Introduction to chemical reactions

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Key points

  • Chemical reactions make new chemicals.
  • Atoms are rearranged during a chemical reaction, but the number of atoms does not change.
  • Evidence of chemical reactions includes a large temperature change, bubbles, or a colour change.
  • Chemical reactions can be represented using equations.


Watch this video to find out what happens to atoms in a chemical reaction.

Have you ever baked a cake? Do you think it would be easy to get the ingredients back after cooking? Explain your reasoning in terms of a chemical reaction.

It would be impossible to get the ingredients back from the cooked cake. This is because a chemical reaction has taken place between ingredients such as flour, baking powder, sugar, eggs and butter.

Evidence for chemical reactions

Evidence for a chemical reaction can include any of the following:

  • Bubbles – Many chemical reactions you see in the science lab make a chemical which is a gas, so you see bubbles.
  • A colour change – If the new chemicals are a different colour from the original chemicals, there will be a colour change.
  • A large energy change – Many chemical reactions give off lots of energy, like burning, and a few absorb energy, so they feel cold.

A new substance must be produced for a chemical reaction to take place.

The water inside a kettle has bubbles but that’s just because it is boiling. Bubbles of steam form in boiling water. These bubbles do not indicate a new substance has been formed because, when cooled, the steam condenses back into liquid water. Boiling is a , not a chemical reaction.

There is a colour change when you add a teabag to a mug of boiling water. Why is this not a chemical reaction?

The chemicals in the tea are dissolving in the water, and dissolving is not a chemical reaction – it is a physical change.

How are chemical reactions used?

  • Lots of chemical reactions are used to make useful new chemicals, like plastics and medicines.

  • Many useful chemical reactions involve burning fuels to release energy. These reactions heat our homes, power our cars and generate lots of the electricity that we use.

  • Cooking uses chemical reactions to make foods which are safe and tasty to eat.

Cooking involves chemical reactions


Watch this video about how a chef uses in their kitchen to make honeycomb.

A case study video on how a chef applies the concept of reactions to their job

What happened when the chef added the bicarbonate of soda to the hot liquid in their pan?

There were lots of bubbles.

Making CO₂ from vinegar and baking soda

If you mix vinegar and baking soda, a reaction will occur, where lots of bubbles of carbon dioxide gas form.

You can do this reaction at home but make sure you don’t get any of the chemicals in your eyes. Ethanoic acid is the chemical name for vinegar. Sodium bicarbonate is also known as baking soda.

Vinegar, bicarbonate of soda, a small glass, a teaspoon and a candle

WHAT YOU NEED: vinegar, bicarbonate of soda, a small glass, a teaspoon and a candle.

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A particle model of a chemical reaction

In this animation, watch how sodium bicarbonate reacts with ethanoic acid.

A model of sodium bicarbonate and ethanoic acid reacting

Notice that…

  • there are the same number of particles at the beginning of the reaction as there are after it
  • the two reactants have broken down and reformed as three products
  • sodium bicarbonate + ethanoic acid → sodium ethanoate + water + carbon dioxide

Can you recall the formula for water and carbon dioxide?

The formula for water is H₂O. The formula for carbon dioxide is CO₂.

Chemical reactions and physical changes

  • Physical changes, such as melting, boiling and dissolving, do not make new chemicals. They are usually easy to reverse.
  • In a chemical reaction, chemical bonds between atoms are broken and made, so the atoms get rearranged into new substances.
  • The simplest kind of chemical reactions involve two reacting together to make a .

Which two elements react together to make the compound sulfur dioxide?

Sulfur and oxygen.

Representing a chemical reaction

See what happens in the chemical reaction when iron and sulfur are heated. A sulfide is a chemical compound containing the element sulfur.

The test tube is partly filled with a mixture of iron and sulfur.

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What has happened to the total number of atoms during the chemical reaction to form iron sulfide?

It has not changed. There are 16 atoms before the reaction and 16 atoms after the reaction.

Working scientifically

Types of data

If you carry out an investigation you will record evidence of any changes. The evidence you record is called data and it may be quantitative or qualitative.

Quantitative data

Quantitative data can be counted, measured, and expressed using numbers.

Quantitative data can be discrete (counted) or continuous (measured).

For example, if a reaction releases a lot of energy we can use a thermometer and measure the temperature rise in degrees Celsius (C).

Qualitative data (words)

Qualitative data describes qualities or characteristics which are often observable.

For example, “the colour changed from white to blue”.

You must always state what the colour was and what it changed too.

Data recorded in scientific experiments can be quantitative or qualitative.

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