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Chemistry Flashcards: Key Terms With Short Definitions

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Matter and its states 16

Boiling point The temperature at which a liquid boils into a gas. It changes with the pressure around it.
Condensation A gas turning into a liquid, as when water vapor forms drops on a cold surface.
Density How much mass is packed into a given volume: mass divided by volume.
Evaporation A liquid turning into a gas at its surface, as when a puddle dries up.
Gas Matter with neither a fixed shape nor a fixed volume, such as the air.
Liquid Matter with a definite volume but no fixed shape: it takes the shape of its container.
Matter Anything that has mass and takes up space, from a grain of sand to the air in a room.
Melting point The temperature at which a solid turns into a liquid.
Mixture Two or more substances together that can be separated again by physical means.
Pure substance Matter with a constant makeup: a single element or a single compound, not a mixture.
Solid Matter that keeps its own shape and volume because its particles are packed closely together.
Solubility How much of a solute can dissolve in a given amount of a solvent.
Solute The substance that is dissolved in a solution. In salt water it is the salt.
Solution A mixture in which one substance is dissolved evenly in another, like salt in water.
Solvent The substance that does the dissolving in a solution. In salt water it is the water.
Sublimation A solid turning straight into a gas without becoming a liquid first, as dry ice does.

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Atoms 16

Atom The smallest particle that still counts as an element: a nucleus with electrons around it.
Atomic number The number of protons in the nucleus of an atom. Each element has its own.
Electron A particle with a negative charge, far lighter than a proton, found around the nucleus.
Electron shell A layer around the nucleus that electrons are pictured as moving in, numbered from the inside.
Element A substance whose atoms all have the same number of protons, such as oxygen.
Group A column of the periodic table. The elements in a group have similar properties.
Ion An atom or group of atoms with an electric charge: its electrons do not match its protons.
Isotopes Atoms of one element with the same number of protons but different numbers of neutrons.
Mass number The total number of protons and neutrons in the nucleus of one atom.
Neutron A particle with no electric charge, found in the nucleus beside the protons.
Nucleus The small, dense center of an atom. It holds the protons and neutrons and almost all the mass.
Period A row of the periodic table. Each element along it has one more proton than the one before.
Periodic table A chart of the elements in order of atomic number, set out in rows and columns.
Proton A particle with a positive charge in the nucleus. The number of protons sets the element.
Relative atomic mass The average mass of an element's atoms, compared with one twelfth of a carbon-12 atom.
Valence electron An electron in the outermost shell of an atom. Valence electrons can take part in bonding.

Bonding 16

Alloy A mixture of elements, at least one a metal, that acts as a metal. Steel and bronze are alloys.
Anion An ion with a negative charge: it has more electrons than protons.
Cation An ion with a positive charge: it has fewer electrons than protons.
Chemical bond A lasting attraction that holds atoms or ions together in a molecule or a crystal.
Chemical formula Element symbols and small numbers that show which atoms a substance holds and how many of each.
Compound A substance made of atoms of two or more elements held together by chemical bonds.
Covalent bond A bond in which two atoms share one or more pairs of electrons.
Crystal lattice The ordered, repeating arrangement of atoms, ions or molecules in a crystal.
Double bond A covalent bond in which two atoms share two pairs of electrons: four electrons in all.
Electronegativity How strongly an atom pulls shared electrons toward itself in a chemical bond.
Hydrogen bond A pull between a hydrogen atom bonded to nitrogen, oxygen or fluorine and another such atom.
Ionic bond A bond made by the pull between ions of opposite charge.
Lone pair A pair of outer-shell electrons that is not shared with another atom in a bond.
Metallic bond The bonding in a metal: positive metal ions held together by electrons free to move among them.
Molecule A group of two or more atoms held together by chemical bonds, with no overall charge.
Polar molecule A molecule with a slightly negative end and a slightly positive end.

Reactions 16

Activation energy The least energy the reactants must have before a reaction can happen.
Catalyst A substance that speeds up a reaction and is not used up by it.
Chemical equation Formulas and an arrow that show the reactants of a reaction turning into its products.
Chemical equilibrium The state in which a reaction and its reverse go at the same rate, so amounts stop changing.
Chemical reaction A process that changes one set of substances into another as their atoms are rearranged.
Combustion Burning: a hot reaction of a fuel, usually with oxygen from the air, that gives out heat.
Conservation of mass In a closed system a reaction neither makes nor destroys matter: the total mass stays the same.
Endothermic reaction A reaction that absorbs heat from its surroundings.
Exothermic reaction A reaction that releases energy, usually as heat.
Mole The unit for amount of substance: exactly 6.02214076 × 10²³ particles.
Oxidation The loss of electrons by an atom, ion or molecule in a reaction.
Precipitate A solid that forms in a liquid solution and settles out of it.
Product A substance that is formed in a chemical reaction.
Reactant A substance that is used up in a chemical reaction.
Redox reaction A reaction in which oxidation and reduction happen together as electrons are transferred.
Reduction The gain of electrons by an atom, ion or molecule in a reaction: the opposite of oxidation.

Acids and bases 16

Acid A substance that can give up a hydrogen ion, also called a proton.
Alkali A base that dissolves in water. Its solution has a pH above 7.
Aqueous solution A solution in which the solvent is water. Equations mark it (aq).
Base A substance that can accept a hydrogen ion. Bases react with acids.
Buffer solution A solution whose pH hardly changes when a small amount of acid or base is added.
Concentration How much of a substance there is in a given volume of a mixture.
Dilution Making a solution less concentrated by mixing in more solvent, usually water.
Endpoint The point in a titration at which the indicator changes color: the sign to stop adding.
Indicator A substance added to a solution in small amounts that shows the pH by its color.
Litmus A dye from lichens that turns red in an acid and blue in a base.
Neutralization A reaction in which an acid and a base react in matching amounts, leaving no excess of either.
pH A scale for how acidic or basic a water solution is. At 25 °C, 7 is neutral and less is acidic.
Salt A compound made of positive ions and negative ions, with no overall charge.
Strong acid An acid that breaks up into ions almost completely in water, such as hydrochloric acid.
Titration Finding the concentration of a solution by adding a solution of known concentration to it.
Weak acid An acid that only partly breaks up into ions in water, such as acetic acid.

The lab 16

Beaker A cylinder-shaped container with a flat bottom and a small spout for pouring.
Bunsen burner A gas burner that gives a single open flame, used for heating.
Burette A long marked glass tube with a tap at the bottom, for letting out measured volumes of liquid.
Crucible A container that withstands very high temperatures, in which substances are heated or melted.
Erlenmeyer flask A flask with a flat bottom, a cone-shaped body and a narrow neck. Also called a conical flask.
Evaporating dish A dish in which a solution is heated so its liquid evaporates, leaving a solid or less liquid.
Filter paper Paper that lets a liquid pass through and holds back the fine solid particles in it.
Fume hood A ventilated enclosure that keeps harmful fumes and dust away from the person working at it.
Funnel A cone that narrows to a tube, for guiding a liquid or powder into a small opening.
Graduated cylinder A tall, narrow cylinder with marked lines, used to measure the volume of a liquid.
Mortar and pestle A bowl and a blunt club-shaped tool, used to crush and grind solids into a powder.
Pipette A narrow tube used to draw up and move a measured volume of liquid.
Safety goggles Eye protection that closes off the area around the eyes, so chemicals cannot splash into them.
Test tube A finger-shaped tube of glass or clear plastic, open at the top and closed at the bottom.
Volumetric flask A flask made to hold one exact volume, used to make up solutions of known concentration.
Watch glass A round, slightly hollow piece of glass, used to hold a small sample or to cover a beaker.

How this list was compiled: Own work: 96 terms from a first chemistry course, each with a gist and a one-sentence definition written for this page. On 2026-09-30 every definition was read against two references: the opening of the article on the term at https://en.wikipedia.org/ and the entry for the term at https://en.wiktionary.org/ (for strong acid, weak acid and aqueous solution, the entries strong, weak and aqueous). 26 of the definitions were also read against the IUPAC Compendium of Chemical Terminology, known as the Gold Book, in the copies of goldbook.iupac.org kept at https://web.archive.org/. A term with fewer than two references was left out. The deck has no card for a single element or ion. Burette and pipette are also spelled buret and pipet. Licence: own work

How it works

These chemistry flashcards cover 96 terms from a first course, in six sets of 16. The front shows a term such as Catalyst. The back gives a short gist, speeds a reaction, and a one-sentence definition.

Read the full guide

Six sets of 16 terms

Show only deals one set; the page opens on Matter and its states. Atoms runs from nucleus to relative atomic mass, Bonding from ionic bond to alloy, and Reactions from reactant to chemical equilibrium. Acids and bases covers pH, indicators and titration. The lab names 16 pieces of glassware and kit by what each is for.

Print one set on two sheets

Choose a set, turn a card and press the print button under the card. Twelve cards fill a sheet of Letter or A4, so a set of 16 takes two sheets and all 96 take eight. Print on both sides and flip on the long edge; the backs are mirrored, so each gist lands behind its term.

There is nothing to type here: say the meaning in your own words, then turn the card and compare.
Element names and symbols are a separate deck, the periodic table flashcards.
Burette and pipette are also spelled buret and pipet. Both spellings are in use.

Questions and answers

What topics do these chemistry flashcards cover?

Six: matter and its states, atoms, bonding, reactions, acids and bases, and lab equipment, with 16 terms each. There are no cards for single elements or ions; the periodic table deck holds the elements.

What is the difference between an atom and a molecule?

An atom is the smallest particle that still counts as an element. A molecule is a group of two or more atoms held together by chemical bonds.

What is the difference between oxidation and reduction?

Oxidation is the loss of electrons and reduction is the gain of electrons. The two happen together in one reaction: what one substance loses, another gains.

How were the definitions checked?

Each definition is written in our own words and was read against two references on the day it was written. The section How this list was compiled names them.