In chemical reactions, atoms combine in different ways to make new substances.
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.
Substances which combine together are called reactants. You write reactants on the left hand side of the arrow, in a chemical reaction.
New substances which are made are called products. You write products on the right hand side of the arrow in a chemical reaction.
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
|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: