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Leaving Cert Chemistry glossary

105 key terms in plain English, grouped by strand and labelled Higher or Ordinary. The same definitions the H1 Owl app teaches, written from the published syllabus.

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Nature of Science 13 terms

AccuracyHigher: How close a measured value is to the true or accepted value of the quantity. Accuracy is limited by systematic errors, so it is improved by calibrating apparatus and removing bias from the method rather than by simply repeating the measurement.
AccuracyOrdinary: How close a measured value is to the correct or true value.
Control variableHigher: A factor that is deliberately kept the same throughout an investigation so that it cannot influence the outcome. Holding these constant means any change in the measured quantity can be attributed to the variable that was altered.
Control variableOrdinary: Something in an experiment that is kept the same every time. Keeping it the same makes the test fair.
HypothesisHigher: A testable statement, based on existing observations or theory, that predicts a relationship between variables. It must be capable of being supported or contradicted by the evidence an experiment produces.
HypothesisOrdinary: A testable idea or prediction that you can check by carrying out an experiment.
Percentage errorHigher: The difference between a measured value and the accepted value, expressed as a percentage of the accepted value. It is found by dividing the error by the accepted value and multiplying by 100, which allows results of very different sizes to be compared fairly.
Percentage errorOrdinary: How far a measurement is from the correct value, given as a percentage. A smaller percentage error means a better result.
PrecisionHigher: How close a set of repeated measurements of the same quantity are to one another. Results can be precise and still be wrong, because a systematic error shifts every reading in the same direction.
PrecisionOrdinary: How close repeated measurements are to each other. Precise readings are grouped closely together.
ReliabilityHigher: The extent to which an investigation gives the same result when it is repeated under the same conditions. Reliability is improved by taking repeated readings and working out a mean, which reduces the effect of random error.
Scientific modelHigher: A simplified representation of a system or idea, such as a diagram, an equation or a physical structure, used to explain observations and make predictions. A model is judged by how well it fits the available evidence and is revised or replaced when new data cannot be accounted for.
Significant figuresHigher: The digits in a measurement that carry real information about its size, counted from the first non-zero digit. A calculated answer should be quoted to no more significant figures than the least precise measurement used to obtain it.

The Nature of Matter 27 terms

AtomHigher: The smallest particle of an element that still shows the chemical behaviour of that element. It is made up of a very small central nucleus containing protons and neutrons, surrounded by moving electrons.
AtomOrdinary: The smallest particle of an element. It has a nucleus in the centre with electrons moving around it.
Atomic numberHigher: The number of protons in the nucleus of an atom, given the symbol Z. It identifies the element and, in a neutral atom, also equals the number of electrons present.
Atomic numberOrdinary: The number of protons in the nucleus of an atom. It tells you which element the atom is.
Avogadro constantHigher: The number of particles contained in one mole of any substance, equal to 6.022 x 10^23 per mole. It is the link that converts a count of atoms, molecules or ions into an amount in moles.
ElectronHigher: A negatively charged subatomic particle that occupies the space outside the nucleus, with a relative charge of -1 and a mass roughly 1/1836 that of a proton. Electrons in the outer energy level are the ones involved in bonding and chemical reactions.
ElectronOrdinary: A very light negative particle that moves around outside the nucleus. Electrons are the part of the atom used in bonding.
Electron configurationHigher: A description of how the electrons of an atom or ion are distributed among the principal energy levels and sublevels, written for example as 1s2 2s2 2p6 for neon. Electrons occupy the lowest available sublevels first, which explains an element's position in the periodic table.
ElectronegativityHigher: A measure of how strongly an atom in a molecule attracts the shared pair of electrons in a covalent bond. Values increase across a period and decrease down a group, and the difference in electronegativity between two bonded atoms shows how polar the bond is.
ElementOrdinary: A pure substance made of only one kind of atom. It cannot be split into anything simpler by a chemical reaction.
Empirical formulaHigher: The formula giving the simplest whole number ratio in which the atoms of each element are present in a compound. It is worked out from percentage composition or combining mass data, and may differ from the molecular formula, as CH2 does for ethene, C2H4.
GroupHigher: A vertical column of the periodic table. Elements in the same group have the same number of electrons in their outermost energy level, which is why they show similar chemical properties.
Ionisation energyHigher: The energy needed to pull an electron away from an atom or ion in the gas phase. The first ionisation energy is the energy required to remove one electron from each atom in a mole of gaseous atoms in their ground state, and its value rises across a period and falls down a group.
IsotopeHigher: Atoms of the same element that contain the same number of protons but different numbers of neutrons, and so have different mass numbers. Isotopes of an element behave identically in chemical reactions but differ slightly in physical properties such as density.
IsotopeOrdinary: Atoms of the same element that have the same number of protons but a different number of neutrons.
Mass numberHigher: The total number of protons and neutrons in the nucleus of an atom, given the symbol A. Subtracting the atomic number from the mass number gives the number of neutrons.
Mass numberOrdinary: The number of protons added to the number of neutrons in the nucleus of an atom.
Molar massHigher: The mass of one mole of a substance, normally quoted in grams per mole. It is calculated by adding together the relative atomic masses of every atom shown in the formula.
MoleHigher: The SI unit of amount of substance, containing 6.022 x 10^23 particles, which is about the number of atoms in 12 g of carbon-12. The mass of one mole of a substance in grams is numerically equal to its relative atomic or relative molecular mass.
MoleOrdinary: An amount of substance that contains 6.022 x 10^23 particles. Its mass in grams is the same number as the relative molecular mass.
NeutronHigher: An uncharged subatomic particle found in the nucleus, with a relative mass of 1. Varying the number of neutrons produces different isotopes of the same element without changing its chemistry.
NeutronOrdinary: A particle in the nucleus of an atom that has no charge. It also has a relative mass of 1.
OrbitalHigher: A region of space around the nucleus in which there is a high probability of finding an electron. Each orbital holds a maximum of two electrons and has a characteristic shape, such as the spherical s orbital and the dumbbell shaped p orbital.
PeriodHigher: A horizontal row of the periodic table. Elements in the same period have their outermost electrons in the same highest principal energy level, and properties change steadily from metallic to non-metallic as you move across.
ProtonHigher: A positively charged subatomic particle found in the nucleus, with a relative charge of +1 and a relative mass of 1. The number of protons in the nucleus fixes which element an atom belongs to.
ProtonOrdinary: A positive particle found in the nucleus of an atom. It has a relative mass of 1.
Relative atomic massHigher: The average mass of the atoms of an element, taking the abundance of each isotope into account, measured relative to one twelfth of the mass of a carbon-12 atom. Because it is a ratio of two masses it has no units.

Behaviour of Matter 27 terms

Activation energyHigher: The minimum amount of energy that colliding particles must possess before a reaction between them can take place. A reaction with a high activation energy is slow because only a small fraction of collisions are energetic enough to succeed.
Boyle's lawHigher: At constant temperature the volume of a fixed mass of gas is inversely proportional to the pressure applied to it. Doubling the pressure therefore halves the volume, and a graph of pressure against 1/volume gives a straight line through the origin.
Boyle's lawOrdinary: At a steady temperature, squashing a fixed amount of gas into a smaller volume raises its pressure. Pressure and volume are inversely proportional.
CatalystHigher: A substance that increases the rate of a chemical reaction by offering an alternative route with a lower activation energy, and that is not used up overall. It cannot shift the position of an equilibrium, it only shortens the time taken to reach it.
CatalystOrdinary: A substance that makes a reaction go faster but is not used up itself.
Charles's lawHigher: At constant pressure the volume of a fixed mass of gas is directly proportional to its temperature measured on the kelvin scale. Warming a gas that is free to expand therefore increases its volume in the same proportion.
Collision theoryHigher: The explanation that a reaction occurs only when particles collide with at least the activation energy and with a suitable orientation. Any change that raises the frequency or the energy of collisions, such as higher temperature, greater concentration or smaller particle size, speeds the reaction up.
Covalent bondHigher: A bond formed when two atoms share one or more pairs of electrons, with each shared pair attracted to both nuclei at once. It normally forms between non-metal atoms so that each atom achieves a stable outer electron arrangement.
Covalent bondOrdinary: A bond made when two atoms share a pair of electrons. It usually forms between non-metals.
Dynamic equilibriumHigher: The state reached by a reversible reaction in a closed system when the forward and reverse reactions are proceeding at exactly the same rate. The concentrations of reactants and products stay constant even though both reactions are still going on.
Endothermic reactionHigher: A reaction that absorbs heat energy from its surroundings, so the temperature of the surroundings falls. The products store more energy than the reactants did, which gives a positive value for the enthalpy change.
Endothermic reactionOrdinary: A reaction that takes in heat, so the surroundings get colder.
Enthalpy changeHigher: The heat energy given out or taken in when a reaction is carried out at constant pressure, given the symbol ΔH and usually quoted in kilojoules per mole. Its sign is negative when heat is released and positive when heat is absorbed.
Equilibrium constantHigher: A value, Kc, obtained by dividing the equilibrium concentrations of the products, each raised to the power of its balancing number, by the same expression for the reactants. It changes only with temperature, and a large value shows the equilibrium mixture is rich in products.
Exothermic reactionHigher: A reaction that releases heat energy to its surroundings, so the temperature of the surroundings rises. The products store less energy than the reactants did, which gives a negative value for the enthalpy change.
Exothermic reactionOrdinary: A reaction that gives out heat, so the surroundings get warmer.
Hydrogen bondingHigher: A comparatively strong intermolecular attraction between a hydrogen atom bonded to nitrogen, oxygen or fluorine in one molecule and a lone pair on the nitrogen, oxygen or fluorine atom of a neighbouring molecule. It explains the unusually high boiling point of water and why small alcohols mix freely with water.
Hydrogen bondingOrdinary: A strong pull between molecules that have hydrogen joined to oxygen, nitrogen or fluorine. It is why water has such a high boiling point.
Ionic bondHigher: The electrostatic force of attraction between oppositely charged ions that are formed when electrons are transferred from a metal atom to a non-metal atom. Ionic substances build up giant lattices, which gives them high melting points and lets them conduct when molten or dissolved.
Ionic bondOrdinary: The attraction between a positive ion and a negative ion. The ions form when electrons move from a metal atom to a non-metal atom.
Le Chatelier's principleHigher: If a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts in the direction that opposes that change. It is used to work out the conditions that give the best yield in industrial processes such as ammonia manufacture.
Metallic bondHigher: The force of attraction between a lattice of positive metal ions and the sea of delocalised outer electrons that surrounds them. Because those electrons are free to move, metals conduct electricity and heat well and can be beaten into shape without shattering.
Metallic bondOrdinary: The attraction between positive metal ions and the free electrons moving around them. It is why metals conduct electricity.
MoleculeOrdinary: Two or more atoms joined together by covalent bonds. Water and oxygen gas are made of molecules.
Polar covalent bondHigher: A covalent bond in which the shared pair of electrons is held closer to the more electronegative of the two atoms. This leaves that atom with a partial negative charge and its partner with a partial positive charge, creating a dipole.
Van der Waals forcesHigher: Weak attractions between neighbouring molecules caused by temporary dipoles that appear as electrons move about within a molecule. They get stronger as the number of electrons in a molecule increases, which is why boiling points rise down a group such as the halogens.
VSEPR theoryHigher: Valence shell electron pair repulsion theory, which predicts molecular shape by assuming the electron pairs around a central atom move as far apart as possible because they repel one another. Lone pairs repel more strongly than bonding pairs, so their presence squeezes bond angles slightly closer together.

Interactions of Matter 23 terms

AcidHigher: A substance that releases hydrogen ions when it dissolves in water. Acidic solutions turn blue litmus red, have a pH below 7 and react with bases to give a salt and water.
AcidOrdinary: A substance that gives out hydrogen ions in water. Acids have a pH below 7 and turn blue litmus red.
BaseHigher: A substance that neutralises an acid, accepting hydrogen ions and forming a salt and water. A base that dissolves in water is called an alkali and produces hydroxide ions in solution.
BaseOrdinary: A substance that cancels out an acid to give a salt and water. A base that dissolves in water is called an alkali.
Bronsted-Lowry acidHigher: A species that donates a proton, that is a hydrogen ion, to another species. On this definition a substance can only behave as an acid when a base is present to accept the proton it gives up.
Conjugate baseHigher: The species left behind when a Bronsted-Lowry acid has donated its proton. An acid and its conjugate base always differ by one hydrogen ion, so the chloride ion is the conjugate base of hydrochloric acid.
IndicatorHigher: A substance that changes colour over a particular range of pH and is used to reveal the end point of a titration or to estimate acidity. Methyl orange and phenolphthalein change colour in different pH ranges, so each suits particular acid and base combinations.
NeutralisationHigher: The reaction between an acid and a base in which a salt and water are produced. In aqueous solution it amounts to hydrogen ions joining with hydroxide ions to form water, and the process gives out heat.
NeutralisationOrdinary: The reaction of an acid with a base to give a salt and water. Heat is given out.
OxidationHigher: The loss of electrons by a species, which shows up as an increase in its oxidation number. In many reactions it can also be recognised as the gain of oxygen or the loss of hydrogen.
OxidationOrdinary: The loss of electrons by a substance. It can also mean gaining oxygen.
Oxidation numberHigher: A number assigned to an atom in a compound or ion showing the charge it would carry if all its bonds were treated as fully ionic. The oxidation numbers in a neutral compound add up to zero, and in an ion they add up to the charge on that ion.
Oxidising agentHigher: A substance that causes another substance to be oxidised by taking electrons from it. Because it accepts those electrons, the oxidising agent is itself reduced during the reaction.
pHHigher: A measure of how acidic or basic a solution is, defined as the negative logarithm to base ten of the hydrogen ion concentration in moles per litre. At 25 degrees Celsius a value of 7 is neutral, lower values are acidic and higher values are basic.
pHOrdinary: A scale that shows how acidic or basic a solution is, usually running from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is basic.
Primary standardHigher: A substance pure and stable enough that a standard solution can be prepared directly by weighing it out and dissolving it in a known volume. It must not absorb moisture from the air, must dissolve readily, and ideally has a fairly high relative molecular mass so that weighing errors are small.
Reducing agentHigher: A substance that causes another substance to be reduced by supplying electrons to it. Because it gives those electrons away, the reducing agent is itself oxidised during the reaction.
ReductionHigher: The gain of electrons by a species, which shows up as a decrease in its oxidation number. In many reactions it can also be recognised as the loss of oxygen or the gain of hydrogen.
ReductionOrdinary: The gain of electrons by a substance. It can also mean losing oxygen.
Strong acidHigher: An acid that ionises completely in aqueous solution, so effectively every molecule hands its proton to water. Hydrochloric acid and nitric acid are examples, and dilute solutions of them have low pH values and conduct electricity well.
TitrationHigher: A quantitative procedure in which a solution of accurately known concentration is run from a burette into a measured volume of another solution until the reaction between them is just complete. The volume delivered at the end point is used to calculate the unknown concentration.
TitrationOrdinary: An experiment where liquid is added from a burette to another solution until the reaction is exactly finished. It is used to find an unknown concentration.
Weak acidHigher: An acid that ionises only partially in aqueous solution, leaving an equilibrium between undissociated molecules and ions. Ethanoic acid is a familiar example, and its solution has a higher pH than a strong acid of the same concentration.

Matter in Our World 15 terms

AlcoholHigher: A member of the homologous series whose molecules carry a hydroxyl group, OH, attached to a carbon atom, with the general formula CnH2n+1OH. Hydrogen bonding between these groups gives alcohols much higher boiling points than alkanes of comparable size.
AlcoholOrdinary: An organic compound with an OH group joined to a carbon atom. Ethanol is the alcohol found in drinks.
AlkaneHigher: A saturated hydrocarbon in which every carbon to carbon bond is a single bond, with the general formula CnH2n+2. Alkanes are relatively unreactive but burn readily in oxygen, and they make up most of natural gas and petrol.
AlkaneOrdinary: A hydrocarbon with only single bonds between its carbon atoms. Methane in natural gas is an alkane.
AlkeneHigher: An unsaturated hydrocarbon containing a carbon to carbon double bond, with the general formula CnH2n when a single double bond is present. The double bond makes alkenes reactive towards addition reactions, which is why they decolourise bromine water.
AlkeneOrdinary: A hydrocarbon with a double bond between two of its carbon atoms. Alkenes turn bromine water from orange to colourless.
Carboxylic acidHigher: An organic compound whose molecules contain the carboxyl group, COOH. They behave as weak acids, giving a salt and water with bases and a salt, water and carbon dioxide with carbonates, and they react with alcohols to form esters.
EsterHigher: An organic compound produced when a carboxylic acid reacts with an alcohol, water being lost in the process, and containing the COO link between two carbon groups. Many esters are volatile liquids with pleasant fruity smells and are used as flavourings and solvents.
Functional groupHigher: An atom or group of atoms within an organic molecule that is responsible for its characteristic chemical reactions. The hydroxyl group of alcohols and the carboxyl group of carboxylic acids are two examples.
Hard waterHigher: Water containing dissolved calcium or magnesium ions, usually picked up as it flows over rocks such as limestone or gypsum. It wastes soap by forming a scum and can leave deposits of scale in kettles, pipes and heating systems.
Hard waterOrdinary: Water with calcium or magnesium dissolved in it. It wastes soap and leaves scale inside kettles and pipes.
Homologous seriesHigher: A family of organic compounds sharing the same general formula and the same functional group, in which each member differs from the next by a CH2 unit. Members show very similar chemical behaviour and a gradual trend in physical properties such as boiling point.
PolymerHigher: A very large molecule built by joining many small repeating units, called monomers, into a long chain. Poly(ethene) is formed when many ethene molecules add to one another without losing any atoms.
PolymerOrdinary: A very long molecule made by joining many small molecules, called monomers, into a chain. Plastics are polymers.
Structural isomerHigher: One of two or more compounds that share the same molecular formula but have their atoms joined together in a different order. Structural isomers have different structural formulae and so differ in physical properties and sometimes in chemical properties as well.

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