Which of the following is a heterogeneous mixture? Air. Salt water. Steel. Soil.

Soil, because you can separate it. Dirt, rocks, mulch, etc.

You can classify a mixture to be heterogeneous if the mixture is not uniform all throughout. Some parts of the mixture is not the same in terms of properties with the rest of the parts. Homogeneous mixture is a mixture of were properties and components are distributed uniformly. From that, soil is heterogeneous.