Chemistry,
one step at a time.

Liquid particles are close together with an irregular arrangement and can move past one another. Wide separation is characteristic of a gas and would misrepresent density and compressibility.

Watch for: Do not describe The particle model only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Most alpha particles passed through, showing atoms are mostly empty space, but rare large deflections required a tiny, dense, positively charged nucleus. Later work identified protons and neutrons in the nucleus.

Watch for: Do not describe Atomic structure only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Two spots show the sample contains at least two soluble components. A matching Rf under the same conditions supports the presence of the reference dye, though additional evidence can strengthen identification.

Watch for: Do not describe Purity and separating mixtures only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Metallic bonding is electrostatic attraction between positive metal ions and delocalised electrons. Simple molecules contain covalent bonds within discrete molecules, ionic solids contain giant ion lattices, and polymer chains contain covalent bonds.

Watch for: Do not describe Bonding only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Nanoparticles can have different reactivity and may cross biological barriers. Evidence about dose, exposure, persistence and toxicity is needed; neither complete safety nor universal harm should be assumed.

Watch for: Do not describe Properties of materials only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Atoms are conserved, but carbon dioxide leaves the open flask, so the apparatus reading falls. Including the escaped gas would give the same total mass before and after.

Watch for: Do not describe Introducing chemical reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

An endothermic reaction absorbs energy overall, so the products lie at a higher energy level than the reactants. The profile must also show the activation-energy barrier.

Watch for: Do not describe Energetics only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

An excess of insoluble base ensures no acid remains. Unreacted copper(II) oxide is filtered off, then the filtrate is concentrated and cooled to crystallise hydrated copper(II) sulfate.

Watch for: Do not describe Types of chemical reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Drying and weighing the cathode before and after gives the mass of copper deposited. Current, time, electrode area and concentration should be controlled or measured for a valid comparison.

Watch for: Do not describe Electrolysis only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Magnesium has a greater tendency to lose electrons and form Mg²⁺, so it displaces hydrogen ions from acid. Copper lies below hydrogen and does not react with dilute non-oxidising acids in this way.

Watch for: Do not describe Predicting chemical reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Instrumental methods are sensitive, rapid and can analyse mixtures. Emission wavelengths identify metal ions and calibrated intensity can determine concentration; calibration standards and uncertainty remain important.

Watch for: Do not describe Identifying the products of chemical reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

After quantitative transfer and rinsing, solvent is added carefully until the bottom of the meniscus reaches the mark at eye level. Stoppering and inversion make the concentration uniform.

Watch for: Do not describe Monitoring chemical reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Enzymes are biological catalysts. Above their optimum temperature, bonds maintaining protein shape can break, changing the active site and reducing successful enzyme–substrate binding.

Watch for: Do not describe Controlling reactions only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

The reactant side has four moles of gas per equation amount and the product side has two. Higher pressure favours the side with fewer gas molecules, increasing ammonia yield.

Watch for: Do not describe Equilibria only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

A composite combines materials to obtain useful properties: concrete resists compression while steel provides tensile strength. Suitability of glass, clay ceramics and composites depends on measured physical properties and intended use.

Watch for: Do not describe Improving processes and products only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Hydrogen can avoid carbon dioxide at the vehicle, but its source matters: production may use fossil fuels or renewable electricity. Storage, distribution, flammability, efficiency and infrastructure must also be considered.

Watch for: Do not describe Organic chemistry only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Volcanic outgassing supplied water vapour, carbon dioxide and nitrogen. Cooling condensed water to oceans; carbon dioxide decreased through dissolution, carbonate formation and photosynthesis, while photosynthetic organisms raised oxygen.

Watch for: Do not describe Interpreting and interacting with Earth systems only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

A hazard is a potential source of harm; risk combines likelihood and severity under the conditions used. Controls such as smaller quantities, lower concentrations and eye protection reduce risk.

Watch for: Do not describe Practical skills only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.