What is a Chemical Reaction?

In chemical reactions, atoms combine in different ways to make new substances.

Chemical Change Vs Physical Change

A chemical reaction is quite different from a physical change, because in a physical change there is no new combination of atoms.

For example, as you can see in the example above, when ice (a solid) changes to water (a liquid), there is no new combination of atoms. Therefore no new substances form. As it would just involve the ice melting and changing its state, this is a physical change.

Reactants and Products

Substances which combine together are called reactants. You write reactants on the left hand side of the arrow in a chemical reaction.

  • Reactants are atoms or molecules which react together in a chemical reaction
  • Reactants

New substances which are made are called products. You write products on the right hand side of the arrow in a chemical reaction.

  • Products are atoms or molecules which are made in a chemical reaction
  • Products

Look at the reaction below between chlorine and sodium.

We put Chlorine and sodium on the left of the arrow in the chemical equation. Then we put sodium chloride on the right side of the arrow in the chemical equation. This would look like:

Sodium + ChlorineSodium chloride

However, this is just the word equation. We might also want to write this equation with symbols.

StepsChemical Equation
We put in the symbol for each reactantNa + Cl2
We then write in the symbol for the productsNa + Cl2 ⮕ NaCl
Now, as the equation is not balanced, as there are 2 sodium atoms on the right and only one on the left. We need to balance it2Na + Cl2 ⮕ 2NaCl

In this case, we have reactants combining to form new substances called products.

Observing a chemical change

There are multiple ways we can tell if a chemical reaction has taken place:

  • Temperature change – the temperature of the reactants are different to the temperature of the products. This could be an increase or decrease in temperature
  • Gas given off
  • Precipitate forming – A precipitate is a solid that forms in a reaction
  • Colour change – When the reactants are together, the reaction could trigger a change. Leading to a different colour product.