NSSCO Chemistry (6117) key terms

A pure substance made of only one kind of atom, which cannot be split into simpler substances by chemical means. Elements are listed in the Periodic Table and have unique atomic numbers.

A pure substance formed when two or more different elements are chemically joined in fixed proportions. The elements lose their original properties, and the compound can only be separated by chemical reactions.

Two or more substances mixed together without chemical bonding, in any proportion. Each substance keeps its own properties, and the components can be separated by physical methods such as filtration or distillation.

A separation method that uses a filter paper or mesh to trap solid particles while allowing a liquid or gas to pass through. It separates an insoluble solid from a liquid mixture.

A separation technique that boils a liquid mixture and condenses the vapour back to liquid. It separates a solvent from dissolved solids or liquids with very different boiling points.

The number of protons in the nucleus of an atom. It identifies the element, and in a neutral atom it equals the number of electrons. Atomic number determines an element's position in the Periodic Table.

The total number of protons and neutrons in the nucleus of an atom. Electrons are ignored because their mass is negligible. Mass number minus atomic number gives the number of neutrons.

Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties but slightly different masses and physical properties.

The weighted average mass of the naturally occurring isotopes of an element, compared with one-twelfth of the mass of a carbon-12 atom. It is not a whole number for most elements.

The strong electrostatic attraction between oppositely charged ions, formed when electrons are transferred from a metal atom to a non-metal atom. Ionic compounds form giant lattices with high melting points.

A bond formed when two atoms share one or more pairs of electrons. Covalent bonds usually form between non-metal atoms and can produce simple molecules or giant covalent structures.

The attraction between positively charged metal ions in a lattice and the delocalised electrons that move freely around them. This bonding explains why metals conduct electricity and are malleable.

The amount of substance containing the same number of particles as there are atoms in exactly 12 grams of carbon-12. One mole contains the Avogadro number of particles.

The number of particles in one mole of a substance, equal to 6.02 × 10²³ particles per mole. It links the mass of a substance to the number of atoms, ions or molecules present.

The mass of one mole of a substance, measured in grams per mole (g/mol). It is calculated by adding the relative atomic masses of all atoms shown in the chemical formula.

The simplest whole-number ratio of the atoms of each element present in a compound. It is found from experimental masses or percentage composition, not from the actual molecular formula.

A substance that releases hydrogen ions (H⁺) when dissolved in water. Acids react with bases to form salts and water, and they turn blue litmus paper red.

A substance that can neutralise an acid to form a salt and water. A soluble base is called an alkali, and it releases hydroxide ions (OH⁻) when dissolved in water.

A scale from 0 to 14 that measures the acidity or alkalinity of an aqueous solution. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline.

A chemical reaction between an acid and a base that produces a salt and water. The hydrogen ions from the acid react with hydroxide ions or with the base to form water.

A vertical column of elements in the Periodic Table. Elements in the same group have the same number of electrons in their outer shell, so they show similar chemical properties.

A horizontal row of elements in the Periodic Table. Across a period, the number of outer-shell electrons increases and the elements change from metallic to non-metallic.

Any element in Group 17 of the Periodic Table, such as fluorine, chlorine, bromine or iodine. They are reactive non-metals with seven outer-shell electrons and form salts with metals.

A naturally occurring rock or mineral that contains a metal compound in high enough concentration to make extraction economically worthwhile. Ores are mined and processed to obtain the metal.

A chemical reaction in which a substance gains electrons or loses oxygen. In metal extraction, metal ions are reduced to metal atoms by removing oxygen or adding electrons.

The decomposition of an ionic compound using electricity, usually when molten or dissolved in water. Positive ions move to the cathode and negative ions move to the anode.

Different structural forms of the same element in the same physical state. Diamond and graphite are allotropes of carbon; they have different arrangements of atoms and different properties.

Rainwater that has become more acidic because gases such as sulfur dioxide and nitrogen oxides dissolve in it. These gases come mainly from burning fossil fuels and damage buildings and ecosystems.

A gas in the atmosphere that absorbs infrared radiation and re-emits it, warming the Earth's surface. Carbon dioxide, methane and water vapour are important examples.

The speed at which reactants are used up or products are formed in a chemical reaction. It is usually measured as change in concentration or mass per unit time.

The minimum energy that colliding particles must have for a reaction to happen. Only collisions with enough energy can break bonds and allow products to form.

A substance that speeds up a chemical reaction by providing an alternative pathway with lower activation energy. It is not used up and is chemically unchanged at the end.

The state in a reversible reaction where the forward and reverse reactions happen at the same rate. The concentrations of reactants and products stay constant, but both reactions continue.

A compound containing only carbon and hydrogen atoms. Hydrocarbons can be alkanes, alkenes or other families, and they are the main components of crude oil and natural gas.

A saturated hydrocarbon with only single covalent bonds between carbon atoms. Alkanes have the general formula CₙH₂ₙ₊₂ and are relatively unreactive.

An unsaturated hydrocarbon containing at least one carbon-carbon double bond. Alkenes have the general formula CₙH₂ₙ and are more reactive than alkanes.

A large molecule made by joining many small molecules called monomers. Polymers can be natural, like proteins, or synthetic, like polythene, and are used as plastics and fibres.

A testable statement or prediction about the relationship between variables, based on scientific knowledge. An experiment is designed to collect evidence that supports or rejects it.

The variable that the experimenter deliberately changes or chooses in an experiment. Its effect on the dependent variable is measured while other variables are kept constant.

A result that does not fit the pattern of other measurements in a set. Anomalies may be caused by measurement errors or mistakes and are usually ignored when calculating averages.