[muimerp]

Hello. I'm writing from Portugal, so I hope you'll excuse me for my (bad) english.
I'm reading a book -- Conversations in chemistry --, by Wilhelm Ostwald, which can be downloaded at www.archive.org. It's a book for those who want to begin their studies in chemistry.
In the reading of the book there is -- at least until now -- one thing that I don't understand. In page 71 of his book he describes this experiment: he "puts a tripod with powdered iron on a little floating board, light it, and cover it with a large glass, so placed that it stands on the bottom." He let's the glowing iron extinguish and then comes to this observation: one fifth of the air has vanished. He doesn't tell how he made that conclusion, but I presume that the floating board must have risen so the volume of the gas inside the glass has diminished by one fifth and hence the given conclusion. Then he says that air is a mixture of two gases, oxygen and nitrogen, and that they get separated by burning. What I don't understand is what hapenned to oxygen: did it pour out of the glass? If so, I don't understand the phenomena of combustion. Then there is a question by the pupil: "what if there was only oxygen inside the glass, would it entirely vanish by burning?" Answer by the master: "certainly, if there was no other gas." If this is so, what about the law of conservation of mass? And, would there be a vacuum inside the glass?

Thank you for your help.

[Borek]

If you will weight the iron (before and after the experiment) you will find out what have happened to the oxygen...

[muimerp]

Thanks for the reply.

In the book it is said that in the process of combustion if we weigh the substance that is the to be combust and if weigh the resultant substances of the combustion (that is, if weigh things after the combustion), the resultant substances are heavier than the original substance. So must I conclude that the oxygen mixed itself up with combusted substance? It's like the iron as "ate" the oxygen, incorporate it into its structure and by that mean changed into a new substance. Is that it?

It's a little confusing.

[Borek]

It's like the iron as "ate" the oxygen, incorporate it into its structure and by that mean changed into a new substance. Is that it?

Yes, only the new substance is no longer called iron - it is iron oxide now.

[muimerp]

Thank tou very much once again. I have only one more question about this subject. I hope I'm not overdoing with all this questions.

I don't understand how he manged to conclude that one fifth of the air is gone. The tripod with iron stands on a floating board. The floating board itself is subjected to the weight of tripod with iron and the weight of air. Now, oxygen just passed from air to the iron, making up that new substance, iron oxide. So the floating board is subjected to the same weight and by no means it could have risen in height and, in consequence, diminish the volume of the glass accopied by gas by one fifth; it should have stayed at the same height. And the question remains: how did he manage to conclude that one fifth of the air is gone?

[Borek]

You must be misunderstaning the experimental setup.

Put an empty bootle upisde down into frying pan full of water. Water doesn't enter the bottle - well, perhaps few millimeters.

Now, put thin pipe into the bottle (it can't be rigid one, as it must be bent so that bottle is still upside down).

Suck the air.

What happens, is that water level goes up.

Now imagine that part of the air has reacted with the hot iron and "disappeared" - effect will be the same, water level will rise. By how much? By one fifth...

[muimerp]

I'm sorry for having take so long to write this post. I've finally understand the experimental setup (Not now, but by the time you have written your last post. I'm not that dum (I hope)). Thank you very much for your help.