In chemical reactions, atoms rearrange to form new chemical compounds.
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. As it would just involve the ice melting and changing its state, this is a physical change.
Substances that combine together are called reactants. You write reactants on the left-hand side of the arrow in a chemical equation.
New substances that are made are called products. You write products on the right-hand side of the arrow in a chemical equation.
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, so it looks like this:
Sodium + Chlorine → Sodium chloride
However, this is just the word equation. We might also want to write this equation with symbols.
|We put in the symbol for each reactant||Na + Cl2 →|
|We then write in the symbol for the products||Na + 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 case, reactants combine to form new substances called products.
There are multiple ways we can tell if a chemical reaction has taken place: