A shared skill is one method that appears in biology, chemistry and physics, such as percentage change. Retesting it in a new context checks that you own the method and not just one example. It follows the mixed-question work in avoiding labelling every task within double-award practice design.
How do I retest a skill?
- Name the skill in one line, for example “percentage change = change ÷ original × 100”.
- Find the original value. The change is always compared with the starting figure.
- Do it in the first context until it is secure.
- Change the science and the quantity, keeping the same structure.
- Record the result in your log, noting which context was harder.
Worked example (invented data)
Skill: percentage change = (new − original) ÷ original × 100. All data are invented.
Biology. A potato strip has mass 4.0 g. After soaking in salt solution it has mass 3.4 g. Change = 3.4 − 4.0 = −0.6 g. Percentage change = −0.6 ÷ 4.0 × 100 = −15%.
Chemistry. A sample of copper carbonate has mass 5.0 g. After heating it has mass 3.2 g. Change = 3.2 − 5.0 = −1.8 g. Percentage change = −1.8 ÷ 5.0 × 100 = −36%.
Physics. A car speeds up from 20 m/s to 25 m/s. Change = 25 − 20 = +5 m/s. Percentage change = 5 ÷ 20 × 100 = +25%.
Re-check each: 15% of 4.0 is 0.60; 36% of 5.0 is 1.8; 25% of 20 is 5.0. All three match.
The skill is identical. Only the labels differ: mass of tissue, mass of a solid and speed.
The mistake to watch for
A student divides by the wrong value.
Mistaken working (biology): −0.6 ÷ 3.4 × 100 = −17.6%.
The student divided by the new mass. The percentage change must be compared with the original mass, 4.0 g, which gives −15%.
The correction is to underline “original” in step 2. If the question says “from 4.0 g to 3.4 g”, the first number is the original.
Check yourself
All data are invented.
1. Biology: a leaf disc has mass 8.0 g, then 9.2 g after soaking in water. Find the percentage change.
Show answer
Change = 9.2 − 8.0 = +1.2 g. 1.2 ÷ 8.0 × 100 = +15%. Check: 15% of 8.0 = 1.2.
2. Chemistry: the volume of a gas falls from 25.0 cm³ to 20.0 cm³. Find the percentage change.
Show answer
Change = 20.0 − 25.0 = −5.0 cm³. −5.0 ÷ 25.0 × 100 = −20%. Check: 20% of 25.0 = 5.0.
3. Physics: the resistance of a wire rises from 40 Ω to 50 Ω. Find the percentage change.
Show answer
Change = +10 Ω. 10 ÷ 40 × 100 = +25%. Check: 25% of 40 = 10.
Where this leads next
After skill transfer, see how to review your errors as one picture in reviewing patterns without treating two grades as independent quizzes. The mistake log and retest queue can hold the retest dates, and the module practice set has percentage change questions in two sciences.
If one science keeps breaking a skill you already own, our teachers can work through it in online one-to-one Co-ordinated Sciences tuition.