Myanmar Matriculation Chemistry key terms

The amount of substance containing 6.022 × 10^23 elementary particles (atoms, molecules, or ions); the chemist's counting unit, equal to the mass in grams numerically matching the relative molecular mass.

The number of particles in one mole, 6.022 × 10^23 per mole. It links the microscopic particle count to the macroscopic amount of substance weighed in the laboratory.

The mass of one mole of a substance in grams per mole, found by summing relative atomic masses. Used to convert between mass and moles in calculations.

The weighted average mass of an atom of an element compared with one-twelfth the mass of a carbon-12 atom. It is a ratio with no unit.

Atoms of the same element with the same number of protons but different numbers of neutrons, so they share chemical properties but differ in mass and physical behaviour.

The total number of protons and neutrons in the nucleus of an atom, written as a superscript before the symbol. Neutron number equals mass number minus atomic number.

The number of protons in the nucleus of an atom, which identifies the element and equals the number of electrons in a neutral atom. Written as a subscript before the symbol.

The arrangement of electrons in shells and subshells of an atom, following the Aufbau principle, Pauli exclusion principle, and Hund's rule. It explains an element's position and reactivity.

An electron in the outermost shell of an atom that takes part in bonding. The number of valence electrons determines the group and the typical valency of the element.

The volume occupied by one mole of any gas at the same temperature and pressure; about 22.4 dm³ at standard temperature and pressure. Used to convert gas volume to moles.

The simplest whole-number ratio of atoms of each element in a compound. It is found from percentage composition and may differ from the molecular formula.

The actual number of atoms of each element in one molecule of a compound. It is a whole-number multiple of the empirical formula, found using the relative molecular mass.

The strong electrostatic attraction between oppositely charged ions formed when one atom transfers electrons to another. Typical of metals with non-metals, giving high melting points and conductivity when molten.

A bond formed when two atoms share one or more pairs of electrons. It is directional and typical of non-metal compounds, producing molecules or giant covalent structures.

The attraction between a lattice of positive metal ions and a sea of delocalised electrons. It explains electrical conductivity, malleability, and ductility of metals.

Loss of electrons by a species, or an increase in oxidation number, or gain of oxygen. Always accompanied by reduction in a redox reaction.

Gain of electrons by a species, or a decrease in oxidation number, or loss of oxygen. Always paired with oxidation in a redox reaction.

The charge an atom would carry if its bonds were treated as fully ionic. Rules based on group number and standard values allow balancing of redox equations.

A species that accepts electrons and is itself reduced, causing another substance to be oxidised. Common examples are potassium manganate(VII) and chlorine.

A species that donates electrons and is itself oxidised, causing another substance to be reduced. Common examples are hydrogen and carbon.

A substance that donates a proton to another species (Brønsted–Lowry), or an electron-pair acceptor (Lewis). Aqueous acids release hydrogen ions and turn blue litmus red.

A substance that accepts a proton (Brønsted–Lowry) or donates an electron pair (Lewis). Soluble bases are alkalis, which release hydroxide ions in water.

An acid that ionises almost completely in water, such as hydrochloric or sulfuric acid. It gives a low pH and high electrical conductivity in solution.

An acid that only partially ionises in water, establishing an equilibrium, such as ethanoic acid. It gives a higher pH than a strong acid of the same concentration.

A logarithmic measure of hydrogen ion concentration, pH = −log[H⁺]. Values below 7 are acidic, 7 is neutral at 25 °C, and above 7 is alkaline.

A solution that resists large changes in pH when small amounts of acid or alkali are added, usually made from a weak acid with its salt or a weak base with its salt.

A volumetric procedure in which a solution of known concentration is added from a burette to a measured volume of analyte until the end-point is reached, allowing concentration to be calculated.

The point in a titration at which the indicator changes colour, ideally coinciding with the equivalence point where the reactants have reacted in exact stoichiometric proportions.

A solution of accurately known concentration, prepared by dissolving a precisely weighed mass of a pure substance in a fixed volume or by standardising against a primary standard.

A reaction that releases heat to the surroundings, giving a negative enthalpy change. The products have lower enthalpy than the reactants, as in combustion and neutralisation.

A reaction that absorbs heat from the surroundings, giving a positive enthalpy change. The products have higher enthalpy than the reactants, as in thermal decomposition.

The heat energy exchanged with the surroundings at constant pressure, symbol ΔH, measured in kJ per mole. Negative for exothermic and positive for endothermic reactions.

The average energy needed to break one mole of a given bond in gaseous molecules. Breaking bonds absorbs energy while forming bonds releases it.

The change in concentration of a reactant or product per unit time. It increases with concentration, temperature, surface area, and catalysts, explained by collision theory.

The minimum energy that colliding particles must possess for a reaction to occur. It corresponds to the energy barrier between reactants and the transition state.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being consumed or changing the overall enthalpy change.

The state of a reversible reaction in a closed system where forward and reverse rates are equal, so concentrations remain constant although both reactions continue.

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the equilibrium shifts to oppose the change and restore balance.

The decomposition of an electrolyte by passing a direct electric current through it, with oxidation at the anode and reduction at the cathode. Used for extraction and electroplating.

The electrode at which oxidation occurs. In electrolysis it is the positive electrode; in a galvanic cell it is the negative electrode.

The electrode at which reduction occurs. In electrolysis it is the negative electrode; in a galvanic cell it is the positive electrode.

A substance that conducts electricity when molten or dissolved in water because it contains mobile ions, and is decomposed by the current. Strong electrolytes ionise completely.

A family of organic compounds with the same general formula and functional group, showing a gradual change in physical properties and similar chemical reactions.

An atom or group of atoms that gives an organic compound its characteristic chemical properties, such as the hydroxyl group in alcohols or the carboxyl group in acids.

The existence of compounds with the same molecular formula but different structural arrangements or spatial orientations, giving different physical and sometimes chemical properties.

A large molecule made by joining many small monomer units through addition or condensation polymerisation, as in polythene from ethene or nylon from diamines and diacids.

A small molecule capable of joining with many identical or similar molecules to form a polymer. The repeating unit of the polymer reflects its structure.

The alkaline hydrolysis of an ester, especially a fat or oil with sodium hydroxide, producing glycerol and the sodium salts of fatty acids that form soap.

The reversible reaction of a carboxylic acid with an alcohol, catalysed by concentrated sulfuric acid, to form an ester and water. Esters have characteristic fruity smells.

A volumetric method in which the analyte and titrant react by electron transfer, often using potassium manganate(VII) as a self-indicating oxidising agent to find unknown concentrations.

The formation of an insoluble solid when two solutions are mixed and their ions combine to give a product with low solubility, such as silver chloride from silver nitrate and sodium chloride.

The maximum mass of solute that dissolves in a given amount of solvent at a stated temperature to form a saturated solution. It is usually expressed in grams per 100 g of water.

A solution containing the maximum amount of dissolved solute at a given temperature, in equilibrium with undissolved solute. Cooling a saturated solution often causes crystallisation.