Question

In an experiment a student dipped pH papers in four different solutions A, B, C and D and reported his observations as given below:

Solution	A	B	C	D
Colour of pH paper	Light green	Blue	Green	Red

1. In which one of these solutions is the concentration of
   1. hydrogen/hydronium ions;
   2. hydroxyl ions maximum?
2. Give one example each of the two solutions identified in (i) above.
3. What would be the pH of the resultant mixture obtained when these two solutions are mixed together in equal proportions? Justify your answer.

Answer 1

1. The most hydronium ions (H₃O⁺) are in solution D, which is why the pH paper is red. This means that solution D is very acidic and has a low pH. Since solution B is blue, it means that it is highly basic and has a high pH. This means that it has the most hydroxyl ions (OH⁻).
2. Solution D (red color) signifies a high concentration of hydrogen ions, as observed in hydrochloric acid (HCl). Solution B (blue in color) signifies a high hydroxyl ion concentration, similar to that found in sodium hydroxide (NaOH).
3. When an acidic solution (D) and a basic solution (B) are combined in equal proportions, they will neutralise each other, producing a salt and water. The resulting pH will be around 7 (neutral) since both solutions possess equal strength.
   \[\ce{HCl + NaOH -> NaCl + H2O}\]