Why do thicker lubricating oils have higher melting points?
a. they have longer chains of carbons.
b. higher density always means higher melting point.
c. unsaturated hydrocarbons have a higher density than saturated hydrocarbons.
d. thicker oils are more solid than liquid, so they have another phase change to complete

The correct answer is option A, They have longer chains of carbons.

Explanation:

The bond between the carbon and hydrogen in a hydrocarbon is non-polar as the difference between the electronegativities of the two elements is very small. Thus there exists weak London dispersion forces per C-H bonds. When the hydrocarbon chain is small, this force is also small and thus the melting point of hydrocarbon is also small. But as the chain of hydrocarbon increases the number of C-H bond also increases and thus the combined London dispersion force also increases. Thus, longer hydrocarbon chains have higher melting points.  

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explanation:

ammonium chloride & tallow