G2 Science Chemistry Definitions
G2 Science Chemistry terms by topic, with precise particle meanings and distinctions for recall and explanation.
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Topic-organised revision definitions for G2 Science Chemistry (K223 / K225). Cover the meaning column, recall the definition and explain why the distinction in the last column matters. These are learning definitions, not a guarantee of marks for one fixed sentence.
Particles, atoms and bonding
| Term | Meaning | Remember |
|---|---|---|
| Melting point | Temperature at which a solid and liquid coexist during melting at a stated pressure. | A pure substance has a characteristic melting point; impurities can alter it. |
| Boiling point | Temperature at which a liquid boils throughout when its vapour pressure equals the external pressure. | Depends on pressure; evaporation can occur below boiling point. |
| Physical change | Change that does not form a new substance. | A change of state is a physical change. |
| Chemical change | Change producing one or more new substances. | Look for changed chemical identity, not only a visible change. |
| Element | A substance containing only one type of atom, defined by proton number. | An element can consist of molecules, as in O₂. |
| Compound | A substance containing two or more elements chemically combined in fixed proportions. | Separating its elements requires chemical change. |
| Mixture | Two or more substances together without being chemically combined. | Composition can vary; physical methods can separate its components. |
| Atom | The smallest particle of an element that retains its chemical identity. | A neutral atom has equal numbers of protons and electrons. |
| Molecule | A discrete group of two or more atoms covalently bonded together. | An ionic crystal is not a collection of molecules. |
| Ion | An atom or group of atoms carrying a net electric charge. | Electron loss makes a cation; electron gain makes an anion. |
| Proton number | Number of protons in an atom’s nucleus. | Determines the element; also called atomic number. |
| Nucleon number | Total number of protons and neutrons in a nucleus. | Neutrons = nucleon number − proton number. |
| Isotopes | Atoms of the same element with different numbers of neutrons. | Same proton number; different nucleon numbers. |
| Diffusion | Net spreading of particles from a region of higher concentration to lower concentration through random motion. | Particles do not all move in one direction. |
| Ionic bond | Electrostatic attraction between oppositely charged ions. | Electron transfer forms ions; attraction holds the lattice together. |
| Covalent bond | A shared pair of electrons attracted to the nuclei of the bonded atoms. | Sharing does not mean that electrons cease moving. |
| Giant structure | An extended network of particles held together by bonds or attractions. | Distinguish giant ionic, giant covalent and metallic structures. |
| Alloy | A mixture of a metal with one or more other elements. | It is not a single compound; composition can vary. |
| Simple molecular substance | A substance made of discrete molecules. | Melting usually overcomes intermolecular attractions rather than covalent bonds. |
Masses, amount and equations
| Term | Meaning | Remember |
|---|---|---|
| Relative atomic mass, Aᵣ | Weighted mean mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom. | A ratio with no unit; isotope abundance affects the mean. |
| Relative molecular mass, Mᵣ | Sum of the relative atomic masses of all atoms in a molecule. | Has no unit; for ionic substances use relative formula mass. |
| Relative formula mass | Sum of relative atomic masses in one formula unit. | CaCl₂ includes two chlorine atoms. |
| Molar mass, M | Mass per mole of a specified substance. | Common unit g/mol; numerically equal to Mᵣ when expressed in g/mol. |
| Amount of substance, n | A quantity measured in moles. | Do not confuse mass, amount and particle number. |
| State symbols | (s), (l), (g) and (aq) identify solid, liquid, gas and aqueous solution. | Aqueous means dissolved in water, not simply liquid. |
Separation, acids and evidence
| Term | Meaning | Remember |
|---|---|---|
| Acid | A substance that produces hydrogen ions when dissolved in water. | This is the Secondary aqueous model; H⁺ represents hydrated hydrogen ions. |
| Base | A substance that neutralises an acid to form a salt and water. | Metal oxides and hydroxides are common examples at this level. |
| Alkali | A soluble base that produces hydroxide ions in water. | All alkalis are bases; many bases are insoluble. |
| Neutralisation | Reaction between an acid and a base forming a salt and water. | For aqueous acid and alkali: H + (aq) + OH-(aq) → H₂O(l). |
| Neutral solution | A solution with equal hydrogen-ion and hydroxide-ion concentrations. | pH 7 at the usual room-temperature school reference conditions. |
| Indicator | A substance whose colour depends on the acidity or alkalinity of its surroundings. | Universal Indicator estimates a pH range. |
| Solvent | Substance in which a solute dissolves. | Water is a common solvent, not the only solvent. |
| Solute | Substance dissolved in a solvent. | The solute need not be a solid. |
| Solution | Homogeneous mixture of solute and solvent. | A suspension contains undissolved particles and is different. |
| Solubility | Maximum amount of a substance that dissolves in a specified amount of solvent at a stated temperature. | A saturated solution is in equilibrium with undissolved solute under the stated conditions. |
| Salt | An ionic compound formed when acid hydrogen is replaced by a metal ion or ammonium ion. | Not every salt solution is neutral. |
| Precipitate | An insoluble solid formed when solutions react. | State its colour as the observation; identify the ion as an inference. |
| Filtrate | Liquid that passes through a filter. | The solid remaining on the filter is the residue. |
| Crystallisation | Separation of a dissolved solid by forming crystals from a solution. | Cooling a concentrated solution can cause crystals to form. |
| Chromatography | Separation of mixture components through different distributions between a mobile and a stationary phase. | Compare spots with known samples; spot number is not a guarantee of purity. |
| Distillation | Separation using vaporisation followed by condensation. | Simple distillation recovers a solvent; fractional distillation separates suitable miscible liquids. |
Periodic patterns, organic chemistry and environment
| Term | Meaning | Remember |
|---|---|---|
| Group | A vertical column of the Periodic Table. | Main-group elements have related outer-shell electron arrangements and chemical properties. |
| Period | A horizontal row of the Periodic Table. | Occupied electron-shell number increases from one period to the next. |
| Hydrocarbon | A compound containing carbon and hydrogen only. | Ethanol contains oxygen and is not a hydrocarbon. |
| Functional group | Atom or group responsible for characteristic reactions of an organic compound. | The carbon–carbon double bond characterises alkenes. |
| Homologous series | A family with the same functional group and general formula, similar chemical properties and a gradation of physical properties. | Successive members differ by CH₂. |
| Saturated hydrocarbon | A hydrocarbon containing only single carbon–carbon bonds. | Unbranched, acyclic alkanes have formula CₙH₂ₙ₊₂. |
| Unsaturated hydrocarbon | A hydrocarbon containing a carbon–carbon multiple bond. | Aqueous bromine is decolourised by an alkene under the stated test conditions. |
| Cracking | Breaking large hydrocarbon molecules into smaller molecules, including alkenes. | A chemical change; fractional distillation is a physical separation. |
| Addition reaction | Reaction in which atoms add across a multiple bond to form one product from the reactants. | An alkene’s double bond becomes a single bond. |
| Substitution reaction | Reaction in which an atom or group is replaced by another. | Methane reacts with chlorine in ultraviolet light. |
| Polymer | A large molecule made from many small monomer units. | An addition polymer forms without elimination of a small molecule. |
| Monomer | A small molecule that can join to form a polymer. | Ethene is the monomer for poly(ethene). |
| Complete combustion | Burning in sufficient oxygen to oxidise the fuel fully. | A hydrocarbon produces carbon dioxide and water. |
| Incomplete combustion | Burning with insufficient oxygen for complete oxidation. | Carbon monoxide and/or carbon may form, along with water. |
| Greenhouse gas | A gas that absorbs outgoing infrared radiation and contributes to atmospheric warming. | This mechanism differs from ozone depletion and acid rain. |
| Renewable resource | A resource replenished on a timescale comparable with its use. | Renewable does not automatically mean no environmental impact. |
Test the distinction
“An acid is a substance with a low pH” describes an aqueous observation but omits the particle model. In the Secondary model, an acid produces hydrogen ions in water. An indicator colour is evidence of acidity or alkalinity; it is not a formula for the acid or an exact concentration measurement.
G2 uses the mass–molar mass–amount relationship without reacting-mass or gas-volume stoichiometry. Keep explanations at the G2 stopping point shown in the course lessons.
Return to the course hub for explanations, observations and topic checks.