Unit 4: Practical Skills – Carrying Out an Experiment
Welcome to your study notes for Unit 4: Practical Skills! Practical work is at the very heart of science. In your CCEA GCSE Single Award Science course, your practical skills are assessed in two ways:
• Booklet A: A practical exam where you carry out experiments in the laboratory based on the 9 Prescribed Practicals.
• Booklet B: A written exam that tests your ability to plan, analyse, evaluate, and assess risks for experiments across Biology, Chemistry, and Physics.
Mastering how to carry out an experiment will help you score top marks in both papers!
1. Variables and Making Tests Fair
Whenever you carry out a scientific investigation, you are testing how one thing affects another. To make sure your experiment is valid and reliable, you need to understand three types of variables.
The Three Key Variables
• Independent Variable (IV): The variable that you deliberately change or select. In graphs, the independent variable is always plotted on the horizontal \(x\)-axis.
• Dependent Variable (DV): The variable that you measure for every change in the independent variable. In graphs, the dependent variable is always plotted on the vertical \(y\)-axis.
• Controlled Variables: All other variables that must be kept constant throughout the entire experiment.
What is a Fair Test?
A fair test is an investigation where only the independent variable is allowed to affect the dependent variable. If you do not keep the controlled variables the same, you cannot tell whether your results were caused by changing your independent variable or by an uncontrolled factor!
Memory Trick:
• Independent variable = I change it.
• Dependent variable = Data you collect.
Key Takeaway: Change only one variable at a time (independent), measure the outcome (dependent), and keep everything else strictly the same (controlled) to ensure a fair test.
---2. The 9 CCEA Prescribed Practicals
CCEA specifies nine core practical investigations across Units 1, 2, and 3. You must be familiar with their methods, observations, and key equipment.
Biology Practicals (Unit 1)
1. Food Tests: Identifying nutrients in food samples using standard chemical reagents:
• Starch: Add iodine solution. A positive result changes colour from yellow-brown/orange to blue-black.
• Reducing Sugars (Glucose): Add Benedict's reagent and heat in a water bath at \(\ge 80^\circ\text{C}\). A positive result changes from blue to green, yellow, and finally a brick-red precipitate.
• Protein (Biuret Test): Add Biuret reagent (sodium hydroxide + copper sulphate). A positive result turns from blue to purple/mauve/lilac.
• Fats/Lipids (Emulsion Test): Dissolve the food sample in ethanol, then pour it into water. A positive result produces a cloudy white emulsion.
2. Photosynthesis:
Investigating how light intensity affects the rate of photosynthesis. You place pondweed (such as Elodea) at measured distances from a light source and count the number of oxygen bubbles produced per minute or collect and measure the volume of gas produced.
3. Ecological Sampling:
Using quadrats to sample the abundance and distribution of plant or stationary animal species. Quadrats can be placed randomly using random coordinates to avoid bias, or along a transect line to see how distribution changes across an area.
Chemistry Practicals (Unit 2)
4. Acids, Bases, and Salt Preparation:
Investigating the reaction of acids with insoluble metal oxides or carbonates to make a soluble salt. The general steps are:
1. Gently warm the acid.
2. Add excess insoluble base until no more reacts.
3. Filter off the unreacted excess base using filter paper and a funnel.
4. Gently heat the filtered solution to evaporate the water and leave salt crystals.
5. Reactivity of Metals:
Comparing how different metals react with water, dilute acids, or metal salt solutions (displacement reactions). By observing reactions and temperature changes, you deduce the reactivity order: \(\text{Mg} > \text{Zn} > \text{Fe} > \text{Cu}\).
6. Rates of Reaction:
Investigating how changing factors like temperature, concentration, or surface area affects reaction speed. Common methods include:
• Reacting magnesium or calcium carbonate with hydrochloric acid and measuring gas volume over time using a gas syringe.
• Measuring mass loss on a balance over time as gas escapes.
• The disappearing cross reaction (reacting sodium thiosulfate with acid and timing how long it takes for a drawn cross beneath the flask to be obscured by a precipitate).
Physics Practicals (Unit 3)
7. Resistance / Ohm’s Law:
Investigating the relationship between current (\(I\)) and voltage (\(V\)) for a metallic wire or resistor at constant temperature. An ammeter is connected in series to measure current, and a voltmeter is connected in parallel across the component to measure potential difference (\(V = I \times R\)).
8. Density Determination:
• Regular solids: Measure dimensions with a ruler to calculate volume (\(\text{length} \times \text{width} \times \text{height}\)), measure mass on a balance, then calculate: \(\text{Density} = \frac{\text{mass}}{\text{volume}}\).
• Irregular solids: Measure mass on a balance. Submerge the object in a displacement/Eureka can filled with water; the \(\text{Volume of the object} = \text{volume of water displaced}\) into a measuring cylinder.
9. Forces and Hooke’s Law:
Investigating the extension of a spring as load is added (\(F = k \times e\)).
• Measure original length (\(L_0\)).
• Add slotted masses and record new extended length (\(L\)).
• Calculate extension: \(\Delta L = L - L_0\).
Key Takeaway: Learn the exact reagents, colour changes, and apparatus setups for all 9 Prescribed Practicals, as they form the foundation for Booklet A and Booklet B questions.
---3. Laboratory Safety and Risk Assessment
In Booklet B, you will often be asked to identify hazards, assess risks, and state specific safety precautions. Never write vague answers like "be careful" or "wear a lab coat". Always link the precaution to the specific hazard!
• Corrosive Acids and Alkalis:
Risk: Can cause chemical burns to skin or permanent damage to eyes.
Precaution: Wear safety goggles/safety glasses, wear gloves, or use lower concentrations.
• Flammable Liquids (e.g., Ethanol in Food Tests):
Risk: Ethanol catches fire easily.
Precaution: Never heat ethanol directly with an open Bunsen burner flame. Instead, heat it using a water bath (e.g., water heated from a kettle or hot water with the Bunsen turned off).
• Hot Glassware and Burn Hazards:
Risk: Skin burns from touching hot beakers or tripods.
Precaution: Use tongs or heat-resistant gloves, and leave apparatus on a heatproof mat to cool before touching.
• Toxic or Irritating Gases/Chemicals:
Risk: Inhaling harmful fumes causing breathing difficulties.
Precaution: Work in a well-ventilated room or carry out the experiment in a fume cupboard.
Key Takeaway: Always name the specific hazard and give the exact safety control (e.g., safety goggles for corrosive acids, water bath for flammable ethanol).
---4. Data Collection, Tables, and Graphing
Designing Results Tables
When recording experimental data, follow CCEA table rules:
1. Independent variable goes in the first column (left).
2. Dependent variable readings (including repeat columns and a mean column) go in the columns to the right.
3. Header format: Every column header must include the quantity name and standard unit (e.g., \(\text{Time / s}\) or \(\text{Temperature / }^\circ\text{C}\)).
4. Data cells: Write only raw numbers inside table cells—never write units inside the data cells.
Drawing Perfect Graphs
Graph drawing in Booklet B carries high marks. Follow these rules to get full credit:
• Axes: Plot the independent variable on the \(x\)-axis (bottom) and dependent variable on the \(y\)-axis (side). Label both axes with quantity and units matching your table header.
• Scale: Must be linear, go up in regular intervals (such as \(1, 2, 5, 10\)), and take up more than half (\(>50\%\)) of the graph grid in both directions.
• Plotting: Mark data points neatly and accurately using a small sharp cross '\(\times\)' or a circled dot '\(\odot\)'.
• Line of Best Fit: Draw a single smooth line using a ruler (if linear) or a smooth curved line. Balance points evenly above and below the line, and ignore any anomalies.
• Never connect points dot-to-dot with a zigzag line!
Key Takeaway: Tables must have units in headers only. Graphs must cover more than half the grid, have labeled axes with units, and feature a single smooth line or curve of best fit.
---5. Evaluating Experiments and Scientific Terms
Understanding scientific terminology allows you to evaluate your data like a true scientist.
Handling Anomalies and Calculating the Mean
• Anomalous Result (Outlier): A measurement that does not fit the general pattern or trend shown by other repeats. Anomalies are caused by experimental errors.
• How to calculate the Mean: Always identify and exclude anomalies before calculating an average!
$$\text{Mean} = \frac{\text{Sum of valid/concordant repeats}}{\text{Number of valid repeats}}$$
Example: Three readings for reaction time are \(12.1\text{ s}\), \(12.3\text{ s}\), and \(18.9\text{ s}\).
• Identify \(18.9\text{ s}\) as an anomaly.
• Exclude \(18.9\text{ s}\) and calculate: \(\text{Mean} = \frac{12.1 + 12.3}{2} = 12.2\text{ s}\).
Accuracy, Precision, Repeatability, and Reproducibility
• Accuracy: How close a measured value is to the true value.
• Precision: How close repeated measurements are to each other.
• Repeatability: The experiment is repeated by the same person using the same equipment and method, yielding similar results.
• Reproducibility: The experiment is repeated by different people or using different equipment/laboratories, yielding similar results.
Key Takeaway: Exclude outliers before averaging. Repeatability is the same experimenter getting similar results; reproducibility is different experimenters getting similar results.
---6. Common Exam Pitfalls to Avoid
Examiners frequently highlight the same simple mistakes each year. Make sure you don't lose marks on these:
• Swapping Graph Axes: Remember, Independent variable goes on the bottom (\(x\)-axis), Dependent goes on the side (\(y\)-axis).
• Including Anomalies in the Mean: Always cross out obvious outliers before dividing by the count of remaining valid trials.
• Vague Safety Advice: Stating "be careful with glass" gets 0 marks. Write: "Wear heat-resistant gloves when handling hot beakers to prevent burns."
• Putting Units Inside Data Cells: Units belong only in the table header column.
• Connecting Data Points Dot-to-Dot: Draw a single smooth straight line or curve of best fit.
• Vague Control Variable Answers: Writing "keep everything else the same" gets 0 marks. You must name the exact variable (e.g., "keep the volume and concentration of acid constant").