What is a Chemical Reaction?

In chemical reactions, atoms rearrange to form new chemical compounds.

Chemical Change Vs Physical Change

A chemical change is different from a physical change, as it involves the formation of new chemical compounds through the rearrangement of atoms. However, physical changes, such as the melting of ice, involve a change in the physical state and there is no new combination of atoms.

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. This is considered a physical change because it simply involves the ice melting and changing its state.

Reactants and Products

Substances that combine together are called reactants. You write reactants on the left-hand side of the arrow in a chemical equation.

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

New substances that are made are called products. You write products on the right-hand side of the arrow in a chemical equation.

  • Products are atoms or molecules that are made in a chemical reaction.
  • Products

Consider the reaction between chlorine and sodium as illustrated below:

We place 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, so it looks like this:

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
The equation is currently unbalanced, meaning the number of atoms of each element on the left side does not match the number of atoms on the right side.

To balance the equation, we need to adjust the coefficients (numbers in front of the elements) so that the number of atoms of each element is equal on both sides.

In this case, we can see that there are 2 chlorine atoms on the left side and only 1 on the right side. To balance the chlorine, we add a coefficient of 2 in front of the chlorine on the right side.
Na + Cl2 → 2NaCl

As a result, we can see that there are 2 sodium atoms on the right side and only 1 on the left side, so we add a coefficient of 2 in front of the sodium on the left side to balance it.2Na + Cl2 → 2NaCl

In this example, reactants combine 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 is different to the temperature of the products. This could be an increase or decrease in temperature.
  • Gas is given off
  • Precipitate forming – A precipitate is a solid that forms during a chemical reaction.
  • Colour change – When the reactants are combined, the reaction could trigger a change, leading to a product with a different colour.