Descriptions of Change

Chemists describe chemical and physical changes in a variety of ways. Chemical equations convey how reactants are converted into products during a change and represent how a macroscopic observation relates to processes occurring on the microscopic (atomic) level. 

Example 1.

Which is a balanced chemical equation for the formation of \(\require{mhchem}\ce{AB3}\) from \(\require{mhchem}\ce{A2}\) and \(\require{mhchem}\ce{B2}\) that corresponds to the diagram drawn below?


A. \(\require{mhchem}\ce{2A + 2B->2AB3}\)

B. \(\require{mhchem}\ce{A2 + B2->2AB3}\)

C. \(\require{mhchem}\ce{2 A2 + 6 B2->4AB3}\)








C. \(\require{mhchem}\ce{2 A2 + 6B2->4AB3}\)

The diagram shows \(2\) molecules of \(\require{mhchem}\ce{A2}\) reacting with \(6\) molecules of \(\require{mhchem}\ce{B2}\). This forms a total of \(4\) \(\require{mhchem}\ce{AB3}\). Note: the simplest balanced equation for this process (with smallest whole number coefficients) is \(\require{mhchem}\ce{A2 + 3 B2 -> 2 AB3}\).

Example 2.

Which diagram best describes the melting of solid ice to liquid water?









In a solid, the particles are close together and fixed in position. In a liquid, the particles are farther apart and have more freedom of motion. This is a physical change, as there is no change in composition (still \(\require{mhchem}\ce{H2O}\)).

Example 3.

Sodium metal reacts with chlorine gas to form solid sodium chloride. Write a balanced chemical equation to describe this change.


A. \(\require{mhchem}\ce{Na(s) + Cl(g) -> NaCl(s)}\)

B. \(\require{mhchem}\ce{2 Na(s) + Cl2(g) -> 2NaCl(s)}\)

C. \(\require{mhchem}\ce{S(s) + Cl2(g) -> SCl2(s)}\)




B. \(\require{mhchem}\ce{2Na(s) + Cl2(g) -> 2NaCl(s)}\)

Sodium is atomic symbol \(\require{mhchem}\ce{Na}\) and chlorine gas exists as a diatomic molecule (\(\require{mhchem}\ce{Cl2}\)).