You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7.
(a) Identify the most acidic and most basic solutions.
(b) Arrange the above four solutions in the increasing order of `H^+` ion concentration.
(c) State the change in colour of pH paper on dipping in solution C and D.

Explanation

Let's solve the question step by step.

### Step 1: Identify the most acidic and most basic solutions.
To determine the most acidic and most basic solutions, we need to look at the pH values of each solution:
- Solution A: pH = 6
- Solution B: pH = 9
- Solution C: pH = 12
- Solution D: pH = 7
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