Question

What quantities must be conserved in all chemical reactions?

Answer 1

When the number of atoms reacting in a chemical reaction then the atoms on the reactant side must be equal to the atoms on the product side and if the reactions have ions then the ions must also be balanced.

Complete answer:
We know that a chemical reaction is a process in which compounds or molecules combine and react to form new molecules or compounds. So, the compounds or molecules that combine are called reactants and the compounds or molecules that are formed are known as products. So, in the chemical reactions, some quantities must be conserved, i.e., mass and charge will remain the same on both sides of the reaction.
Let us understand the properties by the examples. Let us consider a reaction in which hydrochloric acid and sodium hydroxide are the reactants while sodium chloride and water are the products. The reaction is given below:
$HCl+NaOH\to NaCl+{{H}_{2}}O$
The mass of HCl is 36.5 and the mass of NaOH is 40, so the total mass on the reactant side is 76.5. The mass of NaCl is 58.5 and the mass of ${{H}_{2}}O$ is 18, so the total mass on the product side is 76.5. As we can see the mass is the same on both sides, so mass is conserved.
Now, let us consider the dissociation of sulfuric acid. The reaction is given below:
${{H}_{2}}S{{O}_{4}}\to 2{{H}^{+}}+SO_{4}^{2-}$
In the above reaction, on the reactant side, the total charge is 0. On the product side, there are 2 positive charges and 2 negative charges which cancel each other forming 0 charges. So, the charge is conserved.

Note:
If the charge is not balanced in any reaction, then we can add electrons on the side where the charge is higher because electrons have a negative charge and will cancel the positive charge.