Top Mistakes OxfordAQA IGCSE Chemistry Students Make (and How to Avoid Them)

Every One of These Is Fixable

If you are working through this list and recognising yourself in more than one section, take a breath: every mistake here is fixable, and most of them take far less revision time to correct than a student expects. OxfordAQA IGCSE chemistry common mistakes tend to cluster in a small number of topics rather than being spread evenly across the whole specification, and five areas account for a disproportionate share of lost marks: rate of reaction, metals, chemical bonds (ionic, covalent and metallic), the properties of acids and bases, and carbon compounds as fuels. This guide walks through each one honestly: the wrong approach students often take, why it costs marks, and the correct technique to replace it with.

Rate of Reaction

Mistake: Describing "speed" Without Measurable Quantities

A very common answer to a rate-of-reaction question says something like "the reaction goes faster" without stating what is actually being measured. Rate of reaction is always the amount of reactant used, or the amount of product formed, over a given time. If a question asks you to describe how you would measure a rate, name a measurable quantity: volume of gas collected, mass lost from a reacting flask, or time taken for a solution to become cloudy. "It goes faster" earns nothing on its own; "the volume of gas produced per minute increases" does.

Mistake: Confusing the Factors That Increase Rate With Their Reasons

Temperature, concentration, pressure, surface area and catalysts all increase the rate of reaction, but students frequently state the factor without the collision-theory reasoning behind it, or vice versa. The correct technique links both together every time: "increasing the concentration increases the frequency of collisions between reacting particles, which increases the rate of reaction." Drop either half and you lose a mark, because the mark scheme usually splits credit between the factor and the mechanism.

Wrong: "Catalysts make reactions faster."
Right: "A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react, increasing the rate of reaction without the catalyst itself being used up."

Metals

Mistake: Muddling Oxidation and Reduction in Displacement Reactions

Displacement reactions, where a more reactive metal displaces a less reactive one from a compound, are a reliable source of lost marks because students reverse which metal is oxidised and which is reduced. The rule is consistent: the metal that loses electrons is oxidised, and the metal ion that gains electrons is reduced. If you are asked to write an ionic equation for a displacement reaction, write out the half-equations for each metal separately first, decide which one is losing electrons, and only then combine them; trying to write the full ionic equation in one step is where errors creep in.

Mistake: Forgetting Which Metals React With Which Reagents

You are expected to recall and describe the reactions, if any, of potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper with water or dilute acids, and to place them in order of reactivity. A frequent error is assuming every metal on the list reacts with both water and dilute acid; copper reacts with neither, and several metals react only slowly or only with one of the two reagents. Learn the reactivity series as an ordered list with the specific reactions attached to each metal, not as a vague sense of "more reactive metals react more."

Mistake: Getting Extraction Method Backwards

Metals less reactive than carbon (such as iron) can be extracted from their oxides by reduction with carbon. Metals more reactive than carbon (such as aluminium) need electrolysis instead, because carbon cannot remove the oxygen from their compounds. Students sometimes state the extraction method without connecting it to the metal's position relative to carbon in the reactivity series, which is exactly the reasoning the mark scheme wants to see stated explicitly.

Chemical Bonds: Ionic, Covalent and Metallic

Mistake: Drawing Ionic Bonding as Sharing

This is one of the most persistent OxfordAQA IGCSE chemistry errors: students draw dot-and-cross diagrams for an ionic compound showing shared electron pairs, which is covalent bonding, not ionic bonding. In ionic bonding, electrons transfer completely from a metal atom to a non-metal atom, forming charged ions held together by electrostatic attraction; there is no sharing at all. If your diagram shows an electron pair sitting between two atoms, check whether you meant to be drawing a covalent molecule instead.

Mistake: Explaining Properties Without Naming the Correct Force

When asked to explain why a simple molecular substance has a low melting point, a common wrong answer talks about "weak covalent bonds" breaking. This is incorrect: the covalent bonds inside the molecule are strong, and it is the weak intermolecular forces between separate molecules that are overcome on melting or boiling. Mixing these two up is one of the most heavily penalised chemical bonds ionic covalent and metallic errors, because it shows a genuine misunderstanding rather than a slip of phrasing.

Wrong: "Simple molecules melt easily because the covalent bonds are weak."
Right: "Simple molecules melt easily because the intermolecular forces between molecules are weak and require little energy to overcome; the covalent bonds within each molecule remain intact."

Mistake: Forgetting Why Metals Conduct

Metals conduct electricity and heat because of delocalised electrons that are free to move throughout the structure. A common shorthand answer, "metals conduct because they are metals," earns nothing; you must name the delocalised electrons specifically and describe them as free to move.

The Properties of Acids and Bases

Mistake: Mixing Up Acid and Base Products

The salt produced in a neutralisation reaction depends on both the acid used and the metal in the base or alkali: hydrochloric acid produces chlorides, nitric acid produces nitrates, and sulfuric acid produces sulfates. Students frequently name the wrong salt by forgetting which acid produces which anion, especially under time pressure. Build a fixed association (hydrochloric, chloride; nitric, nitrate; sulfuric, sulfate) and check it every time rather than trying to reason it out from scratch in the exam.

Mistake: Writing the Neutralisation Equation Incompletely

The ionic equation for neutralisation is H+(aq) + OH-(aq) → H2O(l). A common error is omitting state symbols, or writing the full formula equation when the ionic equation was specifically requested. Read the question carefully to see whether it wants the full balanced equation, the ionic equation, or a word equation, since these are marked differently even though they describe the same reaction.

Mistake: Confusing pH Direction

Some students reverse the pH scale under pressure, describing a low pH as alkaline or a high pH as acidic. Anchor it firmly: pH below 7 is acidic, pH 7 is neutral, and pH above 7 is alkaline, and hydrogen ions (H+) are what make a solution acidic while hydroxide ions (OH-) make it alkaline.

Carbon Compounds as Fuels

Mistake: Listing Combustion Products Without Linking Them to the Fuel's Composition

A frequent error is reciting a memorised list of combustion products (carbon dioxide, water, carbon monoxide, sulfur dioxide, oxides of nitrogen, particulates) without connecting which products come from which elements in the fuel. Carbon dioxide and carbon monoxide come from the carbon in the fuel; water comes from the hydrogen; sulfur dioxide comes from any sulfur impurity present. If a question gives you the composition of a specific fuel, use that information rather than reciting the general list unfiltered.

Mistake: Assuming Complete Combustion by Default

Carbon monoxide and soot (particulates) only form when combustion is incomplete, meaning there is insufficient oxygen. Students sometimes list carbon monoxide as a standard product of burning a hydrocarbon fuel without qualifying that this only happens under incomplete combustion conditions, which is exactly the qualification the mark scheme is checking for.

Mistake: Muddling Alkanes and Alkenes in the Bromine Water Test

Alkenes decolourise bromine water (orange to colourless) because of their carbon-carbon double bond; alkanes do not react with bromine water because they are saturated. Getting this test backwards, and describing an alkane as decolourising bromine water, is a common carbon compounds as fuels error that examiners see repeatedly, usually because students remember that "something changes colour" without remembering which hydrocarbon family causes the change.

Timing and Command-Word Mistakes That Cut Across Every Topic

Beyond the topic-specific errors above, a handful of habits show up in every single one of these five areas, and they are worth naming separately because fixing them once fixes them everywhere.

Mistake: Answering the Question You Expected, Not the One Asked

Under time pressure, it is tempting to see a familiar keyword ("rate," "bonding," "pH") and write out a memorised paragraph about that topic in general, rather than answering the specific question in front of you. If a question asks you to explain why increasing surface area increases rate, do not write everything you know about all five rate factors; answer the one asked, in the depth the marks suggest, and stop.

Mistake: Not Using the Data You Are Given

Structured questions in chemistry very often supply a table, a graph, or a short description of results specifically so that you use it in your answer. Writing a generic textbook explanation instead of referring to the actual numbers or observations given in the question is a reliable way to miss marks that were sitting there for the taking. If the question gives you data, your answer should mention it by name or value.

Mistake: Treating Every Blank as a Lost Cause

Some students leave a question blank rather than risk writing something wrong. This is almost always the wrong call. A partial answer, even one you are not confident in, can pick up marks for a correct definition, a sensible unit, or one correct step in a calculation. Examiners cannot award marks for a blank space, no matter how good the reasoning would have been in your head.

A Quick Way to Check Your Own Work

Before you consider a revision session finished, pick one answer you wrote for each of the five topics above and check it against the correct technique described here. If you can spot your own version of the mistake in your own working, you have found something worth fixing now rather than in the exam itself, which is a far better time to make the correction.

These oxfordaqa igcse chemistry mistakes are not a sign you are behind; they are simply the ones that repeat often enough to be worth naming and fixing directly, rather than hoping revision alone will smooth them out on its own. Working through a short list of oxfordaqa igcse chemistry exam tips like these, topic by topic, tends to close more of the mark gap in less time than open-ended revision of content you already understand reasonably well.