What observation indicates the presence of an alkane when mixed with bromine water?

Alkanes are saturated hydrocarbons that do not readily react with bromine water, which is a solution of bromine in water and typically appears orange-brown. When an alkane is mixed with bromine water, there is no reaction because alkanes lack the double or triple bonds present in alkenes and alkynes that facilitate bromination. Therefore, the bromine water remains unchanged, retaining its orange-brown color.

The indication that there is no color change is what confirms the presence of an alkane. In contrast, alkenes would react with bromine water, causing the solution to turn colorless due to the formation of dibrominated products. A color change to purple or green is not characteristic of any reaction involving alkanes and bromine water. This reinforces the understanding that the lack of reaction (no color change) is specifically linked to alkanes and is key in distinguishing them from other types of hydrocarbons.