Acid addition shifts indicator equilibrium to acidic form

Steps:

• Pure water has pH 7; universal indicator shows neutral color (assume X).
• Adding acid increases [H⁺(aq)], decreasing pH below 7.
• Equilibrium HIn(aq) ⇌ H⁺(aq) + In⁻(aq) shifts left (Le Châtelier's principle), increasing HIn.
• HIn form gives acidic color Y.

Why B is correct:

• Acid addition raises [H⁺], lowering pH and producing acidic color Y via HIn dominance.

Why the others are wrong:

• A: Color X is neutral/basic, not acidic.
• C: Acid decreases pH, does not increase it.
• D: Acid decreases pH and gives color Y, not X.

Final answer: B