Chemistry at IGCSE splits into two bodies of skill that fail for different reasons. The qualitative side — tests for gases, colours of precipitates, trends in the periodic table — fails when students have not consolidated the descriptive material. The quantitative side — equations, moles, concentrations, yields — fails for a completely different reason: it fails in sequence.

A student who cannot balance an equation cannot get the mole ratio, and a student who cannot get the mole ratio cannot answer any part of a calculation question. One error early produces a cascade of zeros that looks like a total misunderstanding but is really a single unconsolidated step.

This is why quantitative chemistry is the highest-return area for tutoring. Fix the first link and four subsequent marks come back.

Why Equations Cost More Marks Than They Should

In a chemistry paper, the equation is rarely the whole question. It is the gate. Every part that follows — mass calculations, gas volumes, concentration, percentage yield — depends on the equation being correct, and a wrong equation guarantees wrong answers downstream even when the arithmetic is flawless.

Students lose equations in three ways. They fail to balance, usually by forgetting a diatomic element on the left. They omit state symbols where required. And they write the wrong product because they have memorised the reaction's appearance rather than its chemistry — copper plus dilute acid producing a salt and water, when copper does not react with dilute hydrochloric acid at all.

Balancing as Conservation Rather Than Guessing

Students balance by trial and error, which works for simple equations and becomes unreliable at the exact point where the exam becomes interesting.

The reliable method is a count. Write the equation with correct formulas. Count each element on both sides. Adjust coefficients, largest groups first. Recount. Never change a subscript, because that changes the substance.

For ionic equations the additional discipline is charge: a balanced ionic equation conserves charge as well as atoms, which is the check students forget. An equation with equal atoms and unequal charge is wrong, and it will be marked wrong even when the atoms are correct.

State Symbols and the Marks They Carry

State symbols attract marks in two places: in equations where the question explicitly asks for them, and in ionic equations where the distinction between aqueous and solid is the whole point of the reaction.

The convention is four symbols: solid, liquid, gas, and aqueous. The trap is the difference between liquid and aqueous, which students collapse because both involve water. Water itself is liquid; a solution of a salt in water is aqueous. A precipitation reaction works only because the ionic equation shows aqueous ions becoming a solid.

Ionic Equations and the Spectator Problem

Writing the net ionic equation requires deleting the ions that do not change, and students delete the wrong ones or delete nothing.

The procedure is mechanical: write the full equation, split every aqueous ionic compound into its ions, keep solids and liquids and gases intact, then cancel ions appearing identically on both sides. What remains is the net ionic equation.

Students who skip the splitting step try to do it by inspection and produce equations that balance on paper but describe the wrong chemistry. The step takes twenty seconds and is worth several marks across a paper.

The Mole as a Conversion Device

The mole is not a topic. It is a device for converting between the three things chemistry measures: mass, concentration and volume of solution, and volume of gas.

Once a student can move between mass and moles using molar mass, between concentration, volume, and moles, and between gas volume and moles using molar volume, every calculation question becomes an exercise in choosing the right route.

Most students learn the three conversions separately and then cannot choose between them. The fix is to practise classification rather than calculation: given a question, state which of the three starting points it provides and which it asks for, without calculating anything. Twenty such classifications teach the decision, and the decision is what students actually get wrong.

Limiting Reagent Questions

The question that separates grades is the one where two reactant quantities are given and one runs out.

The procedure is fixed. Calculate moles of each reactant. Divide each by its coefficient in the balanced equation. The smaller result identifies the limiting reagent. Use that reactant's moles to calculate the product.

Students who skip the division step assume the larger mass is in excess, which is frequently wrong because molar masses differ. Students who do divide but then use the wrong reactant's moles lose everything after the second step.

Percentage Yield and Atom Economy

These two are taught together and confused constantly, because both produce a percentage that measures efficiency and neither answers the other's question.

Percentage yield compares what was obtained with what could theoretically be obtained from the limiting reagent. It measures the success of a procedure. Atom economy compares the mass of the desired product with the total mass of all products, from the balanced equation. It measures the efficiency of a reaction as designed.

A reaction can have a hundred percent yield and poor atom economy, which is why the distinction exists. Students who can state that difference in one sentence rarely lose the marks.

Concentration, Dilution, and the Units

Concentration questions fail on units more than on chemistry. Concentration in moles per cubic decimetre requires the volume in cubic decimetres, and students substitute cubic centimetres without converting.

The conversion is a factor of a thousand, and it should be written as a separate line every time. Students who do it mentally get it right most of the time, and the times they get it wrong cost several marks each.

Dilution questions add a second step: the moles are unchanged, the volume has increased, so the concentration falls in proportion. Stating that principle before calculating prevents the common error of applying the dilution factor to the moles.

Titration Calculations and the Average Titre

Titration calculation questions give three or four titre values, one of which is anomalous.

The first mark is for selecting the concordant values and averaging them, discarding the outlier. Students who average all readings lose the mark and then carry a slightly wrong volume into the rest of the calculation.

The second is for converting the average titre to moles of the acid or alkali, using concentration. The third is for using the equation's ratio. The fourth is for the final answer with units. Each mark is separate, and a student who states the steps clearly earns them even with an arithmetic slip at the end.

Organic Naming and the Small Errors

Organic chemistry at this level is mostly naming and recognising functional groups, and the marks are lost on small conventions: the position number in a name, the difference between a structural formula and a displayed formula, the distinction between an alkane and an alkene in a test-tube reaction.

Students who can draw every functional group and name a chain of six carbons with a substituent rarely struggle here. Students who have learned the reactions without the naming system lose marks steadily, a few at a time, which is harder to notice than a single collapse.

Electrolysis and the Electrode Conventions

Electrolysis questions mix recall and reasoning, and the reasoning depends on knowing which electrode attracts which ion.

The convention: negative electrode attracts positive ions, positive electrode attracts negative ions. The products depend on the electrolyte and on the reactivity of the ions present, and the exam expects students to justify the product rather than just state it.

The recurring errors are reversing the electrodes and forgetting that in aqueous solution, the presence of hydrogen and hydroxide ions changes what is discharged. Students who write the half-equations for both possible products and then apply the rule choose correctly almost every time.

Rates and the Collision Explanation

Rate of reaction questions are the one place in the course where students are asked to explain at the level of particles, and the explanation has a fixed shape.

A rate increases because particles collide more frequently, or because a greater proportion of collisions have enough energy to react. Every factor — concentration, temperature, surface area, pressure, catalyst — must be explained in those terms, not in terms of the observation.

Students who write "it reacts faster because it is hotter" have described the observation rather than explained it, and earn nothing. Students who write that particles move faster and therefore collide more often, and more of those collisions exceed the activation energy, earn both marks.

A catalyst is the exception that must be stated correctly: it provides an alternative route of lower activation energy, and is not used up.

Bonding, Structures, and the Property Questions

Questions linking structure to property recur every year, and the marks depend on naming the structure and then naming the force that is broken.

A giant ionic lattice has high melting point because strong electrostatic forces between oppositely charged ions must be overcome. A simple molecular substance has a low melting point because only weak intermolecular forces are overcome, not the covalent bonds. A giant covalent structure such as diamond has a very high melting point because many strong covalent bonds must be broken.

The single most common error is saying that covalent bonds are broken when a simple molecular substance melts or boils. That error alone accounts for a steady trickle of lost marks across every paper, and it is fixed by one sentence understood rather than memorised.

Quantitative chemistry fails in sequence: an unbalanced equation or an un-converted unit produces a cascade of zeros that disguises a single broken step.

Balance by counting, never by guessing. Write state symbols where asked. Split ionic equations before cancelling. Treat the mole as a conversion device and practise choosing the route rather than doing the arithmetic.

For limiting reagent, divide moles by coefficients. For titration, discard the outlier before averaging. For every calculation, write the unit conversion as its own line.