Comparing Similarities and Differences by Feature

Hello again. Last lesson focused on explain answers: linking a cause or feature to a scientific mechanism and then to an effect. A compare question has a different job. You are not mainly explaining why something happens; you are placing two things side by side and making clear, matched statements about them.

In this command-word module, you are building a set of answer structures you can use in any Biology, Chemistry, or Physics topic. Today’s aim is to answer compare questions with paired similarities and differences about the same feature, rather than writing two disconnected lists.

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What “compare” asks you to do

Pearson’s science command-word guide defines “compare and contrast” as giving similarities and differences, rather than writing only about one item.

A reliable working definition is:

Compare = state a similarity and/or difference between two things, using the same feature for both.

The wording of the question matters:

Question wording	What you must include
“State one difference between…”	One clear difference only
“State one similarity between…”	One clear similarity only
“Compare X and Y”	Similarities and differences, unless the question gives a more specific instruction
“Compare the ways…”	Compare the stated process or feature, not unrelated facts
“Compare and explain…”	Make comparisons and give reasons for them

For example, if a question says:

State one difference between salmon and human red blood cells. $(1)$

then one precise comparison is enough:

Salmon red blood cells contain a nucleus, whereas human red blood cells do not.

Do not add shape if the question says “other than shape.” Examiners often include this phrase specifically because an obvious difference has already been ruled out.

Pearson’s examiner report for an International GCSE Biology paper uses exactly this red-blood-cell example.

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The paired-feature rule

The most important comparison habit is this:

Choose one feature, then say what applies to both items for that same feature.

Think of every sentence as having three parts:

Part	Example
Feature being compared	Presence of a nucleus
First item	Salmon red blood cells contain a nucleus
Second item	Human red blood cells do not contain a nucleus

A useful sentence frame is:

[Thing A] has / does / is …, whereas [Thing B] has / does / is …

For similarities, use:

Both [Thing A] and [Thing B] …

Strong paired comparisons

Feature	Strong comparison
Nucleus in red blood cells	Salmon red blood cells contain a nucleus, whereas human red blood cells lack a nucleus.
Energy transfer	Photosynthesis transfers light energy into a chemical energy store, whereas respiration transfers energy from glucose.
State at room temperature	Bromine is a liquid at room temperature, whereas chlorine is a gas.
Circuit arrangement	In a series circuit, components are arranged in one loop, whereas in a parallel circuit they are placed on separate branches.
Wave type	Both sound and light transfer energy, but sound is longitudinal whereas light is transverse.

Each statement compares one clear feature. This allows an examiner to spot the mark point quickly.

Weak, unpaired writing

Photosynthesis uses carbon dioxide. Respiration releases energy. Chlorophyll is needed for photosynthesis. Respiration happens in cells.

There may be correct science here, but it is not clearly structured as a comparison. The statements jump between different features and do not directly tell the examiner how the processes are alike or different.

Repair it by pairing:

Photosynthesis uses carbon dioxide, whereas aerobic respiration produces carbon dioxide.
Photosynthesis requires light energy, whereas aerobic respiration releases energy from glucose.
Both are chemical reactions that occur in living cells.

Now every point is a comparison.

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Do not write two separate lists

A common answer format is:

Plant cells: cell wall, chloroplasts, vacuole.
Animal cells: no cell wall, no chloroplasts, small vacuole.

This may contain useful knowledge, but it makes comparison harder to see. It also makes it easy to forget similarities.

Instead, organise the answer by feature, not by organism:

Feature	Plant cell	Animal cell
Cell membrane	Present	Present
Nucleus	Present	Present
Cell wall	Present	Absent
Chloroplasts	May be present in photosynthesising cells	Absent

You do not need to draw a table in the exam unless it is useful. Convert the table into concise paired sentences:

Both plant and animal cells have a cell membrane and a nucleus. However, plant cells have a cellulose cell wall and may contain chloroplasts, whereas animal cells do not.

This is efficient because it includes a similarity and two differences without repeating whole sentences.

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Choosing comparison points that can earn marks

Before answering, identify the comparison category in the question. The category tells you what is relevant.

If the question asks you to compare…	Useful features might include…
Two cells or organisms	Structures, functions, size, shape, adaptations
Two processes	Reactants, products, energy change, location, conditions
Two materials	Bonding or structure, melting point, conductivity, flexibility
Two circuits	Arrangement, current, potential difference, effect of a component failing
Two energy resources	Renewability, reliability, carbon dioxide emissions, cost
Alpha, beta, and gamma	Nature, charge, penetration, ionisation, behaviour in fields

Suppose the question is:

Compare ionic and covalent substances. $(4)$

A poor plan would be to write everything remembered about ions and molecules. A better plan is to choose four matched categories:

1. What particles or bonds are present
2. Structure
3. Melting or boiling point
4. Electrical conductivity

An answer might be:

Ionic substances contain oppositely charged ions, whereas covalent substances contain atoms joined by shared electrons. Ionic compounds usually have giant ionic lattices, whereas many covalent substances consist of simple molecules. Ionic compounds generally have high melting points, while simple molecular substances have low melting points. Ionic compounds conduct electricity when molten or dissolved because their ions can move, whereas simple covalent substances usually do not conduct.

For a four-mark question, four distinct comparisons are a sensible target. You do not need exactly one sentence per mark, but each extra comparison must add a new feature.

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Similarities count too

Students often treat “compare” as meaning “give differences.” Differences are important, but a full compare question can credit similarities as well. Look actively for them.

For example:

Compare diffusion and osmosis.

A well-balanced response could be:

Both diffusion and osmosis involve the net movement of particles from a region of higher concentration to a region of lower concentration. Diffusion can involve many types of particle, whereas osmosis involves water molecules only. Osmosis requires a partially permeable membrane, whereas diffusion does not necessarily require one.

Notice that the similarity is still precise. Avoid vague statements such as:

Both are important.
Both happen in cells.
Both move things around.

These may be true in some contexts, but they are too broad to show secure scientific understanding.

A short visual planning method can help you separate shared features from differences before you write. The example in this video uses non-science examples, but the planning principle transfers directly to science questions.

For revision notes, a simple three-column organiser is useful:

Only A	Both	Only B
Features unique to the first item	Genuine shared features	Features unique to the second item

But in the exam, turn your notes into full comparative sentences. Do not simply write isolated words in three columns unless the question provides a table.

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Comparison language that makes marks visible

Use comparison words deliberately. They make your meaning unambiguous.

For differences

• whereas
• while
• unlike
• in contrast
• however
• more or less than
• higher or lower than

Examples:

Magnesium is more reactive than copper.

In a parallel circuit, current has more than one path, whereas in a series circuit there is only one path.

Unlike a metal, a simple molecular substance does not usually conduct electricity.

For similarities

• both
• similarly
• each
• in both cases

Examples:

Both acids and alkalis can conduct electricity when dissolved in water because they contain mobile ions.

Both transverse and longitudinal waves transfer energy without transferring matter overall.

Be careful: words such as more, faster, larger, and higher need a reference point.

Weak:

The wire has more resistance.

Better:

The longer wire has greater resistance than the shorter wire.

Best, if the question asks for explanation too:

The longer wire has greater resistance than the shorter wire because charge carriers collide with ions more often along the longer length.

The first two are comparisons. The final sentence adds an explanation, which should only be included when it is asked for or clearly useful for the marks.

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Compare is not explain

This is an especially important distinction after the previous lesson.

Command word	Main task	Example prompt
Describe	Report features or a pattern	Describe the graph.
Explain	Give why or how, using linked science	Explain why the rate increases.
Compare	Give matched similarities and differences	Compare the two cells.

Consider:

Compare conduction and convection. $(2)$

A direct comparison answer is:

Conduction transfers thermal energy through collisions between particles, whereas convection transfers thermal energy by the bulk movement of a fluid. Both transfer thermal energy from hotter regions to cooler regions.

If the question instead says:

Explain why convection currents form in a liquid,

you need causal links:

Liquid at the bottom is heated, so its particles move further apart and the liquid becomes less dense. It rises while cooler, denser liquid sinks to replace it.

The difference is not length. It is the job of the answer.

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A fast method for any compare question

Use this routine in past-paper practice.

1. Underline the two things being compared

For example: a series circuit and a parallel circuit.

If three things are named, make sure your statements cover the relevant pairings. Do not accidentally compare only the first two.

2. Circle the comparison focus

If the question says “Compare their electrical conductivity,” do not write about melting point or appearance. The focus controls what counts as relevant.

3. Count the marks

For a one-mark “state one difference” question, write one complete paired difference.

For a three- or four-mark comparison, aim for several distinct features. Brief planning prevents repeating the same idea in different words.

4. Write paired sentences

Use “both” for similarities and “whereas” or “while” for differences.

5. Check each point with this question

Have I said something about both items for the same feature?

If not, rewrite the statement.

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Common comparison errors

Error	Why it is risky	Repair
Writing only about one item	It is description, not comparison	Add the corresponding feature of the other item
Giving two facts on unrelated features	The comparison is unclear	Match the feature first: structure with structure, product with product
Repeating one idea	It may earn only one mark	Choose a new category, such as structure, function, or condition
Ignoring similarities	It misses part of “compare”	Include at least one accurate shared feature when the wording is broad
Giving an explanation when only comparison is needed	It wastes time and can introduce errors	State the matched difference first; explain only if asked
Ignoring a restriction	The obvious answer may be excluded	Read phrases such as “other than,” “using the data,” or “in terms of…” carefully
Vague vocabulary	“Better,” “stronger,” or “different” do not identify a feature	Name the measurable or scientific feature precisely

The red-blood-cell examiner report gives a real example of the restriction problem: candidates lost credit when they used shape after being told to give a difference other than shape.

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Build compare into your flashcards and error log

Add this command-word flashcard:

Flashcard front	Flashcard back
How do I answer “compare”?	Choose the same feature for both items. Use “both” for a similarity and “whereas” or “unlike” for a difference. Give enough distinct features for the marks and follow any stated focus or restriction.

When you mark topic questions, use short error codes:

• CP1 — wrote about only one item
• CP2 — features were not paired
• CP3 — missed an obvious similarity
• CP4 — repeated the same comparison
• CP5 — ignored the focus or restriction
• CP6 — used vague scientific vocabulary

For each comparison mistake, make a correction in this format:

Feature: …
A: …
B: …

Then cover it and write one paired exam sentence from memory. This turns a past-paper error into a reusable comparison pattern rather than a one-off correction.

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Key takeaways

• A compare answer places two or more things side by side using the same feature.
• Use both to show a similarity and whereas, while, or unlike to show a difference.
• A clear comparison is usually better than two separate lists of facts.
• Follow the question’s exact scope: it may ask for one difference, one similarity, or a broader comparison.
• Match your number of distinct comparison points to the mark allocation.
• Do not turn a compare answer into an explanation unless the question asks for reasons as well.

Next, you will practise the command words used in quantitative questions: calculate, determine, estimate, and show that.