Which of the following must be true for a chemical equation to be balanced?

Consider the following equations.

$\mathrm{Pb}\left({\mathrm{NO}}_3{)}_2\right(\mathrm{aq}) +2\mathrm{KI} (\mathrm{aq}) \to {\mathrm{Pbl}}_2(\mathrm{w}) +2{\mathrm{KNO}}_3$(x)

$\mathrm{Zn}\left(\mathrm{y}\right) +{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right) +{\mathrm{H}}_2\left(\mathrm{z}\right)$

What are w, x, y and z in the above equations?

The chemical reaction, 

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\stackrel{∆}{\to }{\mathrm{N}}_2+4{\mathrm{H}}_2\mathrm{O}+{\mathrm{Cr}}_2{\mathrm{O}}_3$ is an example of 

Consider the following statements.

I. A chemical equation tells us about the substances involved in a reaction.

II. It gives information about the symbols and formula of substances involved in a reaction.

III. It tells us about the atoms or molecules of the reactants and products involved in a reaction.

The correct statement(s) is/are