[Jen]

Ok, i'm pretty sure that there is something in petrol that 'decreases' over time because if not stored properly petrol can 'go off'.  Any idea what that might be and how to possibly test for that substance? Thanks

[hmx9123]

By petrol, you mean gasoline, that you put in a car?  And by 'go off', you mean explode?  There's nothing that I know of that can cause gasoline to explode.  Gasoline vapors can build up over time if you let the gasoline sit in a container for a little while, and if you light it, it can explode.  If you are using something like diethtyl ether, you can get peroxides that form and make the container dangerous to handle, or if it gets too hot, likely to explode, but in gasoline you're talking about mostly long-chaing hydrocarbons and benzene rings.  They're close to the bottom of the potential energy well so to speak.

[billnotgatez]

I think they meant deteriorate in quality

[Jen]

Yeah, sorry I'm a New Zealander, petrol is gasoline and by 'go off' I do mean decrease in quality.  Any ideas?? It needs to be reasonably simple 'cos its for a high school chemistry experiment.

[hemispheredancer]

Anyone living in a cooler climate and puts the lawnmower away for the winter season knows that gasoline will definately degrade over time.  The best I can understand is that some of the more volatile components of gasoline evaporate, leaving the heavier components behind.  This leaves a gummy residue.  As far as a way to identify the residues, it may be tough because of the number of actual components in gasoline.  

[jdurg]

There's also the fact that a small bit of water will dissolve into the petrol.  It's not a whole lot that you can visibly see, but it is enough to really put a hampering on the effectiveness of the gasoline.

[constant thinker]

Isn't there already water dilluted into the gasoline? I think it is because some lighter hydrocarbons either evaporate or combine with other ones to form longer chains and this creates a more viscous gas. Maybe not enough to be noticable, but maybe enough to effect the burning quality. 1 thing I'm sure of is for the winter always drain or burn excess gas and keep it at room temperature. I didn't once and the gas in the lawn mower was all screwed up. It looked like there were different substances in the gas because of color variations throughout the liquid. It may have to deal with the changes in temperature.

[eugenedakin]

Hello Constant Thinker,

You almost have the complete answer from the posts that people have submitted.  I'll fill in a few of the concepts.  Petol or gasoline is a petroleum distillate, meaning that it is a blend of hydrocarbons.  This blend of hydrocarbons consists of light ends (pentanes, hexanes, etc) and heavy ends (asphaltenes and paraffins over C15+).  Over time, the pentanes and hexanes evaporate (commonly called flashing off or evaporating).  Repeated warming and cooling does form water condensate in most containers which will increase the water content of the gasoline (not good).  

The heavy hydrocarbons (asphaltenes and paraffins) have higher boiling points and are relatively viscous.  This is the material that remains in gasoline engines that plugs orifices and other areas of the carburator that can require cleaning if the engine does not start.

Gasoline should have almost no water (less than 10 parts per million).  Eventual accumulation of water will be deposited on the storage tank bottom and cause corrosion.  The corrosion particles will then drain in to the carburator and eventually plug it with solid material (usually iron oxide of sorts).

Gasoline antifreeze (commonly methanol) is added during cold winter months to car gasoline tanks to remove low levels of water and emulsify it with fuel to lower corrosion rates.

Great work everyone, I hope this helps.

Eugene Dakin Ph.D., P.Chem.

[Dude]

Gasoline contains hydrocarbons (branched and linear) concentrated in carbon atoms ranging from four carbon atoms (C4) to twelve carbon atoms (C12) (For an actual breakdown of components, get an Agilent catalog and take a look at the complexity- Agilent is a gas chromatograph manufacturer 1-800-227-9770 and will send you a free copy of their catalog.      
     I would postulate that the gumming up of gasoline over time is generally attributed to two factors.  One, the polymerization/crosslinking of low levels of unsaturates (alkenes, alkynes)and other polymerizable materials (certain aromatics, etc) in the gasoline.  Two, most engines have some level of oxygen present which can slowly create oxygenates (especially in the presence of the interior metal surfaces in the engine) and ultimately lead to peroxide formation which also causes polymerization.  If an engine is stored for a long time interval, there are stabilizers that inhibit most of the deposit formation.  The metal surfaces in the engine probably catalyze the gumming up of gasoline more quickly than what would be observed if the gasoline was stored in an airtight polyethylene container.  I don't believe the octane number changes significantly over time.