Question

hcl(aq) + naoh(aq) → nacl(aq) + h₂o(l)
a student had two dilute, colorless solutions, hcl(aq) and naoh(aq), which were at the same
temperature. the student combined the solutions, and the reaction represented above occurred. which of
the following results would be evidence that a chemical reaction took place?
(a) the resulting solution is colorless.
(b) the temperature of the reaction mixture increases.
(c) the total volume of the mixture is approximately equal to the sum of the initial
volumes.
(d) the resulting solution conducts electricity.

Explanation:

• Option A: Both reactants are colorless, and the products are also colorless. A colorless solution does not necessarily mean a chemical reaction has occurred. It could just be a physical mixing of two colorless substances.
• Option B: An increase in temperature indicates that heat is released during the process. In a chemical reaction like the neutralization of \(HCl\) and \(NaOH\) (\(HCl(aq)+NaOH(aq)

ightarrow NaCl(aq) + H_{2}O(l)\)), it is an exothermic reaction. Heat release is a sign of a chemical change as new bonds are formed (in \(H_{2}O\) and \(NaCl\)) and energy is re - distributed.

• Option C: The total volume being approximately equal to the sum of the initial volumes is a physical property related to the mixing of two liquids. It does not provide evidence of a chemical reaction. Physical mixtures can also have volumes that are approximately additive.
• Option D: Both \(HCl\) (a strong acid, dissociates into \(H^{+}\) and \(Cl^{-}\)) and \(NaOH\) (a strong base, dissociates into \(Na^{+}\) and \(OH^{-}\)) are strong electrolytes. The product \(NaCl\) (dissociates into \(Na^{+}\) and \(Cl^{-}\)) is also a strong electrolyte. The ability to conduct electricity is a property related to the presence of ions in solution. Whether it is the reactants or the products (all are strong electrolytes), the solution can conduct electricity. So, this does not prove a chemical reaction has taken place.

Answer:

B. The temperature of the reaction mixture increases.