Seawater is water (H2O which is a compound) containing soluble compounds but also insoluble materials. We can consider, with approximation, that the seawater is a homogeneous solution.
heterogeneous
compound and element
compound
It is an element.
heterogeneous
Concrete is a heterogeneous mixture.
it is a heterogeneous
Homogeneous mixture
Vinegar is a homogeneous mixture.
heterogeneous
This is a heterogeneous mixture.
Cheerios is a heterogeneous mixture.