Final answer:
When concentrated hydrochloric acid is added to potassium dichromate, a redox reaction occurs, leading to the reduction of dichromate ions and the oxidation of chloride ions, forming products including chlorides, chlorine gas, and water.
Step-by-step explanation:
When concentrated hydrochloric acid is poured into a solution of potassium dichromate, a redox reaction occurs. Hydrochloric acid acts as a reducing agent, while potassium dichromate serves as an oxidizing agent. In this redox reaction, the dichromate ion (Cr₂O₇²⁻) is reduced to Cr³⁻, and the chloride ions (Cl⁻) are oxidized to chlorine gas (Cl₂). This reaction is quite complex and can lead to the formation of chromium(III) chloride and water, among other products. The equation for the redox reaction showcases the species involved in the reaction:
14 HCl + K₂Cr₂O₇ → 2 KCl + 2 CrCl₃ + 3 Cl₂ + 7 H₂O
This equation shows the balanced stoichiometry of the reaction, with potassium dichromate and hydrochloric acid as reactants and various chlorides, chlorine gas, and water as the products. Therefore, when hydrochloric acid is mixed with potassium dichromate, option (1), a redox reaction occurs, is the correct answer to the student's question.