Topic 2 of 5
Choose a changing quantity
Match the measurement to the reaction.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Choose a changing quantity
Match the measurement to the reaction.
| Measurement | Suitable situation | Rate evidence |
|---|---|---|
| Gas volume against time | Gas-producing reaction; gas can be collected | Volume increase per unit time |
| Mass against time | A gas escapes from an open reaction vessel | Mass loss per unit time |
| Time to a fixed visible endpoint | A precipitate obscures a mark or a colour reaches a chosen endpoint | Shorter time means a faster average approach to that same endpoint |
For a gas syringe, check airtight connections and free plunger movement. For mass loss, prevent splashes so that lost liquid is not mistaken for escaped gas. Start timing consistently. A subjective colour or visibility endpoint is less precise than a clear instrumental measurement, so use the same criterion and repeat.
Worked example
Calculate an average rate
A reaction produces 36 cm3 of gas in its first 40 s.
- Average rate = change in gas volume / time interval.
- 36/40 = 0.90 cm3 s-1.
- This is an average over 40 s; the rate may have fallen throughout that interval.
Answer
0.90 cm3 s-1. State both interval and units.
Check your understandingTwo experiments reach the same endpoint in 20 s and 40 s. Which is faster?Think it through, then reveal the answer
The 20 s experiment. For the same defined amount of change, 1/time can compare relative rates; it does not provide an absolute gas-volume rate unless that volume is known.