Dilution is a process of creating solutions of the desired concentration by adding $H_2O$ to another more concentrated solution. The amount of solute remains constant. The equation $M_1V_1=M_2V_2$ is true because the amount of solute is the same before and after dilution.
Work Step by Step
To create a solution of desired concentration, we begin with a more concentrated solution. We then add water to lower the concentration of the solution until we reach our desired concentration hence the process of dilution. The amount of solute remains constant through the process of dilution as we are adding water and not solute into the solution. The equation $M_1V_1=M_2V_2$ is true because each side evaluates to the amount of solute in the solution before and after dilution. However, the amount of solute is constant as we are adding water and not solute to the solution during the process of dilution. Therefore, both sides are equivalent.