What color change indicates the presence of an alkene when treated with MnO4-/H+?

When an alkene is treated with a solution of permanganate ions (MnO4-) in an acidic environment (H+), a distinctive color change occurs. The purple color of the permanganate ion is due to its manganese(VII) oxidation state. When the alkene reacts with the permanganate, it undergoes oxidation, and the manganese is reduced from its +7 oxidation state to +2.

This reduction leads to a significant change in color from purple to colorless because the manganese(II) ion is typically light pink in dilute solutions but often appears colorless in normal lab scenarios where the concentration is low enough.

Thus, the presence of an alkene is indicated by the change of the purple color of the permanganate solution to colorless, confirming its successful reduction through the alkene's reaction. The other colors mentioned do not occur in this specific reaction, which specifically results in the transformation of purple to colorless.