Writing Precise Scientific Definitions

Welcome back. Last lesson covered short recall commands such as name, state, give, and identify. The key habit was to answer the exact question, in the exact quantity needed, then stop.

This lesson focuses on a different short-answer skill: writing a definition. In Edexcel questions, this may be written as “define” or “state what is meant by…”. A definition question is not asking for everything you know about a term. It is asking for the scientific meaning of that term, with enough precision to hit every marking point.

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What Edexcel means by a definition

In Pearson’s command-word taxonomy, the closest official wording is “state what is meant by”. The important point is that there can be more than one acceptable sentence, provided that the scientific meaning is correct.

So your aim is not:

“Can I remember the exact words from CGP?”

Your aim is:

“Have I included every scientific idea that makes this term different from similar terms?”

For example, “a fuel is something that burns” is not quite enough. Lots of substances may burn, but the definition must say what makes burning them useful as a fuel: they release heat energy.

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A definition is a scientific boundary

A good definition draws a boundary around a term. It tells the examiner what must be true for something to count as that thing.

Most GCSE science definitions can be built from these parts:

1. What sort of thing or process is it?
   A substance, particle, force, transfer, reaction, structure, or process.

2. What is the key action or feature?
   For example, moving, releasing, sharing, reacting, or transferring.

3. What condition, direction, or result makes it precise?
   For example, from high to low concentration, using energy, through a membrane, or releasing thermal energy.

A compact template is:

Term = scientific category + defining feature(s).

For example:

An element is a substance made of only one type of atom.

• Category: substance
• Defining feature: only one type of atom

The definition should not be too broad:

“An element is a chemical.”

That could include compounds and mixtures, so it does not draw the right boundary.

It should also not be circular:

“An element is an elemental substance.”

That uses a version of the word without explaining it.

And it should not merely give an example:

“An element is carbon.”

Carbon is an example of an element, not the definition.

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Marks tell you how many ideas are needed

Use the mark allocation as a clue. A one-mark definition is often one precise idea. A two-mark definition usually needs two linked ideas. This is the same mark-scheme thinking you used last lesson, but definitions need especially careful wording.

Here are real examples from a recent Pearson mark scheme.

The fuel question is worth two marks. A mark-scheme-ready answer is:

A fuel is a substance that releases heat energy when burned.

It contains both required ideas:

Marking idea	Where it appears
The substance is burned	“when burned”
Heat or thermal energy is released	“releases heat energy”

The word order can change:

A substance that releases thermal energy when it burns.

This still contains the same scientific meaning. Learn the ideas, not a fragile word-for-word script.

However, do not assume that every long answer scores more. In a definition question, extra writing is useful only if it makes the meaning more precise. Otherwise, it wastes time and can introduce an error.

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Precision: the small words that decide marks

Definitions often differ from almost-correct answers because of one missing scientific word. The missing word is usually a condition, a direction, or the thing that moves.

Here are examples across Biology, Chemistry, and Physics.

Term	Too vague or incomplete	Precise definition
Element	“Made of the same atoms.”	A substance made of only one type of atom.
Fuel	“Something that burns.”	A substance that releases heat energy when burned.
Covalent bond	“Atoms sharing electrons.”	The electrostatic attraction between a shared pair of electrons and the nuclei of bonded atoms.
Diffusion	“Substances spread out.”	The net movement of particles from a region of higher concentration to a region of lower concentration.
Osmosis	“Water moving through a membrane.”	The net movement of water molecules from a region of higher water potential to lower water potential through a partially permeable membrane.
Active transport	“Particles move against the concentration gradient.”	The movement of particles from a lower concentration to a higher concentration using energy.
Power	“Energy used.”	The rate at which energy is transferred or work is done.

The precise versions are not automatically the only accepted wordings. For example, an osmosis definition may use dilute to concentrated solution instead of higher to lower water potential. But the same essential meaning must be there:

• it is water, not any substance
• it is net movement
• there is a direction from higher to lower water potential
• a partially permeable or selectively permeable membrane is involved

The video below models this marking-point approach for transport definitions.

A useful warning: in science, ordinary English can be close but not exact enough. For instance:

“Diffusion is particles moving from where there are lots to where there are fewer.”

This may communicate the general idea, but in an exam the more secure wording is higher concentration to lower concentration. It is the scientific relationship being tested.

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Build definitions from marking points, not from memory alone

When revising a topic in your CGP guide, do not simply highlight a definition and hope it sticks. Break it into the parts an examiner can award.

Take covalent bond. The Pearson mark scheme separates it into two points:

1. a shared pair of electrons
2. electrostatic attraction between that shared pair and the nuclei

A weak answer is:

Covalent bonding is sharing electrons.

It is not entirely wrong, but it describes what happens during bonding rather than defining the bond itself. It also misses “pair” and the electrostatic attraction to nuclei.

A secure answer is:

A covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

This is a good example of why you must not rely on a simplified phrase from a revision note if it leaves out the mark-scheme detail.

Use this method whenever you meet a definition:

1. Find the core noun

Ask what the term is: a substance, a process, a force, a structure, or a quantity.

• Fuel is a substance.
• Diffusion is a process.
• Power is a rate.
• An isotope is an atom of an element with a particular nuclear relationship.

Starting with the right category prevents muddled answers.

2. Identify the features that distinguish it

Ask: what makes this different from similar terms?

For example, active transport differs from diffusion because it:

• moves particles from lower to higher concentration
• requires energy

If you miss energy, your answer becomes incomplete. If you say “high to low concentration,” you have defined diffusion instead.

3. Put the ideas into one clear sentence

You do not need impressive language. You need a sentence the examiner can tick.

Active transport is the movement of particles from a lower concentration to a higher concentration using energy.

4. Check for a missing qualifier

Before moving on, scan for words that definitions commonly need:

• net
• particles or water molecules
• higher and lower
• per unit
• energy
• nuclei
• only
• through a membrane
• attraction

These small words often contain the mark.

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Do not confuse a definition with an explanation

A definition gives the meaning of a term. An explanation gives a reason or mechanism for an event. You will study explain questions properly later, but keeping this distinction now will improve your short answers.

For example:

Define a catalyst. $(2)$

A precise definition is:

A catalyst is a substance that increases the rate of a reaction without being chemically changed at the end.

An explanation question might instead ask why a catalyst increases rate. That would require the mechanism:

It provides an alternative reaction pathway with lower activation energy.

These are related, but they answer different questions. The first defines what a catalyst is; the second explains how it changes reaction rate.

Similarly:

Define power.

A definition:

The rate of energy transfer or the rate of doing work.

Not:

Power is useful because it tells us how quickly machines work.

That is a comment about why power matters, not its scientific meaning.

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A fast exam routine for “define” or “state what is meant by”

Use this routine in a past paper:

1. Read the term and the mark allocation.
   One mark usually needs one key fact. Two or three marks usually need several linked details.

2. Say the definition silently in plain English first.
   This helps you recall the concept rather than panic over exact wording.

3. Replace vague words with scientific vocabulary.
   Replace “lots” with “high concentration,” “goes through” with “net movement,” or “needs effort” with “uses energy.”

4. Check the boundary.
   Could your definition accidentally describe a different term? If yes, add the missing condition.

5. Stop when every marking point is present.
   Do not turn a two-mark definition into a paragraph.

For definitions worth two or more marks, use one complete sentence rather than disconnected notes. The ideas are linked, and the grammar helps you avoid leaving out a condition.

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Turn definitions into useful flashcards

Since you are building flashcards alongside your CGP revision, make definition cards that test recall of the marking points, not recognition of a highlighted sentence.

A good card format is:

Front	Back
Define fuel.	A substance that releases heat energy when burned. Must include: burns; releases heat or thermal energy.
Define element.	A substance made of only one type of atom. Must include: one type of atom.
Define covalent bond.	Electrostatic attraction between a shared pair of electrons and the nuclei of bonded atoms. Must include: shared pair; nuclei.
Define osmosis.	Net movement of water molecules from higher to lower water potential through a partially permeable membrane. Must include: water; direction; membrane.

Keep the back of each card in two parts:

• a full answer you can say or write
• a short must include checklist

This means you can accept your own valid wording while still checking whether you included every scientific idea.

When marking past papers, label definition mistakes clearly in your error log:

• D0 — did not know the definition
• D1 — missed a marking point
• D2 — used vague or inaccurate vocabulary
• D3 — gave an example, function, or explanation instead of a definition
• D4 — wrote a contradictory extra statement

For example, if you write “osmosis is movement of water through a membrane” and lose marks, that is not necessarily a whole-topic knowledge gap. It is mainly D1: missing direction and water potential or concentration detail. Your fix is a definition flashcard, not rereading an entire chapter.

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Key takeaways

• In Edexcel, a definition may be asked as “define” or “state what is meant by.”
• There is usually no single magic sentence. Different wording can score if it includes the required scientific ideas.
• Use the number of marks to judge how many linked parts your definition needs.
• Strong definitions use precise vocabulary for the thing involved, its direction or condition, and the result.
• Avoid definitions that are too broad, circular, examples only, or explanations instead of meanings.
• Build flashcards around must include marking points and record definition mistakes separately in past-paper review.

Next, you will move from defining a term to describe questions, where the task is to report relevant features, changes, trends, or sequences without adding an unsupported cause.