Cambridge IGCSE Combined Science 0653: Frequent mistakes

Cambridge Principal Examiner Reports for Combined Science 0653 identify recurring misconceptions across Biology, Chemistry and Physics components. Combined Science draws from all three—these errors appear frequently.

Biology errors

Cell magnification

Confusing the relationship between actual size, image size and magnification. Candidates multiply when they should divide.

Fix: Actual size = image size ÷ magnification. Check units. Show working.

Protein composition

Selecting proteins as containing only carbon, hydrogen and oxygen. Proteins must contain nitrogen (from amino acids).

Fix: Proteins: C, H, O, N (and often S). Carbohydrates: C, H, O only.

Gas transport

Thinking oxygen enters cells via active transport. Oxygen enters by diffusion (down concentration gradient).

Fix: Oxygen: diffusion. Glucose: can be active transport (e.g. gut). Learn which transport is used where.

Asexual reproduction

Selecting “photographic negatives” of parents instead of identical offspring. Asexual reproduction produces genetically identical clones.

Fix: Asexual = identical to parent. Sexual = variation.

Food chains

Confusing primary consumers with producers. Producers (plants) are first; primary consumers eat them.

Fix: Producer → primary consumer → secondary consumer. Producer = plants.

Photosynthesis vs. respiration

Thinking plant respiration removes CO₂ from air. Photosynthesis removes CO₂; respiration releases it.

Fix: Photosynthesis: CO₂ in, O₂ out. Respiration: O₂ in, CO₂ out.

Chemistry errors

Electrolysis

Uncertainty about which product forms at which electrode. For dilute sulfuric acid, oxygen forms at the positive electrode.

Fix: Learn electrolysis rules for aqueous solutions. Anode: often O₂ (from OH⁻). Cathode: H₂ or metal if less reactive than H.

Electrical conductivity

Not understanding that brass conducts in all states (solid—alloy); sodium chloride only when molten or dissolved; naphtha doesn’t conduct.

Fix: Metals/alloys: conduct when solid. Ionic: conduct when molten or in solution. Covalent (molecular): often don’t conduct.

General exam technique

Using the resource

When questions refer to a diagram, table or graph, answers must use that information. Describing from memory often scores poorly.

Fix: Reference specific details from the stimulus.

Precision in language

Use syllabus terminology. Vague wording (“it gets bigger” instead of “it expands”) scores partial credit.

Fix: Use correct scientific terms from the syllabus.

Show working

For calculations, show all steps. Missing working risks losing method marks.

Fix: Formula → substitute → answer → units.

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Based on Cambridge IGCSE Combined Science 0653 Principal Examiner Reports (2021–2025).

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