A group of students produced a red solution by boiling red cabbage leaves in water - HSC - SSCE Chemistry - Question 7 - 2001 - Paper 1

To determine which substance is most likely to cause the red solution to change color, we need to consider the behavior of indicators in acidic and basic environments. Red cabbage juice is a natural pH indicator that exhibits different colors in response to pH changes.

1. Understanding the previous reactions:

   • When sodium hydroxide (a base) was added, the solution turned purple, indicating that the pH increased.
   • When hydrochloric acid (an acid) was added, there was no color change; hence, the solution remained red, indicating it is still in the acidic range.

2. Analyzing the options:

   • (A) Cleaning solution containing ammonia: Ammonia is a basic substance, which would likely turn the solution purple again.
   • (B) Concentrated hydrochloric acid: This is a strong acid, which could change the color; however, it may reinforce the existing red color.
   • (C) Orange juice: Slightly acidic, which would likely keep the solution red or possibly change its shade without a distinct color change.
   • (D) Vinegar: Another weak acid; it would likely retain the red color but not change it significantly.

3. Conclusion: The most reactive substance in terms of changing the color from red would be the one that can increase the pH dramatically. Since a cleaning solution containing ammonia can significantly increase pH, it is the best choice. Thus, the answer is (A) Cleaning solution containing ammonia.