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Let's imagine a plastic container, sealed closed. We fill half of it with alcohol, then we generate a spark.

The question is: can the mixture of alcohol and air burn, or do we need a hole to let air in/out?

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2 Answers 2

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The question is: does the air+alcohol-gas burn creating a big pressure or does the container need of a little hole to burn?

Strange "need of a little hole", What does that mean?

The question was: do we need of hole to start the ratio of oxygen+alcohol

Another time strange "need of hole". And on the whole two rather different questions.

One thing I can say: In a container with alcohol vapor (I assume ethyl alcohol is meant) and air one will have

  • 40 mmHg partial pressure of ethanol at 19 °C

  • 720 mmHg partial pressure of air

This makes 40/760 = 5.3 vol-% of ethanol.

A view in tables says that ethanol has

Lower explosive limit (LEL) 3.3 vol%

Upper explosive limit (UEL) 19 vol%

Answer is: Your container will explode, the liquid ethanol will be sprayed around, burning. I'd prefer to be several
dozens of meters away from such "experiments", at least.

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I'm Italian so I'm really sorry for grammar mistakes. A part from this you got the point. The experiment was: taking a bottle half filled with alcohol, putting a tube long like the bottle instead of the cap, inserting a wet paper ball inside the tube; then we would start a flame (burn) inside the bottle so that the ball could be shot away. We tried this but a friend of mine says that the bottle need of a little hole to work and we never tried without. Do we in this case need of a hole? –  user3024 Apr 10 '11 at 8:50
    
"do we in this case need a hole?" –  user3024 Apr 10 '11 at 9:13
    
No, You dont need such a hole. The container was full of air before the alcohol was filled in, wasn't it? Because no precautions were made to exclude air afterwards (at least You did not mention some) You have that alcohol/air mixture in gas phase. This is called a "saturated" gas (vapour) phase. Do You discuss Molotow Cocktails? Do You want Your friends discussing Darwin award for You? –  Georg Apr 10 '11 at 10:22

'Homework' answer - depending on the amount of air and the temperature the alcohol-air may form a ratio of alcohol-oxygen that will burn until all the oxygen is consumed.

'real world' - the spark might set fire to the plastic, the alcohol may get hot and spray out of the electrode hole as a flame-thrower. = Don't try this at home.

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The question was: do we need of hole to start the ratio of oxygen+alcohol? –  user3024 Apr 9 '11 at 17:20
    
The air already inside might burn. Every fuel has a specific ratio of fuel:air where it is flammable - this will depend on the size (how much air) and the temperature of the alcohol (how much alcohol vapour is present) –  Martin Beckett Apr 9 '11 at 17:42
    
@ Martin : air cannot burn by definition. The alcohol burns. –  Georg Apr 9 '11 at 21:21
    
@Georg sorry should have said the alcohol might burn with the air inside the container. –  Martin Beckett Apr 10 '11 at 6:18

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