A comparison is fair only when a single factor changes. To compare experiments, pick the standard one and pair it with each experiment that changes one condition.
This lesson is part of rates of reaction. It follows reading rate graphs.
Which experiments can I compare?
List what differs between each pair. If the list has one item, the pair is fair. If it has two or more, the result could be caused by either.
This is the same rule used in comparing food processing methods, applied to reactions.
Worked example: marble chips and acid
The table is original. Each experiment collected 40 cm³ of carbon dioxide from 1 g of marble chips and 50 cm³ of hydrochloric acid.
| Experiment | Chip size | Temperature | Acid | Time to 40 cm³ |
|---|---|---|---|---|
| 1 | Large | 30 °C | Dilute | 100 s |
| 2 | Small | 30 °C | Dilute | 55 s |
| 3 | Large | 50 °C | Dilute | 48 s |
| 4 | Large | 30 °C | Twice as concentrated | 52 s |
Rates. Rate = 40 ÷ time, in cm³ per second.
- Experiment 1: 0.40
- Experiment 2: 0.73
- Experiment 3: 0.83
- Experiment 4: 0.77
Fair pairs. Experiments 1 and 2 differ only in chip size. Experiments 1 and 3 differ only in temperature. Experiments 1 and 4 differ only in concentration.
Conclusions. Smaller chips, higher temperature and stronger acid each raise the rate compared with Experiment 1.
The mistake that costs marks
The common slip is to compare Experiment 2 with Experiment 3 and conclude that temperature matters more than chip size. Two factors differ in that pair, so neither conclusion is valid.
| Step | Wrong | Right |
|---|---|---|
| Choose the pair | Experiments 2 and 3 | Experiments 1 and 3 |
| Factors that differ | Size and temperature | Temperature only |
| Conclusion | Temperature beats size | 50 °C is faster than 30 °C |
| Wording | “It is better” | “The rate is higher because…” |
To rank the factors, a new experiment would change them one at a time with the same starting amounts.
A catalyst example
Hydrogen peroxide breaks down to give oxygen. Without a catalyst, 60 cm³ of oxygen forms in 300 s, which is 60 ÷ 300 = 0.2 cm³ per second.
With manganese(IV) oxide added, 60 cm³ forms in 50 s, which is 60 ÷ 50 = 1.2 cm³ per second. The rate is 6 times higher, yet the total volume is unchanged. The catalyst is not used up, so its mass is the same at the end.
Check yourself
A student compares Experiment 2 (55 s) with Experiment 4 (52 s) and writes that stronger acid speeds up the reaction more than smaller chips. Is the conclusion valid? Which experiment should each factor be compared against?
Answer
It is not valid. Experiments 2 and 4 differ in two factors: chip size (small against large) and acid concentration (dilute against twice as concentrated).
For a fair chip-size comparison, pair Experiment 2 with Experiment 1. For a fair concentration comparison, pair Experiment 4 with Experiment 1. Each pair changes only one factor.
What to study next
Continue with explaining collision ideas at Science depth, which gives the reason behind each factor. Then try the rates of reaction practice set.
If you want a teacher to give you fresh tables to check, see online one-to-one Science tuition.