In a mixed paper, nothing tells you which science the next question is, or whether it needs a sentence or a sum. The first skill is to read the command word and pick the right kind of answer. This is the first lesson of mixed original practice for Combined Science 0653.
What does each kind of answer look like?
A calculation answer has three visible parts: the equation, the substitution with units, and a final value with a unit. An explanation answer has a different shape: an observation, the science idea that causes it, and the link between them, joined by “because” or “so”.
Both are marked on the same paper, so switching quickly matters. A quick label in the margin, such as “calc” or “explain”, costs two seconds and sets the shape of the answer.
How do I decide quickly?
- Underline the command word.
- Ask: is the answer a number, or a reason?
- Check what the stem gives you. Numbers with units point to an equation. A described change points to a model.
- Write the shape of the answer before the detail.
Worked example
All data here is invented for practice.
A cyclist travels 90 m in 12 s. (a) Calculate the average speed. (b) Explain why the cyclist needs more energy to travel at 10 m/s than at 5 m/s.
Part (a) is a calculation. The command word is “calculate”.
Equation: speed = distance ÷ time
Substitute: speed = 90 m ÷ 12 s
Answer: 7.5 m/s
Check: 7.5 × 12 = 90, so the value is consistent.
Part (b) is an explanation. The command word is “explain”, and no equation is requested.
At the higher speed the cyclist has more kinetic energy, so more energy must be transferred from the cyclist to the bicycle. The air resistance is also larger at a higher speed, so more work must be done against it each second. Both effects mean more energy is needed.
Notice the chain: observation (more energy), idea (kinetic energy and air resistance), link (so more work is done).
The mistake to watch for
A common slip is answering an explain question with a number, or a calculate question with a sentence.
Mistaken answer to (b): “10 ÷ 5 = 2, so twice the energy.”
The student reached for arithmetic because the stem had numbers. The question asked for a reason, not a ratio, and kinetic energy depends on speed squared, so the energy does not simply double.
The correction is to name the shape first. “Explain” means a chain of reasons in words. Use the numbers only as supporting evidence.
Check yourself
1. A runner covers 240 m in 30 s. Calculate the average speed.
Show answer
speed = distance ÷ time = 240 m ÷ 30 s = 8 m/s. Check: 8 × 30 = 240.
2. Is each task a calculation or an explanation? (a) Suggest why a plant wilts on a hot day. (b) Work out the density of a block of mass 60 g and volume 20 cm³.
Show answer
(a) Explanation: the command words are “suggest why”. (b) Calculation: density = mass ÷ volume = 60 g ÷ 20 cm³ = 3.0 g/cm³.
3. Write one sentence that uses a number as evidence inside an explanation: “The reaction was faster at 40 °C than at 20 °C.”
Show answer
One possible answer: “At 40 °C the particles have more energy, so more collisions are successful each second, which is why the reaction was faster than at 20 °C.” The numbers support the reasoning but do not replace it.
Where this leads next
The next skill is deciding which science idea an explanation needs: identify the science model needed by each task. When you want to test the switch itself, use the mixed practice set.
If you score on calculations but lose marks on explanations, or the other way round, a teacher in online one-to-one Combined Science tuition can listen to your answers and rebuild the weaker half.