Answer is: the evidence that a chemical reaction occurred is formation of white precipitate (change of color).
Balanced chemical reaction: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
Solutions of silver nitrate and sodium chloride do not have color.
From chemical reaction: n(NaCl) : n(AgCl) = 1 : 1.
n(NaCl) = n(AgCl).
m(AgCl) = m(AgCl) · M(AgCl).