Why is acidic Fe3+ nitrate almost colourless, but as neutral it's brown?

Question

I can't get it.. help!

Answer 1

Lowering the pH supresses the hydrolysis reaction, of which the product is brown. You can see that, according to Le Chatelier's Principle, the following equilibrium will go from tight to left in high concentrations of H+:

[Fe(H2O6]3+ <-----> [Fe(H2O)5OH]2+ + H+

Answer 2

Because the atomic bond is different, and therefore some physical, and chemical, aspects (such as colour) vary...