Reactant Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Reactant.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A starting substance that is consumed and chemically transformed during a chemical reaction, appearing on the left side of a chemical equation before the reaction arrow.

What you start with — the ingredients that get used up to make something new.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Reactants appear on the left side of a chemical equation and are used up in the reaction.

Common stuck point: Catalysts help reactions but aren't reactants—they're not consumed.

Sense of Study hint: When identifying reactants in a problem, look at the left side of the arrow. First read the equation and list everything before the \to symbol. Then check that each substance is actually consumed (not a catalyst or solvent). Finally, note the coefficients — they tell you the mole ratios needed.

Worked Examples

Example 1

easy
Define the term 'reactant' in a chemical reaction. In the reaction 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}, identify all reactants.

Solution

  1. 1
    A reactant is a substance that is consumed (used up) during a chemical reaction. Reactants appear on the left side of the chemical equation.
  2. 2
    In 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}, the reactants are \text{H}_2 (hydrogen gas) and \text{O}_2 (oxygen gas).
  3. 3
    The coefficients (2 and 1) tell us the molar ratio: 2 moles of \text{H}_2 react with 1 mole of \text{O}_2.

Answer

\text{Reactants: H}_2\text{ and O}_2\text{ (left side of the arrow)}
Reactants are the starting materials in a chemical reaction. They are transformed into products through bond breaking and bond forming. The arrow in a chemical equation points from reactants to products.

Example 2

medium
In the reaction \text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2, identify the reactants and determine the mole ratio in which they react.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
In the combustion of propane, \text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}, how many moles of \text{O}_2 are needed to react completely with 2 moles of \text{C}_3\text{H}_8?

Example 2

hard
A student mixes 5.0 g of sodium (\text{Na}) with excess water. The reaction is: 2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2. Calculate the mass of the reactant sodium consumed and the moles of \text{H}_2 produced. (Na = 23.0\,\text{g/mol})
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Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reaction