The substances that start a chemical reaction are called reactants. In a chemical equation, reactants appear on the left side of the arrow, and they are consumed or transformed during the reaction to form new substances called products. Depending on the context, you might also hear reactants referred to as “reagents,” “starting materials,” or “substrates,” but these terms carry slightly different connotations that are worth understanding.
Reactants and Products
Every chemical reaction involves at least one reactant and at least one product. Reactants are the input: the substances you begin with. Products are the output: the new substances that form. When you burn natural gas on a stovetop, the methane in the gas and the oxygen in the air are the reactants. The carbon dioxide and water vapor that drift away are the products. The reactants break apart at the molecular level, their atoms rearrange, and entirely new compounds emerge.
This transformation is what separates a chemical reaction from a simple physical change. If you dissolve sugar in water, the sugar molecules are still sugar molecules. But if you heat sugar until it caramelizes, the sucrose breaks down and forms dozens of new compounds with different flavors, colors, and chemical properties. The sucrose was the reactant; the caramel compounds are the products.
Reactants do not disappear during a reaction. The total number of atoms stays the same. What changes is how those atoms are bonded to each other. This is why chemical equations need to be balanced: the same types and numbers of atoms must appear on both sides of the arrow. The reactants’ atoms are all accounted for in the products, just reorganized.
Reagents, Starting Materials, and Substrates
Chemistry has several near-synonyms for “reactant,” and the differences are real even though people sometimes use the terms interchangeably. A reagent is typically a substance added to cause or detect a chemical change, often in a controlled laboratory setting. If you are running a reaction in a flask, you might call the main compound your “starting material” and call the smaller molecule you add to transform it a “reagent.” In organic chemistry especially, the distinction between starting material and reagent reflects which molecule is the focus of the experiment: the starting material is the one you are trying to modify, and the reagent is the tool you use to modify it.
In biochemistry, the term “substrate” fills a similar role. A substrate is the molecule that an enzyme acts upon. The enzyme speeds up the reaction, and the substrate is the reactant that gets transformed. So when your body digests starch, the starch is the substrate, and the enzyme amylase is the biological machine doing the work.
In industrial chemistry, reactants often go by the name “feedstocks,” a term borrowed from manufacturing that emphasizes the raw-material aspect of the substances. A petroleum refinery’s feedstock is crude oil. A chemical plant producing plastics might use ethylene as its feedstock. The word highlights that these are the raw inputs that get chemically transformed into something more useful or valuable. Research into greener manufacturing has pushed industries to explore alternative feedstocks, such as agricultural waste or recycled plastics, to reduce environmental impact while still producing the same end products.1PubMed Central. Alternative starting materials for industrial processes Recent work evaluating graphene production, for instance, has assessed rice husks, plastic waste, and paper waste as feedstocks alongside conventional coal tar pitch.2PubMed. Environmental life cycle assessment of synthesis routes for industrial-scale graphene production from waste-based feed stocks
All of these terms, reagent, starting material, substrate, feedstock, describe substances that enter a reaction and get transformed. The umbrella term remains “reactant.”
What Reactants Need in Order to React
Just mixing two reactants together does not guarantee a reaction will happen. Molecules need to collide with enough energy, and in the right orientation, before bonds can break and reform. Think of it like catching a ball: you need to be facing the right direction and you need the ball to arrive with enough speed. If the molecules bump into each other too gently or at an awkward angle, they just bounce apart unchanged.
The minimum energy needed to get a reaction going is called the activation energy. Imagine pushing a boulder up and over a hill: the boulder will not roll down the other side unless you push it high enough to clear the top. Even if a reaction would release energy overall, the reactants still need that initial energy boost to start the process. Striking a match is a good example. The chemicals on the match head will burn vigorously once ignited, but you have to provide the initial friction energy to get the reaction started.
Orientation matters too. Experiments using beams of oriented molecules have shown that reactivity depends heavily on which part of the molecule is facing the collision partner. Some orientations lead to a reaction easily, while others barely react at all, even at the same energy.3Science. Reactions of oriented molecules This makes intuitive sense: if a reactive site on one molecule is pointing away from the other molecule during a collision, nothing useful happens. The collision is wasted.
Catalysts Speed Things Up Without Being Reactants
One of the most common points of confusion is the difference between a reactant and a catalyst. A catalyst is a substance that speeds up a chemical reaction but is not consumed by it. It lowers the activation energy, making it easier for reactants to get over that energy hill, but the catalyst comes out the other side chemically unchanged. It is not a reactant because it is not transformed into products.
Catalysts work by providing an alternative pathway for the reaction, one that requires less energy to get started. Research on catalytic reactions inside specialized porous materials, for example, has shown that the activation energy for a reaction can drop by roughly half compared to simply mixing the same catalyst components loosely together.4Angewandte Chemie. Lower Activation Energy for Catalytic Reactions through Host–Guest Cooperation within Metal–Organic Frameworks That is a dramatic reduction, and it explains why catalysts are everywhere in industrial chemistry: they make reactions faster, cheaper, and possible at lower temperatures.
Your body uses biological catalysts called enzymes for virtually every metabolic reaction. Without enzymes, the chemical reactions that keep you alive would proceed so slowly that life as we know it could not exist. The enzymes are not reactants in these processes. The substrates (sugars, fats, amino acids) are the reactants. The enzymes just make the transformations happen on a timescale compatible with being alive.
Because catalysts are not used up, a small amount of catalyst can facilitate the transformation of a much larger quantity of reactants. This is why catalytic converters in car exhaust systems can work for years. The precious metals inside (platinum, palladium, rhodium) keep driving reactions that convert toxic exhaust gases into less harmful ones, without the metals themselves being consumed.
When Reactants and Products Swap Roles
Not all reactions go in one direction. Many chemical reactions are reversible, meaning the products can react with each other to re-form the original reactants. In these cases, the labels “reactant” and “product” are somewhat arbitrary and depend on which direction you are looking at. Water can be split into hydrogen and oxygen, and hydrogen and oxygen can be combined to form water. In the first reaction, water is the reactant. In the second, water is the product.
In reversible reactions, a system can reach a state where the forward and reverse reactions are happening at the same rate. At that point, the concentrations of all the substances stop changing, even though individual molecules are still reacting in both directions. The forward and reverse pathways can involve different intermediate steps and different bottlenecks, so the chemistry going one way is not simply a mirror image of the chemistry going the other way.5Chemical Reviews. Concepts Relevant for the Kinetic Analysis of Reversible Reaction Systems
This matters practically because it means you cannot always push a reaction to completion just by adding more reactants. Sometimes you need to remove products as they form, or change the temperature or pressure, to keep the reaction moving in the direction you want. Industrial ammonia production, for instance, relies on high pressure and the continuous removal of ammonia to keep driving the reaction forward.
Concentration and Proportions
The amount of each reactant you use matters. Reactions proceed according to fixed ratios: a certain number of molecules of one reactant combines with a certain number of molecules of another. If you have too much of one reactant and not enough of the other, the reaction stops when the scarcer reactant runs out. The reactant that runs out first is called the limiting reactant, and it determines how much product you can make.
This concept shows up constantly in everyday life. Baking is essentially applied stoichiometry. A recipe calls for specific proportions of flour, sugar, eggs, and baking soda. If you run out of eggs, it does not matter how much flour you have; the eggs are your limiting reactant, and the batch size is capped. In chemistry, identifying the limiting reactant is one of the first steps in predicting how much product a reaction will yield.
The reactant you have in excess does not vanish after the reaction. It remains in the mixture, unreacted. This is sometimes intentional. Chemists often use an excess of a cheap or easily removed reactant to ensure that the expensive or hard-to-obtain reactant is fully consumed. The leftover excess can then be separated and sometimes recycled.
When Light or Electricity Triggers the Reaction
In most reactions, the activation energy comes from heat: molecules move faster at higher temperatures, so they collide harder and more often. But energy can arrive in other forms. In photochemical reactions, light provides the activation energy. A reactant molecule absorbs a photon, jumps to a higher energy state, and in that excited state undergoes a chemical transformation it would not undergo otherwise.6ScienceDirect. Photochemical Reaction The reactants are still the substances being transformed; the light is the energy source, not a reactant in the traditional sense, because photons are not matter.
Photosynthesis is the most consequential photochemical process on Earth. Carbon dioxide and water are the reactants. Light provides the energy. Glucose and oxygen are the products. Without sunlight, the reaction does not proceed, but the light itself is not listed as a reactant in the chemical equation because it is energy, not a substance.
Electrochemistry works on a similar principle but with electrical energy instead of light. When you charge a battery, electrical energy drives a chemical reaction that converts the products back into reactants (which is why the battery can discharge again later). The electricity is the energy input; the electrode materials and the electrolyte solution are the actual reactants.
Common Misconceptions About What Counts as a Reactant
A few misunderstandings come up repeatedly when people think about reactants. The first is confusing a catalyst with a reactant. As discussed above, catalysts are not consumed. If a substance comes out of the reaction chemically changed, it was a reactant. If it comes out unchanged, it was a catalyst or just a bystander.
A second confusion involves solvents. When you dissolve salt in water to make a saline solution, the water is a solvent, not a reactant, because no chemical change occurs. But when you react sodium metal with water and get hydrogen gas bubbling off violently, the water is a genuine reactant because it is chemically transformed. The same substance can play different roles in different reactions, and the distinction comes down to whether its chemical identity changes.
A third misconception is that heat is a reactant. Heat is energy, not matter. It can be required to start a reaction, and it can even be listed on one side of a thermochemical equation as a bookkeeping device, but it is not a substance and does not count as a reactant. The same goes for light, sound, or any other form of energy. Reactants are always material substances: elements or compounds that undergo chemical transformation.
Finally, some people assume that if a substance is present during a reaction, it must be a reactant. But inert substances are sometimes present without participating at all. Nitrogen makes up about four-fifths of the air, yet it sits out most combustion reactions entirely. When you light a campfire, the oxygen in the air reacts with the wood, while the nitrogen just hangs around and drifts away, unchanged and uninvolved. Being present is not the same as being a reactant. What makes a substance a reactant is that it enters the reaction and comes out as something chemically different.
Physical State and Reactivity
Whether a reactant is a solid, liquid, or gas affects how readily it reacts. Reactions between gases tend to proceed quickly because the molecules are already moving fast and colliding frequently. Reactions involving solids are often slower because only the molecules on the surface of the solid are exposed to the other reactant. This is why grinding a solid into a fine powder, increasing its surface area, speeds up the reaction. A sugar cube burns slowly if you hold a flame to it, but powdered sugar dispersed in air can ignite explosively.
Dissolving a solid reactant in a solvent is another way to increase the contact between reactant molecules. In solution, molecules are free to move and collide with each other from all directions, which is why so many chemical reactions in both laboratories and living organisms take place in liquid solutions rather than between dry solids. Your body, after all, is mostly water, and the biochemical reactions that sustain you happen in aqueous solution.
Temperature and pressure can also change a reactant’s physical state, which in turn changes its behavior. Industrial chemical processes often operate at elevated temperatures and pressures specifically to keep reactants in the gas phase, where they mix and react more efficiently. The choice of conditions is driven by the physical properties of the reactants as much as by the chemistry itself.