Carrying Out an Experiment: Unit 7 Practical Skills
Welcome to your study guide for Carrying out an experiment! Doing experiments is at the very heart of science. It is not just about following a recipe in the lab; it is about asking questions, making accurate observations, staying safe, and gathering trustworthy evidence. Don't worry if experimental skills feel a bit overwhelming at first — once you learn a few golden rules and patterns, you will feel confident tackling both lab work and exam questions!
1. Identifying and Controlling Variables
Whenever you plan or carry out an investigation, you are testing how one thing affects another. To make your test fair and valid, you must understand the three types of variables:
• Independent Variable: The variable that you deliberately change or select. (Memory trick: Independent = the one "I" change).
• Dependent Variable: The variable that you measure or observe. It changes in response to the independent variable. (Memory trick: Dependent = the Data you record).
• Control Variables: All the other factors that must be kept strictly constant (the same) throughout the experiment. If you don't keep them constant, you won't know if your results were caused by the independent variable or by something else!
Real-World Analogy: Testing Plant Growth
Imagine you want to see if adding fertiliser makes a plant grow taller:
• Independent variable: Amount of fertiliser added (\(g\)).
• Dependent variable: Height of the plant after two weeks (\(cm\)).
• Control variables: Amount of water given (\(cm^3\)), type of soil, amount of sunlight, and room temperature (\(^\circ\text{C}\)).
Key Takeaway
A fair test only changes one independent variable at a time while keeping all control variables identical.
2. Choosing Apparatus and Making Accurate Measurements
Selecting the right piece of equipment is essential for getting accurate (close to the true value) and precise (finely detailed and repeatable) results.
Common Lab Apparatus:
• Measuring Cylinder: Used for measuring liquid volumes (e.g. \(10\text{ cm}^3\) or \(50\text{ cm}^3\)). A pipette or burette can be used when even greater precision is needed.
• Top-pan Balance: Used to measure mass in grams (\(g\)). Always press the "tare" (zero) button before weighing!
• Stopwatch: Used to measure time in seconds (\(s\)).
• Thermometer: Used to measure temperature in degrees Celsius (\(^\circ\text{C}\)).
• Gas Syringe: Used to collect and measure the volume of gas produced in a chemical reaction (\(cm^3\)).
Avoiding Common Measurement Errors
1. Parallax Error: This happens when you look at a scale from an angle. To avoid this, always view the scale at eye level.
2. Reading the Meniscus: Liquids in narrow containers curve slightly at the surface (the meniscus). Always read the volume from the bottom of the meniscus at eye level.
3. Zero Error: This occurs when an instrument gives a reading when it should read zero (such as a balance that reads \(0.2\text{ g}\) when empty). Always check and reset your instrument to zero before starting.
Did you know? A standard beaker has volume markings on the side, but they are only rough estimates! For accurate scientific data, always use a measuring cylinder, pipette, or burette instead of a beaker.
Key Takeaway
Choose apparatus suited to the volume or quantity being measured, read scales at eye level at the bottom of the meniscus, and always check for zero errors.
3. Health, Safety, and Hazard Symbols
Science practicals involve heat, glass, and chemicals, so safety is always the number one priority. Before starting an experiment, scientists carry out a risk assessment.
Hazard vs. Risk
• Hazard: Anything that has the potential to cause harm (e.g. a hot Bunsen burner flame, concentrated acid, or broken glass).
• Risk: The chance or likelihood that someone will be harmed by the hazard, along with how severe that harm could be.
• Control Measure (Precaution): An action taken to reduce the risk (e.g. wearing safety goggles, tying long hair back, or using a heatproof mat).
Common Hazard Symbols to Recognise:
• Flammable: Catches fire easily. Precaution: Keep away from naked flames and sparks.
• Corrosive: Attacks and destroys living tissue (such as skin and eyes). Precaution: Wear eye protection and protective gloves.
• Toxic: Poisonous and can cause death if swallowed, inhaled, or absorbed through skin. Precaution: Work in a fume cupboard and wear gloves.
• Caution / Irritant: Can cause redness, blistering, or irritation to the skin or eyes. Precaution: Avoid contact with skin; wash splashes immediately.
• Environmental Hazard: Harmful to aquatic life and ecosystems. Precaution: Do not pour down the sink; dispose of in designated waste bottles.
Key Takeaway
Always identify the specific hazard, state the possible harm, and describe the practical precaution you will take to stay safe.
4. Recording Data, Repeating Trials, and Dealing with Anomalies
Good scientists are organised. Recording your data clearly makes it easy to spot patterns and calculate trustworthy results.
How to Draw a Perfect Results Table:
• Place the independent variable in the very first column.
• Place the dependent variable (including repeat columns and mean) in the columns to the right.
• Put the quantity and unit in the column heading only (e.g. "Time / \(s\)" or "Temperature (\(^\circ\text{C}\))"). Do not write units inside the data cells!
• Keep numerical data to a consistent number of decimal places.
Reliability and Repeating Readings
Why do we repeat measurements?
1. Repeating measurements helps you check that your results are reliable (consistent).
2. It allows you to identify anomalous results (outliers).
3. It allows you to calculate an accurate mean (average).
What is an Anomaly?
An anomaly (or outlier) is a measurement that does not fit the pattern shown by the rest of the data. If you spot an anomaly:
1. Do not include it when calculating your mean.
2. Repeat that measurement if possible.
Calculating the Mean (Step-by-Step):
Suppose you measure the time taken for a reaction three times:
Trial 1 = \(24\text{ s}\), Trial 2 = \(25\text{ s}\), Trial 3 = \(38\text{ s}\) (Anomaly!).
Step 1: Identify and discard the anomaly (\(38\text{ s}\)).
Step 2: Add together the remaining concordant (close) results: \(24 + 25 = 49\text{ s}\).
Step 3: Divide by the number of values used (which is \(2\)):
\(\text{Mean} = \frac{49}{2} = 24.5\text{ s}\)
Key Takeaway
Always repeat trials to improve reliability, discard anomalies before calculating the mean, and never put units inside table data cells.
5. Quick Summary & Exam Checklist
When answering exam questions about carrying out experiments, ask yourself:
• Have I clearly named the independent variable (what I change), the dependent variable (what I measure), and at least two control variables (what I keep constant)?
• Have I named specific equipment (e.g. measuring cylinder, stopwatch, balance) rather than generic terms like "timer" or "container"?
• Have I explained how to make the measurement accurate (e.g. read at eye level, avoid parallax error)?
• Have I stated that trials should be repeated at least three times and an average calculated after excluding anomalies?
• Have I mentioned relevant safety precautions linked to specific hazards?