Final answer:
Adding a strong acid to water will produce a pH less than 7; however, mixing a strong base with a strong acid could result in various pH levels depending on the proportions. The scenario guaranteed to result in an acidic solution is adding a strong acid to water.
Step-by-step explanation:
The question asks which scenarios would produce a pH less than 7, meaning an acidic solution. Adding a strong acid to water is certainly going to produce a solution with a pH less than 7 because strong acids like hydrochloric acid dissociate completely in water, contributing a large number of hydronium ions (H+) and drastically lowering the pH. On the other hand, mixing a strong base with a strong acid, depending on the proportions, could result in an acidic, neutral, or basic solution.
However, if they are mixed in exactly equal amounts, the strong acid will neutralize the strong base, and the solution may be neutral (pH of 7) rather than acidic. Thus, the only scenario that is guaranteed to produce a pH less than 7 is when a strong acid is added to water. The correct answer to the question, therefore, is option B) 2 and 4, assuming there is an excess of strong acid when mixed with the strong base.