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So I was doing a chemistry lab for school, and we were analyzing how the concentration of a solution affects the rate of a reaction. In order to do this, we used the inverse burette method for the reaction between zinc and HCl, which would produce hydrogen gas. We had an Erlenmeyer flask connected to the burette by a red tube, and the reaction would occur in the flask. I noticed that the flask stayed blurred, meaning that the hydrogen gas would be trapped inside of it. I asked my teacher if that meant that the pressure was not enough for the hydrogen to displace the water, but he said that was wrong. I cannot think, however, another reason for that. I would appreciate any help.

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  • $\begingroup$ Ok, we're getting closer... --- What did you mean by "the flask stayed blurred"? Did you mean that the solution was not clear? $\endgroup$ – MaxW May 2 at 23:55
  • $\begingroup$ I also assume that you weighed the zinc and didn't get as much hydrogen gas as you expected? $\endgroup$ – MaxW May 2 at 23:57
  • $\begingroup$ @MaxW I mean that the Erlenmeyer flask was foggy inside. As if there was some kind of gas inside (I suppose it was hydrogen gas). Unfortunately, I did not weight the zinc. All I can say is that it was excess (although it would still be nice to do that). $\endgroup$ – GL RM May 3 at 0:03
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    $\begingroup$ Why do you think you can see hydrogen gas? Can you see air, or propane from a torch? What else might create fog? $\endgroup$ – DrMoishe Pippik May 3 at 1:11
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    $\begingroup$ Again, read the question: why do you think a gas might look foggy? What makes fog in air?? $\endgroup$ – DrMoishe Pippik May 3 at 1:29
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When HCl reacts with zinc or any other active metal such as aluminium, hydrogen is produced. This is what your textbook tells you. Whats is perplexing is the appearance of "fog" in the Erlenmeyer. In the beginning there is air above the liquid. When hydrogen evolution starts, the flask begins to fill with $\ce{H2}$ gas and creates enough pressure to displace the column of water in the buret. So that part of your observation is correct and you see a colorless gas displacing water column in the buret. What is in the conical flask that is causing the fog? If you were to touch the Erlenmeyer you could have noticed that it was hot, secondly there was a very vigorous evolution of hydrogen bubbles. There are several reasons, each contributing to the fogginess in the flask. That "fog" is just not hydrogen, air but vapors and very fine droplets of acid.

(i) creation of very fine droplets of acid by vigorous bubbling process (major contribution) (ii) steam generated by the heat of the reaction (iii) Hydrochloric acid has the tendency to produce fumes in moist air (seen with very concentrated acid). You did not mention the concentration of the acid used.

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    $\begingroup$ I'd think that it is mostly steam. The bubbles of gas are equilibrated water at the temperature of the hot solution. Glass doesn't conduct heat well, so the bottom of Erlenmeyer with the solution is hot, but the top of the Erlenmeyer is cold. So when the hot gas bubble breaks the surface, the gas above the solution is cooled by the relatively colder glass above the liquid level. This creates water condensation in the form of fog. // A important point is that the gas doesn't have much heat capacity, the glass has a lot. $\endgroup$ – MaxW May 3 at 5:40

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