When observed independently, which of the following are indications that a chemical reaction has occurred? Select all that apply
Melting of a solid
Evaporation of a liquid
Temperature change
Change in odor
Formation of a precipitate
Why does benzilic acid only precipitate in acidic solution and why does the pH need to be
The Correct Answer and Explanation is:
Part 1: Indications of a Chemical Reaction
Select all that apply:
✅ Temperature change
✅ Change in odor
✅ Formation of a precipitate
❌ Melting of a solid – Physical change
❌ Evaporation of a liquid – Physical change
Part 2: Why Benzilic Acid Only Precipitates in Acidic Solution
Benzilic acid is a weak organic acid with a carboxylic acid functional group. Its solubility in water is highly dependent on the pH of the solution due to the acid-base equilibrium between its protonated (non-ionic) and deprotonated (ionic) forms.
In basic or neutral solution, benzilic acid donates a proton from its carboxylic acid group, forming the benzilate anion (its conjugate base), which is more soluble in water due to its ionic nature. In this deprotonated state, benzilic acid exists as a water-soluble salt (e.g., sodium benzilate if NaOH was used), and does not precipitate.
In acidic conditions, the concentration of hydrogen ions (H⁺) in the solution increases, shifting the equilibrium toward the protonated (non-ionic) benzilic acid, which is less soluble in water. This reduced solubility causes the compound to precipitate out of solution as a solid. The precipitation is thus a result of the decreased ionic character and increased hydrophobicity of the molecule in its protonated form.
The pH is critical because it determines the position of the acid-base equilibrium. To precipitate benzilic acid effectively, the solution must be acidic enough (typically pH < 5) to ensure most benzilic acid molecules are protonated. If the pH is too high (alkaline), the acid will remain in its ionic, soluble form and no precipitation will occur.
In practical applications, adjusting the pH to acidic conditions is a common method for recovering weak acids from aqueous solution after a reaction or purification step. Therefore, careful pH control is essential for successful precipitation of benzilic acid.
