Scientific Investigation Skills
Grade 11
1. Investigations, Experiments and the Scientific Method
Physical Sciences investigates physical and chemical phenomena through scientific inquiry. The same skills are used in every practical from Grade 10 to Grade 12: asking a question that can be tested, planning a fair test, recording and graphing results, and drawing a conclusion. Both examination papers ask questions on these skills, usually about an experiment described in the question, and CAPS sets aside time for them at the start of Grade 12.
An experiment or an investigation?
- An experiment is conducted to verify or test a known theory, for example verifying Boyle's law or Ohm's law.
- An investigation is an experiment that is conducted to test a hypothesis: the result or outcome is not known beforehand, for example finding out how the length of a wire affects its resistance.
The steps of a scientific investigation
- Identify and question a phenomenon: write an investigative question, list all the possible variables and write a testable hypothesis.
- Plan: identify the independent, dependent and controlled variables, list the apparatus, and plan the steps, with more than one trial, safety precautions, a fair test and a control where one is needed.
- Carry out the plan and record the results in a table.
- Present the results on a graph.
- Analyse the results: look for patterns and relationships, and do the calculations.
- Draw a conclusion that answers the investigative question, say whether the results support the hypothesis, and evaluate the validity of the conclusion.
- Communicate the investigation in a report.
Practical work in Grades 10, 11 and 12
- Grades 10 and 11: two prescribed experiments for formal assessment (one chemistry and one physics) and one project, plus four recommended experiments for informal assessment.
- Grade 12: three prescribed experiments for formal assessment, plus three recommended experiments for informal assessment.
- A project is one of: constructing a device (for example an electric motor), building a physical model to solve a challenge, or a practical investigation. A poster may form part of the presentation.
Every practical in these study guides links back to this guide, so you can plan and write it up in the same way each time.
Writing up a practical report
| Heading | What it contains |
|---|---|
| Aim or investigative question | what the practical sets out to find or verify |
| Hypothesis | a testable prediction (for an investigation) |
| Variables | the independent, dependent and controlled variables |
| Apparatus | a list of the equipment, with measuring instruments |
| Method | numbered steps someone else could follow, with safety precautions |
| Results | a table of measurements with headings and units |
| Analysis | a graph and any calculations |
| Conclusion | the answer to the investigative question |
| Evaluation | sources of error and how to improve the practical |
In the examination
A question describes a practical and gives its results. You are asked to name the variables, write an investigative question or hypothesis, draw or read a graph, and state a conclusion. In the November 2025 Paper 1, for example, learners measured the frequency detected from an ambulance siren at different speeds, and had to write down the independent variable, a controlled variable and the conclusion.
Worked Examples
- 1(a) The heating curve of water is known theory: the practical verifies it.
- 2(b) The outcome is not known beforehand, and the practical tests a hypothesis.
2. Investigative Questions, Hypotheses and Variables
Every investigation looks for a relationship between two quantities: one that you change and one that you measure. Everything else that could affect the result must stay the same.
Definition: independent variable
The independent variable is the variable that is deliberately changed in an investigation.
Definition: dependent variable
The dependent variable is the variable that is measured in an investigation; it changes in response to the independent variable.
Definition: controlled variable
A controlled variable is a variable that is kept constant in an investigation so that it cannot affect the results.
On a graph of the results, the independent variable is plotted on the horizontal axis and the dependent variable on the vertical axis.
List all possible variables
Before planning, list everything that could affect the dependent variable. For the reaction of magnesium with hydrochloric acid, the rate could depend on the concentration of the acid, the temperature, the surface area of the magnesium (ribbon or powder), the mass of magnesium, the volume of acid and whether a catalyst is present. Choose one as the independent variable; the others become controlled variables.
Definition: investigative question
An investigative question is a question about the relationship between the independent variable and the dependent variable that an investigation can answer.
Writing an investigative question
- Name the independent variable and the dependent variable.
- Ask about the relationship between them: "What is the relationship between the pressure and the volume of a fixed amount of gas at constant temperature?" or "How does the temperature of hydrochloric acid affect the rate of its reaction with magnesium?"
- Make it a question that an experiment can answer, ending with a question mark.
Definition: hypothesis
A hypothesis is a testable prediction of the relationship between the independent variable and the dependent variable.
Writing a testable hypothesis
- State the expected relationship between the independent and dependent variables: "If the length of a nichrome wire increases, its resistance will increase."
- It is a statement, not a question, and an experiment must be able to show whether it is supported or not.
- The results support a hypothesis or they do not: they never prove it.
Identifying the variables in a practical
| Practical | Independent variable | Dependent variable | Controlled variables |
|---|---|---|---|
| Ohm's law | current in the resistor (set with a rheostat) | potential difference across the resistor | temperature; the resistor used |
| Boyle's law | pressure of the gas | volume of the gas | temperature; amount (mass) of gas |
| Rate and temperature | temperature of the acid | rate of reaction (time to react) | concentration and volume of acid; mass and surface area of magnesium |
| Doppler effect | speed of the source | frequency detected | frequency of the source; the listener stays at rest |
Common mistakes
- Giving "the results" or "the time" as the dependent variable: name the quantity measured, such as the time for the magnesium to disappear.
- Writing a hypothesis as a question, or one that cannot be tested.
- Writing "everything else" as the controlled variable: name a quantity, such as the temperature.
- Swapping the variables: the one you choose and change is the independent variable.
Worked Examples
- 1The learner chooses and changes the pressure by pushing the plunger.
- 2The volume is measured at each pressure.
- 3The temperature is kept at room temperature and the amount of air stays the same.
- 1The independent variable is the length of the wire and the dependent variable is its resistance.
- 2Investigative question: "What is the relationship between the length of a nichrome wire and its resistance?"
- 3Hypothesis: "If the length of the nichrome wire increases, its resistance will increase."
3. Planning an Investigation and a Fair Test
A good plan lets someone else repeat the practical and get the same results, and makes sure that only the independent variable affects the dependent variable.
Apparatus and the sequence of steps
- List appropriate apparatus, with measuring instruments that suit the quantity and its size: a burette or pipette for accurate volumes of solution, a measuring cylinder for rougher volumes, a thermometer, a stopwatch, an electronic balance, and an ammeter in series and a voltmeter in parallel.
- Write the method as numbered steps: set up the apparatus, change the independent variable to its first value, measure the dependent variable, and repeat for each value.
- Take readings over a wide range of values of the independent variable, with at least five values so that a graph shows the pattern.
Definition: fair test
A fair test is an investigation in which only the independent variable is changed and all the other variables are kept constant.
Conditions that ensure a fair test
- Change only the independent variable.
- Keep every controlled variable constant, and say how: use the same volume of acid, keep the temperature constant by waiting for the gas to cool, or open the switch between readings so the resistor does not heat up.
- Measure the dependent variable in the same way each time.
Setting an appropriate control
A control is a set-up in which the factor being tested is left out, so the results can be compared with it. To show that a catalyst speeds up a reaction, run the same reaction without the catalyst. To show that iron rusts only when both water and oxygen are present, compare a nail in water and air with nails kept from water or from air.
More than one trial
Repeat each measurement at least three times and calculate the average. More than one trial minimises experimental errors: a single misread value stands out, and the average is closer to the true value. A reading that does not fit the pattern (an anomalous reading) should be checked and repeated.
Safety precautions
- Wear safety goggles, and gloves when handling acids or bases.
- Dilute a concentrated acid by adding the acid slowly to water, never water to the acid.
- Work with toxic or irritating gases, such as sulfur dioxide, nitrogen dioxide or chlorine, in a fume cupboard or a well-ventilated room.
- Point a test tube being heated away from yourself and others, and keep flammable liquids such as alcohols away from open flames.
- Open the switch between readings in a circuit, so the wires and the battery do not overheat.
Errors, accuracy and precision
- Random errors scatter readings on both sides of the true value, for example when a stopwatch is started a little early or late. Repeating and averaging reduces them.
- Systematic errors shift every reading in the same direction, for example a meter that does not read zero before it is used, or always reading a scale from an angle (parallax). Repeating does not remove them: check and correct the instrument.
- Accurate results are close to the true value; precise results are close to each other.
- Reliable results give the same values when the practical is repeated; a valid investigation is a fair test that really measures what it sets out to measure.
Common mistakes
- Changing two variables at once, so the effect of either cannot be known.
- Giving a general precaution such as "be careful": name the hazard and what to do about it.
- Thinking that repeating a practical removes a systematic error.
Worked Examples
- 1Independent variable: the concentration of the acid (for example 0,5; 1,0; 1,5 and 2,0 mol·dm⁻³). Dependent variable: the time for the magnesium to disappear.
- 2Controlled variables: the same volume of acid, the same length (mass) of magnesium ribbon, and the same temperature.
- 3Method: add the magnesium to 50 cm³ of the first acid, start the stopwatch and stop it when the magnesium has disappeared. Repeat three times and average, then repeat for each concentration.
- 4Safety: wear goggles, handle the acid with care, and keep flames away because hydrogen gas forms.
- 1Pushing the plunger in quickly does work on the gas and warms it.
- 2The temperature is a controlled variable, so the gas must return to room temperature before the reading.
4. Recording Data and Drawing Graphs
Results are recorded in a table while the practical is done, and then drawn on a graph so the pattern can be seen.
Recording results in a table
- Put the independent variable in the first column and the dependent variable next to it.
- Write the quantity and its unit in each column heading, for example "Current (A)", and write only numbers in the cells.
- Give every reading in a column to the same number of decimal places, as read from the instrument.
- For repeated trials, add a column for each trial and one for the average.
| Current (A) | Potential difference (V) |
|---|---|
| 0 | 0 |
| 0,2 | 1 |
| 0,4 | 2 |
| 0,6 | 3,6 |
| 0,8 | 4 |
| 1 | 5 |
Results of an Ohm's law experiment, recorded in a table with the independent variable first.
Drawing an accurate graph
- Give the graph a heading: "Graph of [dependent variable] versus [independent variable]".
- Put the independent variable on the horizontal axis (x-axis) and the dependent variable on the vertical axis (y-axis).
- Label both axes with the quantity and its unit.
- Choose a scale that goes up in equal steps and uses more than half of the grid on each axis.
- Plot each point accurately, as a small cross or a dot in a circle.
- Draw a line of best fit (or a smooth curve of best fit) that passes through as many points as possible, with the others balanced on both sides. Do not join the points dot to dot.
- Leave an anomalous point out of the line of best fit, and repeat that reading if you can.
The results from the table plotted as a graph: the line of best fit passes through the origin and leaves out the anomalous reading at 0,6 A.
Sketch graphs
A sketch graph shows the shape of a relationship without plotting values. Label both axes with the quantities (and units), draw the correct shape, and mark any important values, such as where the graph cuts an axis. No grid or scale is needed.
Testing for inverse proportion
A curve that goes down may or may not be inversely proportional. To test it, plot the dependent variable against the inverse (1 ÷ x) of the independent variable. For Boyle's law, a graph of volume against pressure is a curve, but a graph of volume against 1 ÷ pressure is a straight line through the origin: the volume is inversely proportional to the pressure.
Boyle's law results: the volume against the pressure is a curve (top diagram); the volume against 1 ÷ pressure is a straight line through the origin (bottom diagram), so pV is constant, 1200 kPa·cm³.
Common mistakes
- Putting the units in the table cells instead of the column heading.
- Axes without units, or a scale that jumps (1, 2, 5, 10) or crowds the points into a corner.
- Joining the points dot to dot instead of drawing a line of best fit.
- Forcing the line of best fit through an anomalous point.
Worked Examples
- 1The length is chosen and changed: it is the independent variable, on the horizontal axis.
- 2The resistance is measured: it is the dependent variable, on the vertical axis.
- 3Label the axes "Length (cm)" and "Resistance (Ω)", and head the graph "Graph of resistance versus length".
5. Interpreting Results and Drawing Conclusions
Once the results are on a graph, identify the pattern, work out what the gradient and intercept mean, and state a conclusion that answers the investigative question.
Patterns and relationships
The four shapes to recognise in a sketch diagram of y against x.
- Directly proportional: a straight line through the origin. Doubling x doubles y, and y ÷ x is constant.
- Linear but not proportional: a straight line that does not pass through the origin. y increases by the same amount each time, but doubling x does not double y.
- Inversely proportional: a curve in which doubling x halves y, so y × x is constant. Confirm it by plotting y against 1 ÷ x: a straight line through the origin.
- No relationship: a horizontal line; y does not change as x changes.
Gradient and intercept
gradient = Δy ÷ Δx
Take two points far apart on the line of best fit, not two of the plotted results, and give the gradient its unit (the y unit divided by the x unit). The gradient often is a quantity: the gradient of a potential difference-current graph is the resistance, the gradient of a velocity-time graph is the acceleration. The intercept is where the graph cuts an axis, for example the initial velocity on a velocity-time graph.
Drawing conclusions from tables and graphs
- The conclusion answers the investigative question and relates the independent variable to the dependent variable.
- Describe the trend: "As the speed of the source increases, the detected frequency increases."
- Only say "directly proportional" or "inversely proportional" when the graph shows it: a straight line through the origin, or a straight line through the origin against 1 ÷ x. In the November 2025 Paper 1 memorandum, a conclusion that said "inversely proportional" for a decreasing trend earned no marks.
- Say whether the results support the hypothesis.
Evaluating the validity of conclusions
A conclusion is valid only if the investigation was a fair test and the data are good enough. Ask:
- Was only the independent variable changed, and were the controlled variables kept constant?
- Were there enough readings, over a wide enough range, to show the pattern?
- Were the trials repeated, and do the repeats agree?
- Are there anomalous readings, and were they explained or repeated?
- Does the conclusion claim more than the data show, for example about values outside the range measured?
Common mistakes
- Calculating the gradient from two plotted points that are not on the line of best fit.
- Saying "proportional" for any increasing graph.
- A conclusion that only repeats the results, without relating the two variables.
- Saying the results "prove" the hypothesis.
Worked Examples
- 1gradient = Δy ÷ Δx = (5 − 0) V ÷ (1 − 0) A = 5 V·A⁻¹ = 5 Ω
- 2The graph is a straight line through the origin, so the potential difference is directly proportional to the current.
- 1R ÷ L = 0,055; 0,055; 0,055; 0,055 Ω·cm⁻¹: the same each time.
- 2A constant ratio means a straight line through the origin.
- 1Two readings are too few to show a pattern, and single trials cannot show whether a reading is anomalous.
- 2The conclusion claims more than the data show: it names every reaction, but only one reaction was tested.