Exam FAQs
The questions students always get wrong.
Mark-scheme-accurate answers to the A-Level and IGCSE questions students get wrong most, across Edexcel (International A Level and IGCSE) and Cambridge, in Mathematics, Physics, Chemistry and Biology. A sample is shown below; the full, filterable bank is inside the app.
Sample exam questions and mark-scheme answers
Define first ionisation energy.
The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions: X(g) → X⁺(g) + e⁻.
Explain why there is a large increase in ionisation energy between the 2nd and 3rd ionisation energies of magnesium.
The third electron is removed from the inner shell (n=1) which is much closer to the nucleus and has less shielding, so the nuclear attraction is much stronger.
Explain why diamond has a very high melting point.
Diamond has a giant covalent (macromolecular) lattice with strong covalent bonds throughout, so a very large amount of energy is required to break all of these bonds.
Why does graphite conduct electricity but diamond does not?
In graphite each carbon bonds to 3 others, leaving one delocalised electron per atom that can move through the structure. In diamond all four outer electrons are used in covalent bonds, so there are no free electrons.
Define electronegativity.
The ability of an atom to attract the bonding pair of electrons in a covalent bond towards itself.
Define lattice enthalpy.
The energy released when one mole of an ionic compound is formed from its gaseous ions at infinite separation: M⁺(g) + X⁻(g) → MX(s).
Describe the test for halide ions using silver nitrate solution and state the observations.
Add dilute nitric acid, then aqueous silver nitrate: AgCl is a white precipitate (soluble in dilute ammonia); AgBr is cream (soluble only in concentrated ammonia); AgI is yellow (insoluble in ammonia).
Explain the effect of a catalyst on rate in terms of the Maxwell-Boltzmann distribution.
A catalyst provides an alternative pathway with a lower activation energy, so a greater proportion of molecules have energy at or above the activation energy and the rate increases without changing the reactants or products.