Chemistry,
one step at a time.

Water and carbon dioxide are removed because they would freeze. The cleaned air is compressed and cooled until it liquefies, then its components are separated by their boiling points.

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

Concentrating to near saturation and then cooling promotes crystal formation. Leaving some mother liquor can reduce contamination by soluble impurities and avoids overheating the product.

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

An ore is a naturally occurring material from which a metal or metal compound can be extracted economically. Many metals occur in compounds rather than in the free state.

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

Element identity is fixed by proton number. Different neutron numbers give different mass numbers without changing the element, which is the definition of isotopes.

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.

The periodic table is ordered by increasing atomic number. Elements in a group share the same outer-shell electron pattern, which produces repeating chemical properties even though physical properties can differ.

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

Metal valence electrons are delocalised across a giant lattice. Their attraction to positive metal ions holds the structure together.

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

The three observations are jointly explained by metallic structure: mobile electrons conduct, strong attraction raises melting point, and non-directional bonding permits deformation.

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

Ionic bonding is the electrostatic attraction between oppositely charged ions. In MgO, electron transfer produces Mg²⁺ and O²⁻ ions held in a giant ionic lattice by attractions in all directions.

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

Polar water molecules hydrate cations and anions. Dissolution is favourable when hydration and entropy effects offset the energy required to disrupt the lattice.

Watch for: Do not describe Structures and properties of giant ionic substances only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Both substances are simple molecular. The larger electron cloud of I₂ produces stronger instantaneous dipole-induced dipole attractions, giving iodine higher melting and boiling points.

Watch for: Do not describe Structures and properties of simple molecular substances only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Each carbon in graphite forms three sigma bonds, leaving one electron delocalised across a layer. In diamond, each carbon forms four localised covalent bonds, so no mobile charge carriers are available.

Watch for: Do not describe Structures and properties of giant covalent substances only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Macroscopic properties such as conductivity, hardness, solubility and melting point follow from particles, bonding and arrangement. Those properties constrain useful applications.

Watch for: Do not describe Comparing structures and properties of substances only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Ease of extraction and accessible raw materials influenced historical discovery. Highly reactive metals generally required later electrochemical technology.

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

Reaction with acid shows M can displace hydrogen ions, so it lies above hydrogen. Lack of cold-water reaction places it below very reactive metals such as sodium and calcium under the stated conditions.

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

An empirical formula reduces element amounts to their simplest whole-number ratio; a molecular formula may be a multiple.

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

At a break in tin coating, iron becomes the more readily oxidised metal in contact with tin. Zinc, by contrast, supplies electrons because it is more reactive and protects exposed iron sacrificially.

Watch for: Do not describe Corrosion of metals and their protection only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Acid + carbonate → salt + water + carbon dioxide. Hydrochloric acid produces a chloride salt. Carbon dioxide turns limewater milky; hydrogen gives a squeaky pop.

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

A calibrated pH meter gives a quantitative reading and avoids subjective colour matching. The probe should be rinsed between samples to reduce contamination.

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

Concentration is the amount of acid per unit volume, whereas strength is the degree of ionisation in water. A concentrated weak acid and a dilute strong acid are both possible.

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

Sulfuric acid supplies two acidic protons, so two moles of NaOH are required per mole of H₂SO₄. The soluble sodium sulfate and water equation is atom- and charge-balanced.

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

A volumetric flask defines the final solution volume accurately. Mixing before and after the final adjustment ensures uniform concentration.

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

A volumetric pipette is calibrated to deliver one fixed volume accurately. Rinsing prevents dilution, viewing the meniscus at eye level avoids parallax and gravity drainage respects its calibration.

Watch for: Do not describe Volumetric analysis involving acids and alkalis only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

At a suitable wavelength and concentration range, absorbance can be calibrated against concentration. Its change with time gives concentration-time data and hence rate.

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

At higher concentration, reacting particles are closer together on average and collide more often. The fraction with sufficient energy is unchanged if temperature is fixed.

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

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. One mole therefore occupies approximately the same volume under those fixed conditions.

Watch for: Do not describe Molar volume of gases at room temperature and pressure only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Catalytic converters can transform CO, nitrogen oxides and unburned hydrocarbons, while low-sulfur fuels and reduced fuel demand address other emissions. Controls do not remove all climate impact.

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

Members of a homologous series share a functional group and general formula, with successive members differing by CH₂. The two primary alcohols meet both conditions.

Watch for: Do not describe Homologous series, structural formulae and naming only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

When heating stops, gases cool and contract. If the delivery tube remains under water, reduced pressure can draw water into the hot reaction tube, risking thermal shock and breakage.

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

Addition polymerisation converts each monomer C=C into two single-bonded backbone carbons. Reversing that representation restores the alkene double bond and keeps its substituents.

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

Electronegativity generally increases across a period and up a group, reaching its maximum at fluorine. Fluorine attracts bonding electrons most strongly on the Pauling scale.

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

Both are non-polar molecules, so London dispersion forces dominate. The larger, more polarisable I₂ electron cloud forms stronger instantaneous and induced dipoles, requiring more energy to separate molecules.

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

Melting does not decompose water molecules. It disrupts part of the extended hydrogen-bond network so molecules can move more freely, while transient hydrogen bonds persist.

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

At this level, PCl₅ and SF₆ are treated as expanded-valence-shell exceptions. Carbon is a second-period element and CH₄ is represented with four bonds and an octet.

Watch for: Do not describe Simple molecular substances with non-octet structures only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Both molecules have four electron pairs around the central atom, but water has two lone pairs. Lone pair-lone pair and lone pair-bond pair repulsions are stronger, so O-H bonds are pushed closer together.

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

Primary cells are normally discarded after their reactants are substantially consumed. Secondary cells are designed so an external electrical supply can reverse the chemical changes during charging.

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

Zinc and copper have different electrode behaviours. Coupled oxidation and reduction reactions separate charge and create a potential difference that the voltmeter detects.

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

Cu²⁺ accepts two electrons and is reduced to Cu. An oxidising agent causes another species to be oxidised and is itself reduced, so Cu²⁺ is the oxidising agent.

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

The anode is the electrode at which oxidation releases electrons. In a galvanic chemical cell those electrons flow through the external circuit toward the cathode.

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

Deposited mass depends on both current and time, so time must be kept constant to isolate the effect of current. Electrode area and electrolyte concentration should also be controlled.

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.

Energy must be supplied to separate bonded atoms against electrostatic attraction, so bond breaking is endothermic. When a bond forms, the atoms move to a lower-energy arrangement and energy is released.

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

Standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states. The equation must therefore form exactly one mole of CH₄ from graphite and H₂ gas.

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

A cooling or warming trend continues while readings are taken. Extrapolating the post-reaction line back to mixing estimates the temperature that would have been reached with less heat exchange, improving ΔT.

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

A catalyst lowers activation barriers for both forward and reverse pathways. It changes the time taken to reach equilibrium but not the final equilibrium position at a fixed temperature.

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

The equilibrium law relates the concentrations present after equilibrium has been established. Initial values are valid only if they happen to be unchanged, which generally they are not.

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

For a specified equation, Kc depends on temperature. Concentration and pressure changes may shift the equilibrium position, but the system re-establishes the same Kc if temperature is unchanged.

Watch for: Do not describe Effects of concentration, pressure and temperature on equilibrium only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Members of a homologous series share a functional group and general formula, show similar chemical properties, and successive members differ by CH₂. Their physical properties change gradually with chain length.

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

Both have formula C₄H₈ and the same connectivity, but the C=C bond cannot freely rotate without breaking the pi bond, so the two geometries persist.

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

An alkene decolourises aqueous bromine by addition. An aldehyde reduces Tollens' reagent to a silver mirror. Using the two tests on separate samples identifies the functional groups through distinct observations.

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

Hydration adds H and OH across the carbon-carbon double bond, converting ethene to ethanol. Industrial hydration uses steam and an acid catalyst under controlled temperature and pressure.

Watch for: Do not describe Inter-conversions of carbon compounds only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Each monomer is supplied in a different immiscible phase. They contact at the boundary, where condensation polymerisation forms nylon that can be drawn out continuously while fresh monomers meet.

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

Silicon forms a giant covalent lattice and has limited charge-carrier availability, giving semiconductor behaviour. Sodium and magnesium are metallic conductors, while monatomic argon does not conduct under ordinary conditions.

Watch for: Do not describe Periodic variation in properties from lithium to argon only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Al₂O₃ is a high-melting ionic solid and is amphoteric, reacting with both acids and strong alkalis. Na₂O is basic, SiO₂ is acidic but largely unreactive with acids, and SO₃ is molecular and acidic.

Watch for: Do not describe Bonding, composition and acid-base properties of oxides from sodium to chlorine only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Cu²⁺ and Fe²⁺ give characteristic solution colours. Not every transition-metal ion is strongly coloured, but coloured aqueous ions are a common general property.

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

Titration can determine exact neutralisation volumes and make a pure soluble salt. Industrial manufacture uses integrated large-scale processes chosen for throughput, heat recovery, raw materials and safe economical separation.

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

For an elementary rate-determining step, the reacting species and powers can match the experimental rate law. Rate equations must be determined from kinetic evidence, not copied from the overall balanced equation.

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

At the same temperature, lowering Ea makes the exponential factor e^(−Ea/RT) larger, other factors being comparable. More collisions are therefore energetically successful.

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

A catalyst provides an alternative reaction mechanism whose highest energy barrier is lower. At the same temperature, a larger fraction of particles has enough energy to react, so effective collisions are more frequent.

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

Ammonia formation is exothermic, so lower temperature favours yield, but particle energies and successful-collision frequency fall. A moderate temperature balances rate and equilibrium yield.

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

Percentage yield compares actual and theoretical amounts of the desired product. Atom economy examines how the balanced equation distributes reactant atoms among desired and unwanted products.

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

Both polymers contain carbon, hydrogen and oxygen, but chitin's N-acetyl groups introduce nitrogen. Elemental or spectroscopic evidence for nitrogen therefore distinguishes it.

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

PLA can be produced from biologically derived lactic acid. Its environmental benefit depends on manufacture, performance, collection and access to suitable composting or treatment conditions.

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

Pure-metal layers can often slide comparatively easily. Solute atoms disrupt the regular lattice, so dislocation or layer movement becomes more difficult.

Watch for: Do not describe Metals and alloys 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 Synthetic materials in modern life only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Mechanical recycling often requires reasonably pure polymer streams. Design choices that simplify identification and disassembly can improve recovery quality and yield.

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

Cleaning removes traces of previous salts, which could otherwise produce a misleading flame colour. The wire should be retested until it gives no colour before the sample is examined.

Watch for: Do not describe Detecting the presence of chemical species 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 Separation and purification methods only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.

Silver ions react stoichiometrically with chloride ions: Ag⁺ + Cl⁻ → AgCl(s). A suitable chromate indicator signals the first slight excess of silver ions after chloride has precipitated.

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

One bromine atom gives molecular ions two mass units apart because ⁷⁹Br and ⁸¹Br have similar abundance. Matching reference patterns supports identification but other structural evidence may still be needed.

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

A signal match can be persuasive but may not exclude interferences or mixtures. Appropriate blanks, standards, validated criteria and complementary methods strengthen identification.

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

A best-fit line or curve represents the overall trend with an even distribution of normal points on either side. A genuine anomaly should be identified rather than forcing the fit through it.

Watch for: Do not describe Investigative study: planning, analysis, evaluation and reporting only at the observable level. Use the correct particles, bonding, energy change or quantitative relationship where the question requires it.