Chemical Reactions

Summary of this page:

  • chemical reactions are vital for life and technology
  • chemists use the words reactants to describe what they have at the start of a reactions and products to describe what they have at the end
  • learn about four reactions that improved people’s lives thousands of years ago
  • a chemical reaction has happened only if at least one new substance is made
  • there are six indications that a chemical reaction may have happened
  • test your knowledge with our questions and answers

How important are chemical reactions?

Long ago, people discovered that:

substances can react to make new substances
these reactions can be controlled

frog chemistry set

Am I just a remarkably sophisticated chemistry set?

As the science of chemistry developed, we learned that living things exist only as a result of chemical reactions – and this includes you.

Every living thing is like a remarkably well-organized, highly sophisticated, self-managing chemistry set.

Furthermore, our high tech society would be impossible without our ability to control chemical reactions – the screen you’re viewing right now is just one example of a device that would be impossible to make.

So, on a one to ten scale of scientific importance, chemical reactions rate eleven ten.

People started reacting chemicals a long time ago

The Colosseum

The Colosseum was built using calcium oxide cement. This building is almost 2000 years old. Image by Gunnar Bach Pedersen

Our ancient ancestors learned:

  • How to harness and control the reaction between oxygen and wood to make fire.
  • How to release metals from their ores. This enabled them to make efficient plows, axes, hammers and chisels.
  • How to disinfect/fumigate rooms and buildings with the gas released by burning sulfur (sulfur dioxide).
  • How to convert limestone into what we now call calcium oxide. Complex, large-scale construction projects such as ancient Rome’s Colosseum became possible, because calcium oxide is the basis of cement and concrete.

Not all uses were peaceful

Our ancestors used new materials, such as iron and steel, to wage war. They discovered explosive chemical reactions and made gunpowder by mixing potassium nitrate, carbon, and sulfur.

We’re still making new discoveries

Although we know a lot more about chemistry than our ancestors did, we continue to study chemical reactions, because this leads to improvements in our technology and our health.

The sheer excitement of making new discoveries also drives chemists to continue investigating reactions.


  • Lithium reactions power our phones, tablet computers and cameras.
  • Biochemical and electrochemical reactions allow our brains to think and store memories.
  • Reactions to produce new antibiotics are needed, because some bacteria are becoming resistant to the antibiotics we’re using today.

Reactants and Products

When chemists talk about reactions, they describe the stuff they begin with as reactants and the stuff they end up with as products.

In all chemical reactions:

reactants → products

For example, we can make water by reacting hydrogen with oxygen.

H2 + O2 → H2O

The reactants are hydrogen and oxygen. The product is water.

How do we know there’s been a chemical reaction?

Did you know?
Fireflies, like the one shown below, glow when a chemical reaction within their bodies releases light.
Firefly glowing
Image by Yikrazuul

There has been a chemical reaction only when one or more new substances are made. Sometimes this is obvious, sometimes less so.

You will also often detect an energy change, because the reaction will heat up or cool down.


Food can react chemically with oxygen. Two ways of reacting food with oxygen are burning and respiration.

If the food is a carbohydrate, the word equation for both burning and respiration is:

carbohydrate + oxygen → carbon dioxide + water

Burning: The ash and flame make it obvious that new substances are being made and that energy has been released. Ash forms because food usually doesn’t burn cleanly. Although the carbon dioxide and water formed by the reaction are less obvious, the ash and flame make it easy to tell that a chemical reaction has taken place.

Respiration: The evidence for a chemical reaction during respiration is more subtle than when we burn food. It’s not obvious that water and carbon dioxide are made by the reaction, although we can detect them. For example, we can use limewater to show that there is more carbon dioxide in the air we breathe out than in the air we breathe in. Breathing on a cold window produces condensation, indicating water is present in our breath. The energy released by respiration keeps all animals on our planet moving and keeps warm-blooded animals warm.

Signs of a chemical reaction

Look for one or more of these:

Appearance Changes

Gas is released check
A solid appears or disappears check
There is a color change check

Energy Changes

The temperature changes check
Light/flames seen check
Sound is heard check

The signs above are all clues, but (except for flames) don’t prove there has been a reaction.

A chemical reaction has happened only if one or more new substances are made.

Test your knowledge:
Question 1

When water freezes, has there been a chemical reaction? Why?

Show Answer

No, there has not been a chemical reaction, because no new substances form – the only substance involved at any time is H20.

Question 2

You toast some bread and end up burning it. Has there been a chemical reaction? Why?

Show Answer

Yes, there has been a chemical reaction, because when you burn toast, the color changes and a new substance forms. (The carbohydrate you started with reacts and forms carbon.)
Note: When anything burns, there is always a chemical reaction.

Question 3

A tree grows a little higher. Has there been a chemical reaction? Why?

Show Answer

Yes, there has been a chemical reaction, because trees and other green plants grow by photosynthesis. During photosynthesis, green plants use energy from the sun to chemically combine water and carbon dioxide into carbohydrates such as cellulose and starch, while releasing oxygen gas.

Question 4

Name three visual clues that a chemical reaction might have happened.

Show Answer

Gas is released
A solid appears or disappears
There is a color change

Question 5

Here is an example of a chemical reaction. Identify the reactant(s) and the product(s).

sodium + chlorine → sodium chloride
Show Answer

The reactants are sodium and chlorine. The product is sodium chloride.