G3 Science Chemistry Definitions
G3 Science Chemistry terms by topic, with precise particle meanings and distinctions for recall and explanation.
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Topic-organised revision definitions for G3 Science Chemistry (K326 / K328). 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. |
| Metallic bond | Electrostatic attraction between positive metal ions and delocalised electrons. | Mobile electrons account for electrical conduction. |
| 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. |
Reactions and calculations
| Term | Meaning | Remember |
|---|---|---|
| Exothermic reaction | A reaction that transfers energy to the surroundings. | The surroundings warm; reaction enthalpy change is negative. |
| Endothermic reaction | A reaction that takes in energy from the surroundings. | The surroundings cool; reaction enthalpy change is positive. |
| Activation energy | Minimum energy that reacting particles need for a successful reaction. | A catalyst provides an alternative pathway with lower activation energy. |
| Rate of reaction | Change in amount of reactant used or product formed per unit time. | A graph’s tangent gives rate at one instant. |
| Catalyst | A substance that increases reaction rate and is chemically unchanged at the end. | It is not used up overall; it may participate in intermediate steps. |
| Oxidation | Gain of oxygen, loss of hydrogen, loss of electrons, or increase in oxidation state, as appropriate to the reaction. | State the model you are using. |
| Reduction | Loss of oxygen, gain of hydrogen, gain of electrons, or decrease in oxidation state, as appropriate. | Oxidation and reduction occur together in a redox reaction. |
| Oxidising agent | A species that causes another species to be oxidised and is itself reduced. | It accepts electrons in the electron-transfer model. |
| Reducing agent | A species that causes another species to be reduced and is itself oxidised. | It donates electrons in the electron-transfer model. |
| Mole | Amount containing exactly 6.02214076 × 10²³ specified entities. | Specify atoms, molecules, ions or other entities. |
| Empirical formula | Simplest whole-number ratio of atoms of each element in a compound. | It need not show the actual atoms in a molecule. |
| Molecular formula | Actual number of atoms of each element in a molecule. | A whole-number multiple of the empirical formula. |
| Concentration | Amount of solute per unit volume of solution. | State mol/dm³ or g/dm³; the solution volume includes the solvent and solute. |
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. A concentrated solution and a strong acid are also different ideas: concentration describes amount per solution volume; strength describes ionisation.
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