What Are Reactants in Chemistry?
Reactants in chemistry are the starting substances that get used up when a chemical reaction takes place. They are written on the left-hand side of a chemical equation, before the arrow, and the products they turn into are written on the right. In the equation CH4 + 2O2 → CO2 + 2H2O, methane and oxygen are the reactants. Once you can spot the reactants in chemistry, the rest, balancing and yield calculations included, falls into place.
Key takeaways
- A reactant is consumed during a chemical reaction; a product is a substance formed by it.
- In a chemical equation, reactants go left of the arrow and products right. The arrow means “yields,” not “equals.”
- Atoms are never created or destroyed, so every atom on the reactant side must reappear on the product side. That is why equations are balanced.
- The limiting reactant runs out first and caps how much product you can make. What is left over is the excess reactant.
- A catalyst is not a reactant: it is regenerated rather than consumed, so it never appears in the balanced equation. Neither do spectator ions.
What is a reactant?
A reactant is any substance that is used up during a chemical reaction. Its bonds break, its atoms are rearranged, and by the end it has become something else. That is the whole reactant definition in one line: what goes in and gets consumed.
The test is not what you put in the flask; it is whether the substance is chemically changed. Stir sugar into water and neither is a reactant, because no new substance forms. Burn the sugar and both it and the oxygen from the air are reactants, because carbon dioxide and water appear in their place.
You will also meet the word reagent. In lab language a reagent is something you add to make a reaction happen or to test for a substance. The two words overlap heavily, and at introductory level you can treat them as near-synonyms.
How do you read a chemical equation?
Read it left to right, like a sentence. The layout itself tells you which substance is which.
- Left of the arrow: the reactants, separated by plus signs.
- The arrow (→): read it as “yields” or “reacts to form,” never as an equals sign.
- Right of the arrow: the products, also separated by plus signs.
Two kinds of number appear, and mixing them up is the classic beginner mistake. Coefficients, the full-size numbers in front of a formula, say how many units take part, and you may change them to balance an equation. Subscripts, the small lowered numbers inside a formula, say how many atoms are in one unit; changing one changes the substance itself, so it is never allowed.
Anything above or below the arrow is a condition, not a reactant: heat, pressure, a solvent or a catalyst. Symbols in brackets after each formula give the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Worked example: the combustion of methane
Natural gas burning on a stove is one of the cleanest examples to start with.
CH4 + 2O2 → CO2 + 2H2O
The reactants are methane (CH4) and oxygen (O2); the products are carbon dioxide (CO2) and water (H2O). Read aloud: one molecule of methane reacts with two molecules of oxygen to yield one molecule of carbon dioxide and two molecules of water. Now count the atoms, multiplying each coefficient by the subscripts inside its formula.
| Element | On the reactant side | On the product side |
|---|---|---|
| Carbon | 1 (in CH4) | 1 (in CO2) |
| Hydrogen | 4 (in CH4) | 4 (2 × H2O) |
| Oxygen | 4 (2 × O2) | 4 (2 in CO2, 2 in 2H2O) |
Every row matches, so the equation is balanced. Follow the four oxygen atoms: two end up in the carbon dioxide and two are shared between the two water molecules.
Why is mass always conserved?
Equations must balance because of the law of conservation of mass: in a chemical reaction, matter is neither created nor destroyed. Atoms are only rearranged into new combinations.
Test that with real numbers. Using molar masses rounded to one decimal place, start with 1 mol of methane and 2 mol of oxygen.
- Reactants: 16.0 g of CH4 plus (2 × 32.0 g) of O2 = 80.0 g
- Products: 44.0 g of CO2 plus (2 × 18.0 g) of H2O = 80.0 g
Nothing has gone missing; the same 80.0 g is simply spread across different molecules. This also settles an old puzzle. A log seems to lose mass as it burns, but most of its products are gases that drift away. Weigh those gases too and the total is unchanged.
What are limiting and excess reactants?
Reactants are hardly ever mixed in exactly the ratio the equation calls for. One runs out first, and the moment it is gone the reaction stops. That substance is the limiting reactant; whatever is left over is the excess reactant.
Try it with the same reaction. Suppose you have 4 mol of methane and 6 mol of oxygen.
- The equation says 1 mol of CH4 needs 2 mol of O2.
- Burning all 4 mol of methane would need 8 mol of oxygen. You only have 6 mol, so oxygen is limiting.
- Checking the other way, 6 mol of oxygen can react with 6 ÷ 2 = 3 mol of methane. You have 4 mol, so methane is in excess.
- Those 3 mol of methane give 3 mol of CO2 and 6 mol of H2O, and 4 − 3 = 1 mol of methane is left unburned.
The limiting reactant alone sets the theoretical yield: adding more of the excess reactant will not produce a single extra molecule of product. Industry uses this on purpose, supplying the cheap reactant in excess.
Why is a catalyst not a reactant?
A catalyst speeds up a reaction by opening a route with a lower activation energy, yet it is not consumed overall. Weigh it before and after and you get essentially the same amount back, which is why it fails the test for a reactant.
What confuses people is that a catalyst genuinely does take part. In most mechanisms it is chemically changed in one step and regenerated in a later one, so it is borrowed and returned rather than used up. Its net change across the whole reaction is zero, which is why it is written above the arrow instead of among the reactants. Enzymes work the same way: an enzyme processes substrate molecule after substrate molecule and emerges unchanged.
Two honest caveats are worth knowing.
- Real catalysts do wear out. Impurities can poison them and they degrade physically, so industrial catalysts are eventually replaced. That is not stoichiometric consumption.
- A catalyst changes the rate, not the position of equilibrium. It gets you to the same destination sooner; it cannot raise the maximum yield.
Reactant, product, or neither?
Plenty of substances are present during a reaction without being reactants. The table sorts them by the role they actually play.
| Role | Where it appears | Is it consumed? | In the balanced equation? |
|---|---|---|---|
| Reactant | Left of the arrow | Yes, used up | Yes |
| Product | Right of the arrow | No, it is formed | Yes |
| Catalyst | Above or below the arrow | No, it is regenerated | No |
| Spectator ion | On both sides, unchanged | No | In the full equation only; cancelled from the net ionic equation |
Spectator ions deserve a closer look. Mix silver nitrate solution with sodium chloride solution and white silver chloride drops out.
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Written out as separate ions, sodium and nitrate drift through untouched. Strip them out and you get the net ionic equation, which shows what actually happened.
Ag+(aq) + Cl−(aq) → AgCl(s)
Silver and chloride ions are the true reactants; sodium and nitrate ions are spectators, present in the beaker but chemically unchanged.
Frequently asked questions
What is the difference between a reactant and a product?
A reactant is a starting substance that is consumed during the reaction, and a product is a new substance formed by it. Reactants are written to the left of the arrow and products to the right. The atoms are the same on both sides; only the bonding changes.
Where are the reactants in a chemical equation?
Always on the left-hand side, before the arrow, with plus signs between them if there is more than one. Anything above or below the arrow, such as heat or a catalyst, is a condition rather than a reactant.
Can a substance be both a reactant and a product?
Yes. In a reversible reaction the products of the forward reaction are the reactants of the reverse, which is why those equations use a double arrow. A substance made in one step of a mechanism and used up in the next is called an intermediate.
Is a catalyst a reactant?
No. A catalyst takes part in the mechanism but is regenerated by the end, so it is not consumed and does not appear in the balanced equation. It is normally written above the arrow instead.