Linking Causes, Mechanisms, and Effects in Explanations

Welcome back. Last lesson focused on describe: reporting what is shown or what happens, without adding reasons. This lesson makes the crucial next step: explain. In an explanation, you must connect scientific ideas so the examiner can see why an outcome happens.

For Edexcel International GCSE Science, this matters across every topic: why a reaction becomes faster, why air enters the lungs, why heat is lost from the skin, or why a material has a particular property. Your goal is not to write a long paragraph. It is to produce enough accurate, linked scientific points for the marks available.

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What “explain” means in Edexcel

Edexcel’s command-word taxonomy says that an explanation needs a justification or example of a point, with an element of reasoning. In other words, an answer must do more than state a fact.

A describe answer might say:

The rate of reaction increases as temperature increases.

An explain answer adds the scientific reason:

At a higher temperature, particles have more kinetic energy and move faster. They collide more frequently, so there are more successful collisions each second and the reaction rate increases.

The second answer is not better because it is longer. It is better because every part has a job: it begins with a cause, gives a mechanism, and reaches the required effect.

A useful exam definition is:

Explain = use scientific ideas to show why or how something happens.

The short clip below models this with a reaction-rate question and also reminds you to use the mark allocation.

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The cause–mechanism–effect structure

Most strong “explain” answers can be built from three linked parts.

Part	What it does	Typical wording
Cause or starting feature	Identifies the relevant condition, change, or adaptation	“At a higher temperature…”, “The diaphragm contracts…”, “The walls are thin…”
Mechanism	States the scientific process that connects cause to outcome	“…particles move faster and collide more often…”, “…the volume increases and pressure falls…”
Effect	Answers what happens as a result	“…so the reaction rate increases.”

The structure is not a rule that every answer must have exactly three sentences. In a two-mark question, you may write one concise sentence containing two linked points. In a three- or four-mark question, you will usually need a longer chain.

The key test is this:

Could you put the word “so” between your ideas and still have correct science?

For example:

The diaphragm contracts and flattens. This increases the volume of the thorax, so the pressure in the lungs decreases and air enters.

The chain works because each statement causes the next one. Compare it with this weak answer:

The diaphragm contracts because we need oxygen.

The statement is vague, skips the pressure mechanism, and does not explain how contraction makes air enter.

Three reliable sentence patterns

Use these as flexible frames, not as wording to memorise blindly.

1. A change in conditions

When [condition changes], [particles or system change in a scientific way], so [observable outcome].

Example:

When temperature increases, reactant particles gain kinetic energy and move faster, so collisions occur more frequently and the reaction rate increases.

2. An adaptation question

[Feature] means that [scientific mechanism], so [function is improved].

Example:

Alveoli have thin walls, which gives a short diffusion distance, so oxygen diffuses into the blood quickly.

3. A control-system question

[Stimulus or change] causes [response], which leads to [effect on the body or system].

Example:

Arterioles supplying the skin widen, increasing blood flow close to the skin surface. More heat is transferred to the surroundings, so heat loss increases.

Notice that the starting point is not always a “cause” in everyday language. In an adaptation question, it may be a feature. What matters is the logical chain from that feature to its benefit.

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Marks are awarded for scientific links, not for filler

Your previous lesson established that marks give a clue about answer depth. This remains true for “explain,” but do not treat it as a word-count rule.

A two-mark explanation often needs two distinct linked scientific ideas:

Explain why reaction rate increases when temperature increases. $(2)$
Particles gain kinetic energy and move faster. They collide more frequently, so the reaction rate increases.

A three-mark explanation normally needs a fuller chain:

Explain why air enters the lungs when breathing in. $(3)$
The diaphragm contracts and flattens, increasing the volume of the thorax. This decreases the pressure in the lungs below atmospheric pressure, so air moves into the lungs.

This could gain credit for:

1. diaphragm contracts or flattens
2. volume increases
3. pressure decreases or air flows in down a pressure gradient

One sentence can contain several marking points. Equally, three long sentences can still earn only one mark if they repeat the same idea.

Plan from the question’s endpoint

Before writing, identify exactly what you must explain.

Question ending	Your final effect should address
“Explain why reaction rate increases”	Increased reaction rate
“Explain how air enters the lungs”	Air moving into lungs
“Explain why heat loss increases”	Greater transfer of heat to surroundings
“Explain why cooling water is needed”	Maintaining temperature and protecting enzymes

This prevents a common mistake: writing correct science that does not answer the actual question.

For example, if the question asks why temperature is monitored in a fermenter, a long account of how microorganisms reproduce may be interesting but irrelevant. The endpoint is temperature control and its consequences for enzyme action.

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Learn from real mark schemes: reconstruct the chain

A mark scheme is not just something to read after you have lost marks. It is evidence of the separate ideas Edexcel is prepared to credit. The extracts below show three explanation chains from an Edexcel International GCSE Science Double Award mark scheme.

Here is how to turn those lists of points into fluent exam answers.

Example 1: temperature control in fermentation

The mark scheme credits ideas including reactions releasing heat, cooling water removing heat, maintenance of optimum temperature, and prevention of enzyme denaturation.

A connected answer could be:

Respiration or fermentation releases heat energy, which could raise the temperature. The temperature is monitored so cooling water can remove excess heat. This maintains an optimum temperature and prevents the enzymes from denaturing.

This is a strong explanation because it has a complete purpose:

• reactions release heat
• heat could cause overheating
• cooling removes heat
• correct temperature is maintained
• enzymes keep their shape and function

For a three-mark answer, you would not necessarily need every point. But including a clear chain makes it much easier to reach the available marks.

Example 2: breathing in

The diaphragm contracts and flattens, increasing the volume of the thorax. This reduces the pressure in the lungs, so air enters down a pressure gradient.

The important detail is volume, not “more space.” Science mark schemes reward terms that have a defined meaning and fit the mechanism.

Example 3: losing heat from the skin

During vasodilation, arterioles supplying the skin widen, increasing blood flow through capillaries near the surface. More thermal energy is transferred from the blood to the surroundings by radiation and convection, so heat loss increases.

Avoid:

Capillaries widen.

At this level, that is not the correct biological mechanism. The small wording difference matters because the vessel that widens is an arteriole.

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Do not merely list features: explain their benefit

Adaptation questions are a frequent source of lost marks in Biology. They may ask you to explain how a structure is adapted for its function.

A list is a description:

Alveoli have a large surface area, thin walls, and a good blood supply.

It could earn some marks if the question asks you to describe alveoli. But if the command is explain, each feature needs its consequence.

Alveoli have a large surface area, so many oxygen molecules can diffuse across at the same time. Their walls are one cell thick, giving a short diffusion distance. A continuous blood supply maintains a concentration gradient, so diffusion into the blood remains rapid.

Each feature has been developed into a reason.

This technique applies widely:

Topic	Description only	Explanation
Root hair cell	“It has a long extension.”	“Its long extension gives a large surface area, so mineral ions and water can be absorbed efficiently.”
Catalyst	“A catalyst is present.”	“A catalyst provides an alternative pathway with lower activation energy, so more collisions are successful each second.”
Fuse	“It has a thin wire.”	“If the current becomes too high, the thin wire heats up and melts, breaking the circuit and preventing overheating.”
Black surface	“It is black and dull.”	“A black, dull surface is a good absorber of infrared radiation, so it gains thermal energy quickly.”

The best final clause always returns to the function or outcome named in the question.

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Common errors and how to repair them

Error	Why it loses marks	Better approach
Giving a description	It states what happens but not why.	Add the underlying process or scientific principle.
Giving a fact with no link	The examiner cannot see why the fact matters.	Follow the fact with “this means…” or “so…” and a valid consequence.
Skipping the mechanism	The answer jumps from cause to result.	Include the middle science, such as pressure, collisions, diffusion distance, or energy transfer.
Repeating a point in different words	Repetition is not a new marking point.	Add the next causal step instead.
Using vague language	“More space,” “it works better,” and “things move faster” are often too imprecise.	Name the relevant quantity or process: volume, pressure, particles, diffusion, current, or energy.
Including incorrect extra science	A correct answer can be weakened by a contradiction.	Give only the chain needed for the marks.

A useful final check is to read your answer as a sequence of because and so links:

Higher temperature gives particles more kinetic energy, so they move faster, so collisions are more frequent, so the rate increases.

If one connection is scientifically wrong, repair it before moving on.

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Build this into your past-paper and flashcard routine

For the next three months of content learning, make one command-word flashcard that is genuinely useful under exam pressure:

Flashcard front	Flashcard back
How do I answer “explain”?	Identify the required outcome. Give a relevant cause or feature, state the scientific mechanism, and link it to the outcome. Include enough distinct scientific points for the marks.
What is the difference between describe and explain?	Describe reports what happens or what is shown. Explain gives scientific reasons for why or how it happens.
How do I check an explanation?	Check the chain: cause or feature, mechanism, effect. Use precise vocabulary. Remove repeated or irrelevant points.

When marking topic questions, use an explanation error code in your error log:

• EX1 — described instead of explained
• EX2 — missing mechanism
• EX3 — correct idea, but no link to the outcome
• EX4 — vague or imprecise vocabulary
• EX5 — incorrect causal link
• EX6 — not enough distinct points for the marks

For each EX error, rewrite the answer once from memory using the mark scheme’s ideas, not its exact wording. This is far more useful than copying a model answer: it trains you to produce the chain independently in a new topic.

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Key takeaways

• Explain requires scientific reasoning, not just a statement of what happens.
• Build answers around a linked cause or feature, mechanism, and effect.
• Use the mark allocation to judge the number of distinct scientific ideas needed, but avoid padding your answer with repeated phrases.
• Be precise: use terms such as volume, pressure, concentration gradient, kinetic energy, and denature when they are the actual mechanism.
• Mark schemes are useful because they reveal the individual points that can be connected into a coherent answer.
• In past papers, always check whether your final clause answers the exact outcome asked for.

Next, you will learn how to answer compare questions using paired similarities and differences about the same scientific feature.