12 Common GCSE Science Mistakes That Cost Marks

1. Confusing "describe" and "explain"

A description tells what happens. An explanation tells why. Answering "explain" with a description loses half the marks.

2. Forgetting units

A numerical answer without units is usually worth zero on the final mark, even if the number is correct. Circle the required unit as you read the question.

3. Wrong significant figures

"Give your answer to 3 significant figures" is a real requirement, not a suggestion. Wrong sig figs lose a mark on top of the correct answer.

4. Missing key phrases in 6-mark answers

Mark schemes reward specific phrasings. "Particles collide more often" earns 1 mark. "The rate of successful collisions increases because more particles have kinetic energy greater than the activation energy" earns 2-3.

5. Not showing working in calculations

Correct answer only = full marks if right, zero if wrong. Full working = partial credit even with a wrong final answer. Always show working.

6. Confusing genotype and phenotype

Genotype = the alleles (e.g. Bb). Phenotype = the observable trait (e.g. brown eyes). Swapping these in genetics questions is one of the most common Grade 5-6 errors.

7. Getting exothermic and endothermic backwards

Exothermic = energy released to surroundings (temperature goes UP). Endothermic = energy absorbed from surroundings (temperature goes DOWN). Under exam pressure, students routinely swap these.

8. Not identifying independent, dependent and control variables

Required practical questions almost always ask about variables. Independent = what you change. Dependent = what you measure. Control = what you keep the same. Learn the three categories cold.

9. Missing the "human error vs equipment error" distinction

Reading a scale incorrectly = human error. A thermometer with 1°C resolution = equipment (systematic) limitation. Asked to "suggest an improvement," specifying which type matters — different suggestions apply to each.

10. Drawing sloppy graphs

Missing axis labels, missing units, wrong scale, plot points not clearly marked, no line of best fit. Any one of these costs a mark on graph questions. Do it properly the first time.

11. Assuming the practical you did in class is exactly what's asked

Exam practical questions often vary the equipment or method from what your school used. The mark scheme wants the principles, not your school's specific method. Read the question's method carefully before answering.

12. Running out of time on the last question

The final application question is often the highest-mark single question on the paper. Students who mismanage time on questions 1-15 arrive at question 16 with 5 minutes left and score 2 marks out of 8. Time budget: 1 minute per mark, and stick to it.

How much this actually costs

Across a Combined Science paper (100 marks), a student making 4-5 of these mistakes typically loses 12-18 marks. That's the difference between two grades. Fixing them isn't glamorous — it's just discipline, developed through past-paper drilling with someone marking honestly.