Understanding vs Memorising in IGCSE Chemistry

Students preparing for 0620 often ask whether chemistry is a “memory subject” or an “understanding subject.” The honest answer is that it is both, and the exam penalises you for treating one kind of content as the other. Memorising an explanation produces a rigid answer that breaks when the question is phrased differently. Trying to “understand” the reactivity series from first principles during the exam wastes time when you should simply know the order.

The skill is knowing which content falls into which category.

Content that must be memorised

Some 0620 content has no underlying logic accessible at IGCSE level. You cannot derive flame colours from theory — you must simply know that potassium gives a lilac flame and sodium gives a yellow one. Attempting to “understand” these wastes time. Learn them, card them, and move on.

Memory-first content includes:

• All qualitative analysis tables: cation tests, anion tests, gas tests, flame colours
• The reactivity series order
• Colours of specific compounds and precipitates
• Precise definitions as stated in the syllabus
• The names, formulae, and functional groups of organic homologous series
• Specific reagents and conditions for reactions (e.g., concentrated sulfuric acid as a catalyst for the hydration of ethene)
• The formulae of common polyatomic ions: hydroxide (OH-), sulfate (SO4 2-), nitrate (NO3-), carbonate (CO3 2-)

For this content, flashcards and spaced repetition are the right tools. Spend no time trying to reason your way to these facts during revision.

Content that must be understood

Other content follows logical principles. If you understand the principle, you can apply it to any question, even one you have never seen before. Memorising the answer to one specific question gives you that question and nothing else.

Understanding-first content includes:

• Why ionic compounds conduct when molten but not when solid. The principle: conduction requires charged particles that are free to move. In a solid ionic lattice, ions are fixed. When melted or dissolved, they are free. Apply this principle to any electrolyte question.

• Why rate of reaction increases with temperature. The principle: higher temperature means particles have more kinetic energy, move faster, collide more frequently, and a greater proportion of collisions exceed the activation energy. This explains every temperature-rate observation in the syllabus.

• Why metals higher in the reactivity series are extracted by electrolysis. The principle: more reactive metals form more stable compounds, so more energy is needed to separate them. Carbon reduction does not provide enough energy for metals above carbon.

• How covalent bonding explains the properties of simple molecular substances. The principle: the molecules themselves are held together by strong covalent bonds, but the forces between molecules (intermolecular forces) are weak. Low melting points and poor conductivity follow logically.

• Why adding water to a concentrated solution decreases the rate of reaction. The principle: fewer solute particles per unit volume means fewer collisions per second.

For these concepts, the revision method is not flashcards but explaining from scratch. Close your book and explain, out loud or in writing, why a particular phenomenon occurs. If you can only state the fact without the reasoning, you have memorised but not understood.

The danger of memorising explanations

The most common mistake is memorising model answers to past paper explanation questions. The student learns that the answer to “Why does the rate increase when temperature increases?” is “Particles have more energy, move faster, collide more often, and more collisions exceed the activation energy.” This works perfectly — until the examiner asks the question differently: “Explain why the reaction between marble chips and acid is faster at 40 degrees C than at 20 degrees C.”

The student who memorised the model answer now hesitates. Is it the same question? It looks different. The student who understands the collision theory recognises it as the same principle in a new context and answers confidently.

How to tell if you have understood rather than memorised: can you explain the concept to a friend who has never studied chemistry? Can you apply it to a scenario you have not seen in a past paper? Can you explain what would happen if the conditions were reversed? If yes to all three, you understand it.

A practical sorting exercise

Go through the syllabus checklist point by point. For each statement, ask: is this a fact I must know, or a concept I must understand?

Mark each one:

• M for memorise: definitions, specific observations, reagents, formulae, colours
• U for understand: explanations of why, how, cause-and-effect relationships
• B for both: some content requires you to know the fact and understand why. Example: you must memorise that Group I reactivity increases down the group (M), and you must understand why — the outer electron is further from the nucleus, so it is lost more easily (U).

This sorting exercise takes about an hour for the full syllabus and gives you a personalised study plan. Apply flashcards to the M items and active recall explanations to the U items.

How the exam tests each type

The command words in the question tell you which type of knowledge is being tested:

• State, give, name, identify: These test recall. The examiner wants a fact.
• Explain, suggest a reason, account for: These test understanding. The examiner wants reasoning.
• Describe: This can test either, depending on context. “Describe the test for hydrogen” is recall. “Describe and explain the trend in reactivity of Group VII” requires both.

Reading the command word correctly prevents you from writing a long explanation when a one-word answer was needed, or from writing a bare fact when the marks are for the reasoning. Both errors are common and both are avoidable.

The revision implication

If you have limited time — and every student does — allocate it based on the type of content:

• Memory content: short, daily sessions using spaced repetition. Ten minutes a day, every day.
• Understanding content: longer, less frequent sessions where you work through explanations, draw diagrams, and attempt practice questions. Thirty minutes, three or four times a week.
• Both: start with understanding (so the fact makes sense), then card the fact for long-term retention.

Students who spend all their time on memory work score well on factual questions but lose marks on explanations. Students who spend all their time on understanding often forget basic facts under exam pressure. The balance produces the highest marks.