Air is a homogeneous mixture. Heterogeneous mixture means that the components are not evenly distributed, just like a mixture of sand and water where we can identify the individual components visually.
The difference between a homogeneous and a heterogeneous mixtures is that in the former, you cannot identify individual components that make up the mixture, and in the latter you can. So, air is a homogeneous mixture because it is uniform even though it contains various elements, such as nitrogen, oxygen, etc.
An example of a heterogeneous mixture would be salt and oil, or sand and water.
