Final answer:
The final solution is 1,000 times more acidic than the initial solution.
Step-by-step explanation:
The pH scale is a logarithmic scale that measures the acidity or basicity of a solution. A decrease of 1 pH unit corresponds to a tenfold increase in acidity. So, if the pH of the initial solution is 9 and the final solution is 6, the final solution is 1,000 times more acidic than the initial solution. This can be calculated by subtracting the pH values (9-6=3) and then calculating 10 raised to the power of the difference (10^3=1,000).