Final answer:
The first evidence that a new substance had formed was observed when the mixture turned bright red, coupled with a temperature decrease, indicating a chemical change.
Step-by-step explanation:
The student observed a chemical change when the mixture of two clear liquids turned bright red, which is an unexpected color change and a strong indication of a chemical reaction. The occurrence of a temperature decrease in the mixture, resulting in it becoming quite cold, further validates that a new substance with different chemical properties has formed.
The endothermic reaction absorbed heat from the surroundings, leading to the temperature drop. This combination of color change and temperature shift signals that a new substance has indeed been created. In a classroom demonstration involving the cooling of a balloon in liquid nitrogen, the appearance of a light blue liquid and snow-like crystals, followed by the sublimation and boiling of these solids as the balloon warms up, exemplifies similar chemical and physical changes. The appearance of a blue liquid and two solids (likely nitrogen in liquid and solid forms) indicate a sequence of physical changes: condensation and solidification followed by melting and sublimation.