How fast does a reaction go? What makes it faster or slower? In this lesson, you will design and conduct a practical investigation to answer these questions like a real scientist — controlling variables, collecting reliable data and drawing evidence-based conclusions.
You have a tablet of antacid, a glass of water and a stopwatch. You want to find out whether crushing the tablet makes it dissolve faster.
Write down your answers before reading on:
Variables, hypotheses and valid tests
A good scientific investigation is like a fair race — only one thing changes at a time, and everything else stays the same.
Every practical investigation has three types of variables:
A hypothesis is a testable prediction. The best hypotheses clearly state the relationship between variables:
All investigations must be safe. Before starting, identify hazards (hot equipment, corrosive chemicals, glassware) and describe how to manage them. Wear safety glasses, tie back long hair, and follow your teacher's instructions. For temperature-based investigations, teacher supervision is essential.
Methods for timing and quantifying speed
Reaction rate can be measured in several ways depending on the reaction. The key is to choose a method that is both measurable and repeatable.
| Method | What you measure | Best for |
|---|---|---|
| Timing | Time for reaction to complete | Reactions with a clear endpoint |
| Gas collection | Volume of gas produced over time | Reactions producing CO2 or H2 |
| Mass loss | Mass decrease as gas escapes | Carbonate + acid reactions |
| Colour change | Intensity of colour over time | Iodine clock reactions |
| Temperature change | Temperature increase or decrease | Exothermic or endothermic reactions |
A classic Stage 5 investigation examines how surface area affects the reaction rate between calcium carbonate (marble chips) and hydrochloric acid:
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
You could measure the volume of CO2 gas collected every 30 seconds, or time how long it takes for the mass to stop decreasing. Powdered marble should react faster than large chips because the powder has a greater surface area.
From raw numbers to scientific insight
Data only becomes knowledge when it is processed, represented and interpreted.
Raw data should be organised in a clear table with headings and units. Calculate averages for repeated trials. If one result is very different from the others (an anomaly), check for experimental error before deciding whether to include it in your average.
Line graphs are ideal for showing how a variable changes over time. Plot the independent variable on the x-axis and the dependent variable on the y-axis. Draw a line of best fit to show the overall trend.
A steep line on a volume-vs-time graph means a fast reaction. A flat line means the reaction has stopped. If you compare two conditions on the same graph, the steeper line shows the faster reaction.
A good conclusion:
"A hypothesis is just a guess." No — a hypothesis is an informed, testable prediction based on scientific understanding. It should specify the relationship between variables.
"One trial is enough if you are careful." No — even careful investigators can have unexpected results. Repeating trials and calculating averages improves reliability and confidence.
Australian industries depend on controlling reaction rates. In aluminium production, the Bayer process uses controlled temperature and concentration to extract alumina from bauxite ore. Engineers carefully monitor reaction conditions to maximise yield and minimise waste.
In agriculture, fertiliser production relies on the Haber process, where nitrogen and hydrogen are combined under controlled temperature and pressure with an iron catalyst. Australian farmers use these fertilisers to improve crop yields — but overuse can lead to unwanted reactions in waterways.
Indigenous fire management also involves controlled reaction rates. By adjusting fuel load (surface area) and temperature, Aboriginal fire practitioners control combustion rate to achieve desired ecological outcomes.
| Time (s) | 0.5 mol/L acid (mL) | 1.0 mol/L acid (mL) | 2.0 mol/L acid (mL) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 30 | 12 | 24 | 48 |
| 60 | 22 | 45 | 88 |
| 90 | 30 | 62 | 95 |
| 120 | 35 | 70 | 96 |
1. In an investigation into how temperature affects reaction rate, what is the dependent variable?
2. Which of the following is a controlled variable in an investigation of how surface area affects the rate of reaction between marble and acid?
3. A student repeats a timing measurement three times and obtains 45 s, 47 s and 28 s. What should the student do with the 28 s result?
4. Which hypothesis is most testable and scientifically sound?
5. A student concludes: "My results did not match my hypothesis, so my experiment was a failure." What is the best evaluation of this statement?
1. Define independent, dependent and controlled variables. For an investigation into how surface area affects the reaction rate between marble chips and hydrochloric acid, give one example of each variable. 4 MARKS
2. A student wants to investigate how the concentration of sodium thiosulfate solution affects the rate of its reaction with hydrochloric acid. Write a hypothesis for this investigation and explain why it is testable. Describe how the student could measure the reaction rate. 4 MARKS
3. Using the data table from Activity 2, describe how you would represent this data on a graph. Explain what the shape of each line would tell you about reaction rate, and how you could use the graph to support a conclusion about the effect of concentration. 4 MARKS
Go back to your Think First answer. Has your understanding changed?
B — The dependent variable is what you measure. In this case, it is the time taken for the reaction to finish (or another measure of reaction rate).
C — The volume and concentration of the acid must be kept constant so that only surface area differs between trials. The size of the chips is the independent variable, and time and mass of CO2 are dependent variables.
A — 28 s is very different from 45 s and 47 s, so it is likely an anomaly caused by an error. The student should investigate what went wrong (e.g., did they start the timer late?) and exclude it if an error is identified.
D — A good hypothesis specifies the variables, predicts a direction of change, and provides a scientific reason. Option D does all three: it identifies concentration and rate, predicts an increase, and explains it using collision theory.
B — Unexpected results are valuable in science. They may reveal problems with the method (such as uncontrolled variables) or they may challenge existing understanding and lead to new discoveries. All results contribute to scientific knowledge.
Model answer: The independent variable is the factor deliberately changed: the surface area of the marble chips (e.g., powdered vs large chips). The dependent variable is what is measured: the time taken for the reaction to finish, or the volume of gas produced per minute. A controlled variable is something kept constant: the volume and concentration of hydrochloric acid, the temperature of the acid, or the mass of marble chips used. Keeping these the same ensures that any difference in reaction rate is due to surface area alone.
Model answer: Hypothesis: "If the concentration of sodium thiosulfate is increased, then the reaction with hydrochloric acid will be faster, because there are more thiosulfate particles per unit volume, leading to more frequent collisions with acid particles." This is testable because concentration can be precisely measured and changed, and reaction rate can be measured by timing how long it takes for a cross placed under the flask to disappear as sulfur precipitate forms. The student could time this for different concentrations and compare the results.
Model answer: I would plot time (seconds) on the x-axis and volume of gas (mL) on the y-axis. Each concentration would have its own line. The steeper the line, the faster the reaction. The 2.0 mol/L line would be steepest at the start and level off first, showing the fastest reaction. The 0.5 mol/L line would be the shallowest and level off last. This supports the conclusion that higher concentration increases reaction rate, because more acid particles per unit volume lead to more frequent successful collisions, as predicted by collision theory.
Race through the lab! Identify variables, spot anomalies and blast your way to experimental mastery.
Tick when you have finished all activities and checked your answers.