🎓 Prepared by students from Boğaziçi University

What is a Chemical Reaction?

A chemical reaction is a process where reactants transform into new substances called products, with atoms rearranging as chemical bonds break and form. Reactions are everywhere — from rusting iron to the metabolism inside your cells — and chemists classify them into a handful of recognizable types.

Short answer

A chemical reaction is a process that converts one or more substances (reactants) into different substances (products) through the breaking and forming of chemical bonds, with atoms conserved but rearranged.

How a Chemical Reaction Happens

1
Reactants collide
Particles of the reactants move and collide with each other.
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2
Activation energy is reached
Colliding particles need enough energy to break existing bonds.
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3
Bonds break and form
Old bonds between atoms break; new bonds form in different arrangements.
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4
Products form
Atoms are rearranged into new substances — the products.

Step-by-step worked examples

Methane burns completely in oxygen. Write and classify the reaction.

CH4 + 2O2 → CO2 + 2H2O
All atoms are conserved: 1 C, 4 H, 4 O on each side
This is a combustion reaction (fuel + O2 → CO2 + H2O)

Hydrogen peroxide decomposes into water and oxygen gas. Write the equation.

2H2O2 → 2H2O + O2
Check atoms: 4 H and 4 O on each side
This is a decomposition reaction (one compound → two or more products)

Sodium metal reacts with chlorine gas to form table salt. Write the equation.

2Na + Cl2 → 2NaCl
Check atoms: 2 Na and 2 Cl on each side
This is a synthesis (combination) reaction (two or more reactants → one product)

Flashcards

Quick quiz

Q1.Which best defines a chemical reaction?

Correct answer: B. A chemical reaction rearranges atoms into new substances by breaking and forming bonds.

Q2.2Na + Cl2 → 2NaCl is an example of which reaction type?

Correct answer: C. Two reactants combine into a single product, so it's a synthesis reaction.

Q3.In CH4 + 2O2 → CO2 + 2H2O, what type of reaction is shown?

Correct answer: A. A fuel (CH4) reacts with O2, releasing energy — a combustion reaction.

Q4.Which law explains why atoms balance on both sides of an equation?

Correct answer: B. Mass (and atoms) is conserved — matter is neither created nor destroyed in a reaction.

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Common mistakes

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Thinking mass or atoms disappear during a reaction. — Correct: Atoms are only rearranged — total mass and atom count are conserved.

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Confusing a physical change (melting, dissolving) with a chemical reaction. — Correct: A chemical reaction forms new substances with different properties; a physical change does not.

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Assuming all reactions release energy. — Correct: Some reactions absorb energy (endothermic), others release it (exothermic).

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Mixing up decomposition and synthesis reactions. — Correct: Synthesis combines substances (A+B→AB); decomposition breaks one apart (AB→A+B).

FAQ

What is a chemical reaction?

A process in which reactants are transformed into different products through the breaking and forming of chemical bonds.

What are the main types of chemical reactions?

Synthesis, decomposition, single replacement, double replacement, and combustion are the five common types.

What are some chemical reaction examples?

Rusting iron, burning methane, photosynthesis, and baking soda reacting with vinegar are everyday examples.

How do you know a chemical reaction happened?

Signs include color change, gas bubbles, temperature change, precipitate formation, or light/energy release.