What is a Chemical Reaction?

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

Difference from Physical Change

A chemical reaction quite different from a physical change, as 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.

Chemical Reactions

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

  • 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.

  • 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 + Chlorine ⮕ Sodium chloride

  • However, this is just the word equation. We might also want to write this equation with symbols.
Steps Chemical Equation
We put in the symbol for each reactant Na + Cl2
We then write in the symbol for the products Na + 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 it 2Na + Cl2 ⮕ 2NaCl

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