* Homogeneous Mixture: A homogeneous mixture has a uniform composition throughout. This means that the components are evenly distributed and indistinguishable from one another.

* Heterogeneous Mixture: A heterogeneous mixture has components that are not evenly distributed and can be visually distinguished from each other.

Leaded gasoline consists of gasoline (a hydrocarbon mixture) and tetraethyllead (TEL) added as an anti-knock agent. While the TEL is a distinct compound, it is dissolved in the gasoline at the molecular level. You cannot visually separate the TEL from the gasoline. This makes it a homogeneous mixture.

Think of it this way: Imagine sugar dissolved in water. You can't see the individual sugar crystals anymore, and the sweetness is evenly distributed. This is similar to how TEL is dispersed in leaded gasoline.