Ask A Scientist Chemistry Archive

name         Montse S.
status       other
age          20s

Question -   We would like to know what chemical changes appear when a
housewife uses oil (for instance, olive oil) in a frying pan? Or what
changes have been produced when you must change your motor oil?
I suppose this question is more concrete so I hope to hear from you as
soon as possible.
Thanks, again.
--------------------------------
Montse,

Olive oil -- a vegetable oil -- is very different in composition from motor
oil -- a mixture of hydrocarbons and additives.

When heated and exposed to air, vegetable oils oxidize to produce sometimes
interesting (and sometimes unpleasant) flavors. Over-used (meaning
over-heated repeatedly), such oils can accumulate polymeric substances and
degradation products that have been shown to be detrimental to health. If
heated for simple frying and not over-used, or when used unheated -- as a
salad oil -- vegetable oils are suitable and safe for consumption.

Petroleum-based motor oils (not the synthetics) are very stable, high
molecular mass hydrocarbon mixtures to which additives are incorporated to
enhance the lubricity and anti-wear properties of the blend. They are
inedible and indigestible. Motor oil molecules are fairly tough and
resistant to breakdown. It's the additives that wear out and make it
necessary to change the oil. In addition, the oil becomes contaminated with
water from combustion of the fuel. The water dissolves acids from the sulfur
in the fuel and from air-derived nitrogen oxides. Acids are detrimental to
engine life. Finally, motor oils accumulate toxic metal particles from the
normal wear of engine bearings and scrubbing surfaces. For that reason, it
is environmentally unwise to burn the oil -- where it will cause air
pollution -- or pour it on the ground -- where it will contaminate
groundwater. Recycle it.

Regards,
ProfHoff 364
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In both cases the process is the oxidation of the oils -- the reaction
products are different because the former oil is a triglyceride and the
latter is a hydrocarbon, but in both cases it is the scission of the oils
that produces smoke and acid fragments.

Vince Calder
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