When a table changes two conditions, only some pairs of rows give a fair comparison. A fair pair differs in exactly one condition, so any change in the rate can be linked to that condition.
This lesson belongs to interpreting enzymes and experimental rates. It builds on separating initial rate from total product formed.
What is the pairing rule?
Follow these steps before you write a comparison.
- List every condition that differs between the two rows.
- If more than one condition differs, the pair is unfair.
- If exactly one differs, name it as the tested factor.
- Compare the rates, with units, and state the direction of change.
Write “rate” in your answer, not just “result”, so the claim is about speed.
Worked example: four trials
An original table shows the initial rate of an enzyme reaction under four conditions.
| Trial | Temperature | Substrate concentration | Initial rate (cm³ per min) |
|---|---|---|---|
| P | 20 °C | 1% | 3 |
| Q | 20 °C | 2% | 6 |
| R | 40 °C | 1% | 9 |
| S | 40 °C | 2% | 14 |
Question 1: Which pair shows the effect of substrate concentration at 20 °C?
Trials P and Q. Both are at 20 °C, and only the substrate concentration differs (1% and 2%). The rate rises from 3 to 6 cm³ per minute, so doubling the concentration doubled the rate in this range.
Question 2: Which pair shows the effect of temperature at 1% substrate?
Trials P and R. Both use 1% substrate, and only temperature differs. The rate rises from 3 to 9 cm³ per minute, a threefold increase.
Question 3: A student compares trial P and trial S and says “concentration makes the rate 14 ÷ 3 times higher.” What is wrong?
Both temperature and concentration changed between P and S. The difference in rate (3 to 14) cannot be linked to one factor, so the pair is unfair.
Now look at R and S, which differ only in concentration. The rate rises from 9 to 14, an increase of 5 (14 − 9), which is less than double. This suggests that at 40 °C something else, such as the amount of enzyme, is starting to limit the rate.
The mistake that costs marks
The common mistake is to compare the largest and smallest values and credit one factor.
| Student wrote | Problem | Better version |
|---|---|---|
| “Temperature raised rate from 3 to 14” | Two factors changed | “At 1% substrate, raising temperature from 20 °C to 40 °C raised the rate from 3 to 9” |
| “Concentration has no effect” | Based on an unfair pair | “At 40 °C, doubling the concentration raised the rate from 9 to 14” |
| “The table proves temperature matters most” | Compares different changes | “Each factor raised the rate in the pairs that test it” |
If two factors changed together in a real experiment, say what the data can and cannot separate.
Check yourself
Use the table above. State the pair of trials that shows the effect of temperature at 2% substrate, and give the change in rate.
Answer
Trials Q and S. Both use 2% substrate, and only temperature differs (20 °C and 40 °C).
The rate rises from 6 to 14 cm³ per minute, an increase of 8 cm³ per minute (14 − 6).
What to study next
Next, learn how to treat a value that does not fit the pattern in explaining an anomalous point. The graph evidence and fair-comparison lab has more tables to practise on.
If you want a teacher to give you new tables and have you justify each pair, see online one-to-one Biology tuition or the one-hour trial class (from RM50).