What Is a Chemical Reaction?
Every day, you see hundreds of chemical reactions — bread toasting in the oven, iron railings turning rusty red, or a match bursting into flame. But what is actually happening at the level of matter?
A chemical reaction is a process in which one or more substances, called reactants, are transformed into one or more completely different substances, called products. The key word here is different — the products are not just the same stuff rearranged in space; they are genuinely new materials with new properties.
We write chemical reactions using a word equation:
Reactants → Products
The arrow means "reacts to produce." For example:
iron + oxygen → iron oxide (rust)
A chemical reaction always produces at least one new substance. The products have different properties from the reactants — different colour, smell, melting point, and chemical behaviour.
Chemical Changes vs. Physical Changes
Not every change you see is a chemical reaction. It is important to tell the difference between chemical changes and physical changes.
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Change reversible? | Usually yes | Usually no |
| Example | Ice melting to water | Wood burning to ash |
| Atoms rearranged? | No | Yes |
When ice melts, you still have H₂O — just in liquid form. When wood burns, you get carbon dioxide, water vapour, and ash — completely new substances. Melting is physical; burning is chemical.
Signs That a Chemical Reaction Is Happening
How do you know a chemical reaction is taking place? Scientists look for these observable signs of a chemical reaction:
- Colour change — iron turns orange-red as it rusts; leaves turn yellow in autumn.
- Gas production — bubbles form when vinegar is mixed with baking soda (CO₂ is released).
- Precipitate formation — a solid appears in a liquid that was previously clear.
- Heat or light released — a candle gives off heat and light as it burns.
- Smell change — milk going sour produces a new, acidic smell.
Not every sign on its own proves a chemical reaction. Boiling water produces bubbles and steam — but that is a physical change. Always look for multiple signs, or check whether a new substance has been formed.
Conservation of Mass
Here is one of the most important rules in all of chemistry: matter is never created or destroyed in a chemical reaction. This is called the Law of Conservation of Mass, first clearly stated by the French scientist Antoine Lavoisier in the 18th century.
When wood burns, it may seem like the wood disappears — but the atoms that made up the wood have not vanished. They have combined with oxygen from the air to form carbon dioxide and water vapour, which float away as invisible gases. If you could collect all the products (including the gases), their total mass would equal the mass of the wood and oxygen that reacted.
Mass of reactants = Mass of products
To test conservation of mass in the lab, always perform reactions in a closed container so no gases escape. Then weigh everything before and after — the scales should balance.
Everyday Examples of Chemical Reactions
Rusting
Iron reacts with oxygen and water in the air to form iron oxide (rust). This is a slow chemical reaction — you can see it happening over days or weeks as metal objects left outside turn orange-red. The iron has been chemically changed; rust has very different properties from shiny iron.
iron + oxygen + water → iron oxide (rust)
Cooking
Cooking an egg is a chemical reaction. The heat causes proteins in the egg to change their structure permanently. You cannot "uncook" a boiled egg — this irreversibility is a clue that a chemical reaction has taken place.
Photosynthesis
Plants perform one of the most important chemical reactions on Earth. Using sunlight as energy, they combine carbon dioxide from the air with water from the soil to produce glucose (a sugar) and oxygen.
carbon dioxide + water → glucose + oxygen
This reaction feeds almost all life on Earth and is also responsible for the oxygen in our atmosphere.
Burning
Burning (also called combustion) is a rapid chemical reaction between a fuel and oxygen that releases energy as heat and light. Whether it is a campfire, a gas cooker, or a car engine, combustion is at work.
fuel + oxygen → carbon dioxide + water (+ energy)
Real world: Rusting Cars
Have you ever seen an old car with orange patches on its body? That is iron oxide — rust — forming as the steel reacts with rain water and oxygen. Car manufacturers use paint, galvanisation (zinc coating), and stainless steel to slow this chemical reaction. Without these protections, a car body can rust through in just a few years. Understanding chemical reactions helps engineers design longer-lasting vehicles.
Real world: Baking Bread
When bread bakes, dozens of chemical reactions happen at once. Yeast converts sugars into carbon dioxide gas — those bubbles are what make the bread light and fluffy. At the same time, heat triggers the Maillard reaction, which browns the crust and produces hundreds of flavour compounds. None of these substances existed in the raw dough. This is chemistry at work in your kitchen every day.
Writing Word Equations
A word equation is a simple way to summarise a chemical reaction using the names of the substances involved. The reactants go on the left of the arrow and the products go on the right.
- Magnesium + oxygen → magnesium oxide
- Hydrogen + oxygen → water
- Sodium + water → sodium hydroxide + hydrogen
Word equations are the first step in describing chemical reactions. Later in chemistry you will learn to write symbol equations using chemical formulas, which give even more precise information.
Practice Problems
What is the difference between a physical change and a chemical change? Give one example of each.
Show answer
List four signs that a chemical reaction is taking place.