Do two compounds form a mixture or a pure substance when you put them together mechanically, such as by stirring?

The answer to your question would be c. mineral oil  

The water cycle increase the water content in the earth atmosphere

So the main goal for the elements is stability which is a complete/full valence shell and typically the magic number for a stable element is 8 valence electron but not always. so elements to the left of the periodic table tend to be way less stable then those on the right, so the elements on the left are much more reactive.