Question: Motor oil is avaliable in various grades (10W30, 10W40, and so on). Is motor oil homogenous mixture or a substance?

Answer: Homogenous mixture because it is not a fixed composition

Question: Iron ore is heterogenous mixture that contains iron oxide. Iron ore can be smelted to produce pure iron. Is ieon smelting a chemical or physical process?

Answer: Chemical because you took a compound and broke it down to an element

Question: Air —> oxygen + nitrogen

Answer: Physical

Question: Water —> hydrogen + oxygen

Answer: Chemical

Question: Salt water —> water + sodium chloride

Answer: Physical