Chemistry: Science Terms Explained
Plain-language definitions of every chemistry term your child meets in school, each with kid-friendly explanations, worked examples, and common mistakes to watch for.
Acid
An acid is a substance that releases hydrogen ions (H+) in water, often tastes sour, and can react with metals and bases to form new substances.
Atomic mass
Atomic mass is the mass of a single atom, measured mostly by adding up the mass of its protons and neutrons, and is usually expressed in atomic mass units (amu).
Atomic number
An atomic number is the number of protons in the nucleus of an atom, which identifies what element it is and helps determine its place on the periodic table.
Catalyst
A catalyst is a substance that speeds up a chemical reaction without being used up or permanently changed by the reaction.
Chemical bond
A chemical bond is a force of attraction that holds atoms together in molecules or crystals by sharing or transferring electrons between them.
Chemical equation
A chemical equation is a written way to show a chemical reaction, using chemical formulas and symbols to tell which substances react, which new substances form, and in what amounts.
Chemical formula
A chemical formula is a short way of writing which elements are in a substance and how many atoms of each element are joined together in one particle of that substance.
Chemical reaction
A chemical reaction is a process where one or more substances change into new substances with different properties because the atoms are rearranged into new combinations.
Combustion
Combustion is a chemical reaction where a substance reacts quickly with oxygen, releasing energy as heat and often light, and forming new substances such as gases or ash.
Compound
A compound is a pure substance made when two or more different elements join together in a fixed ratio by chemical bonds, creating a material with its own unique properties.
Concentration
concentration is how much of one substance is mixed into a certain amount of another substance, usually describing how much solute is dissolved in a given amount of solvent or solution.
Covalent bond
A covalent bond is a chemical bond where two atoms share one or more pairs of electrons so that each atom can reach a more stable electron arrangement.
Electron
An electron is a tiny particle inside an atom that carries a negative electric charge and moves around the atom's center, helping control how atoms bond, form chemical reactions, and conduct electricity.
Electron cloud
An electron cloud is the three-dimensional region around an atom's nucleus where electrons are most likely to be found, described by probability rather than fixed orbits.
Element
An element is a pure substance made of only one kind of atom, which cannot be broken down into simpler substances by ordinary chemical reactions.
Endothermic
Endothermic describes a process or chemical reaction that absorbs energy, usually in the form of heat, from its surroundings, causing the surroundings to get cooler while the system taking in energy gets warmer or changes state.
Exothermic
Exothermic describes a process or chemical reaction that releases energy, usually as heat or light, into its surroundings.
Groups and periods
Groups and periods are the vertical columns and horizontal rows of the periodic table of elements, which organize elements by their atomic number and by patterns in their properties, such as how they react and how many valence electrons they have.
Ion
An ion is an atom or group of atoms that has gained or lost electrons, so it has an overall electric charge instead of being neutral.
Ionic bond
An ionic bond is a chemical bond formed when one atom transfers one or more electrons to another atom, creating oppositely charged ions that attract each other.
Isotope
An isotope is a form of the same chemical element that has the same number of protons but a different number of neutrons in its nucleus, so its atoms have different masses.
Law of conservation of mass
The law of conservation of mass is the science rule that says in a closed system, mass cannot be created or destroyed during a physical or chemical change; the total mass of all substances before the change is equal to the total mass after the change.
Metallic bond
A metallic bond is the attraction between positively charged metal ions and a "sea" of freely moving valence electrons that holds metal atoms together in a solid or liquid metal.
Molarity
Molarity is the concentration of a solution, defined as the number of moles of dissolved substance (solute) in 1 liter of solution.
Neutron
A neutron is a tiny particle found in the nucleus (center) of an atom that has mass but no electric charge, and it helps determine an atom's mass and which isotope of an element it is.
Noble gases
Noble gases are the group of elements in the far right column of the periodic table that are very stable and almost never react with other elements because their outer electron shells are full.
Oxidation
Oxidation is a type of chemical reaction where a substance loses electrons, often while reacting with oxygen, which usually increases the substance’s oxidation number and can release energy.
Periodic table
The periodic table is a chart that organizes all known chemical elements by their atomic number and by patterns in their properties, so scientists and students can quickly see how elements are related and predict how they might behave in chemical reactions.
PH scale
The pH scale is a numerical scale from 0 to 14 that measures how acidic or basic (alkaline) a water-based solution is, based on the concentration of hydrogen ions in the solution.
Precipitate
A precipitate is a solid that forms and separates out of a liquid mixture during a chemical reaction, often appearing as tiny particles that make the liquid look cloudy or as a solid that settles to the bottom.
Proton
A proton is a tiny positively charged particle found in the nucleus (center) of an atom, and the number of protons in the nucleus determines which element the atom is.
Reactant
A reactant is a starting substance in a chemical reaction, the material that is used up or changed to form new substances called products.
Every term here links to practice activities. Create an account to get started with your child.
Get Started Free