Complete the missing components/variables given as x and y in the following reactions.
${{Pb}}{\left( {{{N}}{{{O}}_3}} \right)_{2\left( {{{aq}}} \right)}} + 2{{K}}{{{I}}_{\left( {{{aq}}} \right)}} \to {{Pb}}{{{I}}_{2\left( {{x}} \right)}} + 2{{KN}}{{{O}}_{3\left( {{y}} \right)}}$
A. ${{x}} \to {{s; y}} \to {{g}}$
B. ${{x}} \to {{s; y}} \to {{aq}}$
C. ${{x}} \to {{aq; y}} \to {{g}}$
D. None of these

Hint:The missing components or variables in the question explains about the state or phase of the compound. ${{s}}$ denotes that a certain compound is a solid. ${{aq}}$ denotes that the compound is an aqueous solution. While ${{g}}$ denotes that the compound is a gaseous compound.

Complete answer:
Chemical equation is the simplest form of representing the chemical reaction by writing in the form of an equation. The reactants are generally written in the left side and the products are written in the right side. The phase of each reactant and products are written near to each compound. It also shows the phases. It shows the relationship of mass of the substances.
Solid phase is formed when the substance is rigid and has definite shape and volume. Liquid phase is formed when the substance has no definite volume, shape etc. Gaseous phase is formed when the substance has no definite shape or volume. It shows little response to gravity.
Now let’s consider the given reaction. This reaction occurs when an aqueous solution of lead nitrate reacts with an aqueous solution of potassium iodide to give a solid lead iodide and an aqueous solution of potassium nitrate.
Thus we can say that ${{x}}$ is solid and ${{y}}$ is aqueous.
Hence the given reaction can be rewritten in the form of balanced chemical equation as:
${{Pb}}{\left( {{{N}}{{{O}}_3}} \right)_{2\left( {{{aq}}} \right)}} + 2{{K}}{{{I}}_{\left( {{{aq}}} \right)}} \to {{Pb}}{{{I}}_{2\left( {{s}} \right)}} + 2{{KN}}{{{O}}_{3\left( {{{aq}}} \right)}}$

So, option B is correct.

Note:
Major characteristics of chemical equations are:
-Only known facts must be represented in chemical equations.
-It must have the correct formulas for the reactants and products.
-It should satisfy the law of conservation of mass.