Thursday, May 6, 2010

The Durian Smell of LPG


I just would like to highlight a letter written to the The Sun by Physics Teacher about SPM syllabus. While I do not wish to get involved if the topic mentioned by the writer is considered as part of the syllabus or not, I wish to highlight some misconception raised by the topic mentioned.

Let's read his/her email published by the The Sun newspaper today 6 May 2010:

Keep to the syllabus

I WOULD like to draw the attention of the Examination Syndicate to the SPM Physics papers.

Question No 12 (Paper 1, 2009) is definitely out of the present syllabus. The question is: Butane gas leakage happens in a laboratory preparation room. A teacher in the adjacent laboratory smells the odour even though no wind is blowing. The phenomenon happens because of: A condensation, B diffusion, C evaporation, D expansion.

The expected answer I suppose is B diffusion. Before I discuss the answer, I would first like to point out that this question IS NOT in the SPM Physics syllabus.

Incidentally, Question 11 (Paper 1, 2008) on Brownian Motion is also not in the syllabus.

Both diffusion and Brownian Motion were in the syllabus before 2006 under the section of properties of matter (Chapter 3, Form 4 ). But this section had been taken out from the syllabus.

Should topics not mentioned in the current syllabus be tested? The Physics panel may argue that these topics are in the Form 1 syllabus, or that diffusion is in Chemistry syllabus! Then what is the syllabus for?

Perhaps the panel does not know the syllabus well. They might have just chosen the questions from the Question Bank without reviewing the syllabus. The Question Bank may still have some questions from the previous syllabus.

Did the syndicate take the trouble to throw out those questions which are no longer in the syllabus?

Now back to Q12 2009.

Is the supposed answer diffusion the correct one? In his book: Physics (Fifth edition, Prentice Hall), pg 410, Giancoli shows that air currents (convection) are more important than diffusion in transmitting odours. Giancoli shows that it takes about two minutes for ammonia (NH 3) to be detected 10cm away from a bottle containing ammonia after it is opened. The diffusion rate of butane (C4H10 ) is even slower.

Diffusion, which is rather slow, alone would not have enabled the teacher to smell the odour.

Cutnell and Johnson also use the same reasoning in Physics (Fourth Edition, Wiley).

So now when Mrs Smith is preparing a cup of coffee, you smell the aroma right away in the question hall. Is this due to convection or diffusion alone?


Let's get one thing straight. Butane (LPG) has no odour. You can't smell it, if it is released to the atmosphere irrespective of the volume or if it would reach your nose by convection or by diffusion.

Just like you would not be able to smell the air you breath, unless of course you are breathing in a restaurant, or in a toilet! ;-)

But that's not the smell of oxygen.

Technically though, the question is not wrong.
Caption: LPG Terminal. I have always been fascinated by the shape of the LPG ''tank'. Why would they build it spherically and not cylindrically?

So the question above is a misrepresentation of the physical property of Butane. Anyone reading the question, even if he did not know about transport phenomena, would believe that he could smell Butane should he be in contact with the hydrocarbon.

Actually you can't.

During processing, once butane - LPG if you must - is produced, a chemical called Mercaptan (sulphur compound) is added to the hydrocarbon. Mercaptan, as with most sulphur compound, has a peculiar smell; some says like rotten egg. For us in the refining industry, I like to pass by the Mercaptans tank, as it reminds me of the durian smell, so it was enjoyable for us to breeze through the area.

I swear, I always thought that the operator would be having a durian party in my rookie years as an engineer there. Later on I found it was the chemical compound Mercaptan. May be in bulk it would smell like durian.

To my nose, its smell is more pleasing than that of expensive perfumes - Poison, Christian Dior's, any brands.

So Mercaptan would help us detect leak should we forgot to turn off the stove after we finished cooking. Or in the old days, if you had a flame out due to the wind. You would know the your LPG is leaking out and you better switch the stove off quickly.

Should we be worried that we don't really understand the physical phenomenon and are teaching wrong facts to our students. NO wonder a recent studies by Monash University found that only 15% of our students who got an A for Physics really understood the concept and deserve the As they are getting.

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