Which addition reaction requires a catalyst and results in no visible change initially?

The addition reaction that requires a catalyst and results in no visible change initially is hydrogenation. This reaction involves the addition of hydrogen across a double bond in an alkene or alkyne, typically in the presence of a metal catalyst such as palladium, platinum, or nickel.

The key aspect of hydrogenation is that the process can proceed without initially producing a visible change in the mixture. For example, when an alkene is hydrogenated, the reaction can be asymptomatic at first because the hydrogen gas is colorless and does not alter the visual properties of the alkene under typical conditions.

In contrast, reactions like bromination involve the addition of bromine, which produces a color change due to the presence of brownish-red bromine, allowing for immediate visual confirmation of the reaction. Similarly, hydration reactions involving water may also lead to a change in physical properties, particularly if the product is significantly different from the reactant. Reaction with hydrogen bromide, while also a notable addition reaction, typically alters the characteristics of the starting materials, making a visible change more likely.

Thus, hydrogenation is distinct in its initial lack of visible change and its reliance on a catalyst to facilitate the reaction, making it the correct choice here.