Cambridge IGCSE Combined Science 0653: Command words and keywords

Cambridge IGCSE Combined Science (0653) and Co-ordinated Science (0654) cover Biology, Chemistry and Physics in a single qualification. The syllabus includes a glossary of command words that may differ slightly from the standard Cambridge list. Command word meaning depends in part on context—check the question and mark allocation.

Command words in Combined Science 0653/0654

Command word	Combined Science definition	What to do
Define	Formal statement or equivalent paraphrase of the term(s)	One clear definition
What do you understand by / What is meant by	Definition plus relevant comment on significance or context	Definition + brief comment; match mark value
State	Concise answer with little or no supporting argument	Short; e.g. numerical answer from inspection
List	Number of points; generally one word each; no elaboration	Do not exceed specified number
Describe	State in words (diagrams where appropriate) the main points	Account of what/how; no “why”
Explain / Give a reason / Give reasons	Reasoning or reference to theory; make clear why something happens	Use “because”; cause and effect
Compare	Similarities and/or differences	Comparative language
Suggest	Apply knowledge to context	Apply to scenario
Calculate	Work out; show working; units	Correct formula; units
Outline	Main points	No detail
Evaluate	Judge; weigh up	Conclusion with evidence

Describe and Explain may be coupled

When a question says “Describe and explain”, you must do both—describe what happens and explain why. Do not do only one.

Keywords across Biology, Chemistry and Physics

Combined Science draws from all three sciences. Key terms from each:

Biology (Combined Science)

Term	Use correctly	Avoid
Osmosis	Water; partially permeable membrane	Including solutes
Diffusion	Down concentration gradient	Against gradient
Active transport	Against gradient; energy	Without membrane
Enzyme	Biological catalyst; protein	”Chemical”
Optimum	Best conditions	”Ideal”
Denature	Shape change; enzyme fails	”Destroy”
Absorption	Into blood	Assimilation
Assimilation	Into cells	Absorption
Kidney	Excretes urea	Bladder
Photosynthesis	Light + CO₂ + water → glucose + oxygen	Incomplete equation
Respiration	Glucose + oxygen → CO₂ + water + energy	Fermentation
Mitochondria	Aerobic respiration	Ribosome
Allele	Form of a gene	Gene
Genotype	Genetic makeup	Phenotype
Phenotype	Observable characteristic	Genotype

Chemistry (Combined Science)

Term	Use correctly	Avoid
Ionic bond	Electrostatic attraction between ions	”Shared”
Covalent bond	Shared pair of electrons	”Transfer” for covalent
Alkane	Saturated; C–C single bonds	Alkene
Alkene	Unsaturated; C=C double bond	Alkane
Combustion	Reaction with oxygen	”Burning”
Oxidation	Loss of electrons	Gain of oxygen only
Reduction	Gain of electrons	Loss of oxygen only
Neutralisation	Acid + base → salt + water	Incomplete
Exothermic	Releases energy	”Hot”
Endothermic	Takes in energy	”Cold”
Catalyst	Speeds up; unchanged	”Used up”

Physics (Combined Science)

Term	Use correctly	Avoid
Speed	Distance / time; scalar	Velocity
Velocity	Displacement / time; vector	Speed
Weight	Force; N	Mass (kg)
Mass	kg	Weight (N)
Work done	F × d; J	”Effort”
Power	Energy / time; W	”Strength”
Efficiency	(useful / total) × 100%	Wrong formula
Current	A	”Flow”
Potential difference	V	”Voltage”
Resistance	V/I; Ω	Wrong formula
Conduction	Through solids	Convection
Convection	In fluids	Conduction
Radiation	Infrared; no medium	Conduction
Refraction	Change in speed; bending	”Bending” only

What examiner reports say

• Define—precise definition; “What do you understand by” needs definition + context.
• List—do not exceed the number asked; one word per point typically.
• State—concise; no lengthy explanation.
• Describe and explain—both required when coupled.
• Suggest—apply to the scenario; generic answers score poorly.
• Syllabus terminology—use correct scientific terms across all three sciences.

Tips for Combined Science 0653

1. Check the command word—List = brief; Explain = why; Describe = what.
2. “What do you understand by”—definition + brief comment on significance.
3. Describe and explain—do both when asked.
4. List—only the number asked for; no extra.
5. Use syllabus terms—Biology, Chemistry and Physics each have specific vocabulary.
6. Past papers + mark schemes show acceptable wording for each science.

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Based on Cambridge IGCSE Combined Science 0653 syllabus. Check the latest syllabus for your exam series.