atomThe unit of an element that a reaction cannot split. Carbon atoms that react with oxygen atoms form carbon dioxide molecules.
catalystA helper that speeds a reaction yet ends unchanged. A catalyst usually works for one particular reaction. Enzymes are the catalysts of living cells.
chemical bondThe grip that fastens an atom to its neighbor. In a reaction, bonds are broken and new ones are made.
chemical equationA line of formulas that sums up a reaction. A plus sign stands between the substances on each side.
chemical reactionWhat happens when atoms regroup into new substances. Atoms are rearranged, but their number does not change. Burning fuel and cooking food are chemical reactions.
chemistryThe science of matter and of its reactions. It studies what matter is made of, how it is built, and how it changes in reactions. Its nickname is the central science: it sits between physics and biology.
endothermicTaking in heat, so that things nearby cool down. Citric acid mixed with baking soda is an endothermic reaction: the flask feels cold.
Show all 36 terms (29 more)
Reaction basics, continued
exothermicLetting energy out, so that things nearby warm up. Combustion is exothermic. A glow stick is too, though its energy comes out as light.
physical changeA change in look or state, not in makeup. Boiling is one: cool the steam and it is liquid water again. Dissolving is another.
precipitateA cloud of solid bits formed as two liquids react. The solid appears because it is insoluble. The clear liquid left above it is called the supernatant.
productA substance that a reaction brings into being. A reaction can have more than one. Burning methane has two: carbon dioxide and water.
reactantAn ingredient of a reaction, changed as it proceeds. It is used up as the reaction goes on. Reagent is often used to mean the same thing.
Reaction types and energy 12
activation energyThe smallest dose of energy that sets a reaction off. A spark supplies it for a gas flame, which then keeps itself going.
by-productA second product made alongside the wanted one. It may be useful or it may be waste. Bran is a by-product of milling wheat into refined flour.
calorimeterA device that gauges heat released or absorbed. Antoine Lavoisier and Pierre-Simon Laplace used the first ice calorimeter in the winter of 1782 to 1783, to find the heat of chemical changes.
carbon dioxideThe gas that turns limewater cloudy. Its formula is CO2. It bubbles out when vinegar is mixed with baking soda, and fuels that contain carbon give it off as they burn.
chemiluminescenceLight given off by a chemical reaction. A glow stick works this way: once the glass capsule inside it is broken, two solutions mix and glow without warming up.
combustionThe fast, hot reaction that people call burning. The fuel combines with oxygen, and energy is given out. A fire needs fuel, oxygen and heat: take one away and it goes out.
decompositionA reaction that breaks one substance into several. Hydrogen peroxide slowly decomposes into water and oxygen. Heating green copper carbonate gives black copper oxide and carbon dioxide.
double replacementAn exchange of ions between a pair of compounds. Its pattern is AB + CD gives AD + CB. Often one of the products is a precipitate.
hydrocarbonA compound built from just two elements, H and C. Methane, CH4, is one. Most of the world's energy comes from burning hydrocarbons, which gives carbon dioxide and water.
single replacementA reaction where one element pushes out another. Its pattern is A + BC gives AC + B. Zinc dropped into copper sulfate solution takes the place of the copper.
surroundingsAll that lies outside the reaction being studied. They include the flask and the air around it. An insulated cup slows the flow of heat to or from them.
synthesisA reaction in which several things combine into one. Its pattern is A + B gives AB: two or more simple substances combine into a more complex one.
Formulas, matter and systems 12
carbonateA compound holding carbon joined to three oxygens. Calcium carbonate is the main part of limestone, of seashells and of coral skeletons.
catalytic converterA car part that uses a catalyst to clean exhaust. It sits in the exhaust system and speeds up reactions that reduce the pollution released.
chemical formulaA short code, such as H2O, naming a substance. H2O says that a molecule of water has two atoms of hydrogen and one of oxygen. The small numbers are called subscripts.
closed systemA setup that no matter can enter or leave. It can still exchange heat, though not matter, with its surroundings.
compositionThe makeup of a substance: which elements, how much. A chemical change alters composition; a physical change does not.
flammabilityThe readiness of a material to burst into flame. A flammable material ignites easily at room temperature. A combustible one takes more effort to light.
metabolismThe sum of the reactions going on in a living body. Enzymes speed these reactions up. Digestion and respiration are part of it.
open systemA setup that matter can enter or leave. A burning log is one: it takes in oxygen from the air and gives off carbon dioxide and water vapor.
particleA bit of matter, whether an electron or a dust grain. Particles vary greatly in size. Atoms are particles too.
spectator ionAn ion that sits through a reaction unchanged. It appears on both sides of the full equation. A net ionic equation leaves the spectator ions out.
stabilityHow strongly a substance resists changing. In everyday use it can mean shelf life: how long a substance lasts before it starts to break down.
state symbolA bracketed letter that tells solid, liquid or gas. (s) marks a solid, (l) a liquid, (g) a gas and (aq) something dissolved in water. Each is written after a formula.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Chemical reaction, Reagent, Product (chemistry), Chemical equation, Chemistry, Physical change, Precipitation (chemistry), Catalysis, Exothermic process, Endothermic process, Chemical bond, Atom, Chemical decomposition, Single displacement reaction, Salt metathesis reaction, Combustion, Activation energy, Thermodynamic system, Calorimeter, Chemiluminescence, Glow stick, Hydrocarbon, Carbon dioxide, By-product, Chemical formula, Spectator ion, Particle, Chemical composition, Chemical stability, Combustibility and flammability, Carbonate, Closed system, Open system (systems theory), Conservation of mass and Catalytic converter. Surroundings, composition, stability, flammability, particle, by-product, metabolism, closed system, open system, spectator ion and physical change rest on their entries in Wiktionary, the free dictionary (https://en.wiktionary.org/). What a reaction is, its signs, physical change, catalysts and the catalytic converter, exothermic and endothermic reactions and the surroundings, combustion and the fire triangle, decomposition, the zinc and copper sulfate reaction, state symbols and subscripts, limewater, and mass in a burning candle and a burning log were checked against nine BBC Bitesize pages (https://www.bbc.co.uk/bitesize/): Introduction to chemical reactions, Chemical reactions, Exothermic and endothermic reactions, What is combustion?, Catalysts, Thermal decomposition, Types of chemical reactions, Writing symbol equations and Conservation of mass. The fact after each meaning is from the same pages. Licence: own work
How it works
This worksheet lists reactant, product, catalyst, exothermic, endothermic, precipitate and six more chemical reaction terms beside their meanings in another order. The answer key prints as the last page.
Read the full guide
Reaction types and energy
Set opens on Reaction basics. The second set has synthesis, decomposition, single replacement, double replacement, combustion, activation energy, surroundings, calorimeter, chemiluminescence, hydrocarbon, carbon dioxide and by-product.
Formulas, matter and systems
The third set has chemical formula, state symbol, spectator ion, particle, composition, stability, flammability, carbonate, closed system, open system, metabolism and catalytic converter.
Coefficient, balanced equation and conservation of mass are terms on the stoichiometry vocabulary worksheets.
Questions and answers
What is the difference between a reactant and a product?
A reactant is there at the start and is used up; a product is what the reaction makes. A chemical equation shows the reactants before the arrow and the products after it.
Is mass lost in a chemical reaction?
No. The total mass before and after is the same, because no atoms are created or destroyed. A burning candle only seems to lose mass: the gases it makes escape into the air.
Is baking a cake a chemical change?
Yes. The ingredients react and new substances form, so the change cannot be undone: there is no getting the flour, the eggs and the sugar back out of a cake.