, the dependent variable might be the volume of \( \text{CO}_2 \) gas produced every 30 seconds.
Control Variables
These are the factors you must keep constant. If these change, you won't know if your results were caused by your independent variable or by a random mistake.
Common controls include:
• Temperature (use a water bath).
• Total volume of the solution.
• Surface area of a solid (e.g., using "lumps" vs. "powder").
• Pressure (especially for gas reactions).
Quick Review:
When asked to "comment on a method," always look for a variable the student forgot to control! If they didn't mention using a water bath, the temperature might have fluctuated, making the data unreliable.
2. Choosing the Right Apparatus
The syllabus requires you to select apparatus and judge its range and resolution.
• Range: The minimum and maximum values the tool can measure (e.g., a \( 50\text{ cm}^3 \) burette).
• Resolution: The smallest change the tool can detect (e.g., a balance that weighs to \( 0.01\text{ g} \) has a higher resolution than one that weighs to \( 0.1\text{ g} \)).
Don't worry if this seems tricky! Just remember: always choose the tool that gives you the most precision for the amount you are measuring. You wouldn't use a \( 100\text{ cm}^3 \) measuring cylinder to measure \( 1\text{ cm}^3 \) of liquid; a \( 1\text{ cm}^3 \) pipette or a syringe would be much better!
3. Hazards, Risks, and Safety
In Topic 4 and Topic 20, the syllabus distinguishes between a hazard and a risk. This is a classic exam favorite!
The Difference
• Hazard: An inherent property of a substance that has the potential to cause harm. (e.g., "Concentrated \( \text{HCl} \) is corrosive"). It doesn't change based on how you use it.
• Risk: The likelihood that the hazard will actually cause harm under specific conditions. (e.g., "There is a risk of \( \text{HCl} \) splashing into your eyes during a titration").
Risk Management (Control Measures)
When planning, you must suggest control measures to reduce the risk.
• Hazard: Reagent is flammable (like ethanol). Control: Use an electric heating mantle instead of a Bunsen burner.
• Hazard: Toxic gases produced (like \( \text{Cl}_2 \) or \( \text{NO}_2 \)). Control: Carry out the experiment in a fume cupboard.
• Hazard: Corrosive liquids (like concentrated \( \text{H}_2\text{SO}_4 \)). Control: Wear gloves and safety goggles.
Did you know? Some chemicals mentioned in your syllabus need very specific handling. For example, \( \text{KCN} \) (used in nucleophilic addition) is highly toxic, and \( \text{PCl}_5 \) (used to test for \( \text{-OH} \) groups) reacts violently with water!
4. Step-by-Step Planning: The "Checklist"
If you are asked to devise an experiment (like Core Practical 16: Preparation of Aspirin), follow this mental checklist:
1. The Goal: State clearly what you are trying to find or make.
2. The Quantities: Use specific amounts. Don't just say "add some acid." Say "Add \( 25.0\text{ cm}^3 \) of \( 1.0\text{ mol dm}^{-3} \) \( \text{HCl} \)."
3. The Method: List the steps in a logical order (e.g., Reflux \( \rightarrow \) Distill \( \rightarrow \) Purify \( \rightarrow \) Dry).
4. The Measurements: State what you will record. "Record the mass of the precipitate using a 2-decimal place balance."
5. Repeatability: Always mention repeating the experiment (usually three times) to identify anomalies and calculate a mean.
5. Common Mistakes to Avoid
• Vague Language: Avoid words like "take a bit" or "wait a while." Use "measure \( 5\text{ g} \)" or "record the time for \( 60\text{ seconds} \)."
• Confusing Resolution: Don't claim a measurement is \( 25.00\text{ cm}^3 \) if you used a measuring cylinder that only has marks every \( 1\text{ cm}^3 \).
• Ignoring State Symbols: When planning reactions, remember that the physical state (solid, liquid, gas, aqueous) affects how you handle the substance.
• Mathematical Units: Always use the correct conventions. Concentration is \( \text{mol dm}^{-3} \), and temperature in calorimetry is measured in \( ^\circ\text{C} \) for the change (\( \Delta T \)), even if the standard units are \( \text{Kelvin (K)} \).
Key Takeaways:
• Independent = You change it. Dependent = You measure it. Control = You keep it the same.
• Hazards are what the chemical is; Risks are what could happen.
• Use a fume cupboard for toxic gases and a water bath for flammable liquids.
• Always specify the precision of your equipment (e.g., "a volumetric pipette").