hydrogen bubbles which dont float

[hermanntrude]

I've been attempting to recreate this experiment (link), but without using a hydrogen cylinder. The only problem is, that no matter which acid or metal i use, no matter how fast i generate the gas, my bubbles don't float. They burst into flame nicely but they don't float. I'm not sure what's causing the bubbles to be so heavy... is it gaseous water?

[mississippichem]

I've been attempting to recreate this experiment (link), but without using a hydrogen cylinder. The only problem is, that no matter which acid or metal i use, no matter how fast i generate the gas, my bubbles don't float. They burst into flame nicely but they don't float. I'm not sure what's causing the bubbles to be so heavy... is it gaseous water?

 

-Hmm...strange. I would try de-gassing the water.

 

-Or, you're soap could be so concentrated that it's messing around with colligative properties, just a guess though.

 

-Perhaps your metal isn't finely divided enough. Maximizing surface area should increase hydrogen production per unit time.

Edited December 2, 2010 by mississippichem

[Mr Skeptic]

What about the size of your bubbles? If your bubbles are too small, the weight of the bubble will hold it down. Are you trying to generate the hydrogen in the soap water, or in a separate reaction bubbled into the soap water? In either case, your rate of hydrogen generation can't really compete with a hydrogen cylinder.

[hermanntrude]

What about the size of your bubbles? If your bubbles are too small, the weight of the bubble will hold it down. Are you trying to generate the hydrogen in the soap water, or in a separate reaction bubbled into the soap water? In either case, your rate of hydrogen generation can't really compete with a hydrogen cylinder.

 

The bubbles are smallish but should be big enough. i've tried a number of setups but all involving an acid and a metal (i've settled on 1M HCl and Mg for speed of delivery without getting dangerously hot), and in each case i've run the hydrogen through a separate bubbler. I'm thinking about this a lot and i'm thinking the only thing it can be is water vapor weighing down the bubbles. The bubbles are of comparable size to those in videos ive seen. I found only one video involving floating bubbles which didn't use a cylinder, and that used aluminum, which I will try tomorrow. i'm thinking perhaps aluminum has a lower enthalpy change for the reaction with HCl and so less water gets vaporised. I'll also try running it through a longer tube before bubbling it and if i get desperate i'll try a drying tube

[insane_alien]

your bubbles are too much water, not enough hydrogen.

 

probably due to small bubbles.

 

a quick fix for this(if it is indeed to cause) would be to put a small cap over the metal shavings to catch the bubbles and allow them to form larger bubbles before getting to the surface. although i'm not sure how that will be affected by the soap, they might not join as easily.

[Mr Skeptic]

If water vapor is your problem, how about passing the gas through a condenser? In any case, that will give an idea of just how much water is there. However, water vapor is less dense than air so I don't really see it being a problem.

[hermanntrude]

your bubbles are too much water, not enough hydrogen.

 

probably due to small bubbles.

 

a quick fix for this(if it is indeed to cause) would be to put a small cap over the metal shavings to catch the bubbles and allow them to form larger bubbles before getting to the surface. although i'm not sure how that will be affected by the soap, they might not join as easily.

 

the bubbles are already doing that. I have a side-arm conical flask which has the acid and metal in, with a stopper in it, a tube running from th side arm to another conical which has an inlet tub below the surface of some soapy water and an outlet leading to a small glass funnel. the bubbles are probably a centimeter or so in diameter on average

[mississippichem]

What concentration of acid are you using? I've seen the [ce] HCl_{(aq)} + Zn_{(s)} [/ce] reaction proceed sluggishly with dilute acid. Have you tried icing the far end of the tube to get faster hydrogen flow?

[hermanntrude]

I tried all the above suggestions and also a few more.

 

I managed to get floating bubbles by using zinc and fairly concentrated H2SO4 (created by putting zinc in water and adding swigs of conc acid as needed), but it wasn't reproducible... the bubbles floated sometimes and sometimes not. And i could never get the bubbles to break off of the larger mass, they just made snakes which stood up

[insane_alien]

maybe the hydrogen is getting diluted by air thats in the headspace of the first conical flask. if you reduce the headspace(prefferably eliminate it entirely) then it could work.

[hermanntrude]

maybe the hydrogen is getting diluted by air thats in the headspace of the first conical flask. if you reduce the headspace(prefferably eliminate it entirely) then it could work.

 

this is one of the things i tried. it failed horribly. i think the headspace is necessary to allow cooling of the water vapor. i did notice that with a smallish headspace was best, and gave heavy bubbles which after a few minutes became lighter.

 

i'm going to buy a cylinder and be done with it, but i've also decided to get rid of the horribly rusty chlorine cylinder first

[mississippichem]

How many chemists does it take to make a bubble float...?

[Mr Skeptic]

Well, another thing to consider is your soap/detergent, which might affect how thick or how large the bubbles are.

[CaptainPanic]

I put my money on the type of surfactant / detergent.

 

Detergent

The detergent (or soap, surfactant) will determine the weight of the liquid part of the bubble. It will probably also determine the size of the bubbles, and how easy they break off... and I think that the latter is your real problem. According to wikipedia, soap generally decreases surface tension, and soap only stabilizes the bubbles (link). I'm not an expert, but luckily it should be relatively easy to try a number of different ones as almost everybody has several types of soap at home (shower gel / shampoo / washing detergent for laundry / dish wash soap etc).

 

The concentration of the detergent also has a large influence on the behavior...

 

In short, you have a number of parameters to play with, and I am sure that at some point you'll get it right.

 

Gas phase

Your gas phase is fine. No need for condensers or anything like that. Assuming you keep the temperature somewhere under 40 deg C, the vapor pressure of the water is too low to heavily influence the buoyancy of the gas. (Buoyancy is the difference between the air and the other gas here - while the hydrogen gas will become a lot heavier because of water vapor, the buoyancy will hardly change).

For example:

Air has a density of about 1.2 kg/m3

Hydrogen has a density of about 0.08 kg/m3

The difference is about 1.12 kg/m3

 

Now, if we saturate the hydrogen gas with water at 40 deg C, we get 7374.1 Pa (or 0.07 bar) of water. The new density is now about 0.12 kg/m3, because of the added water... it's more than 50% heavier! The difference however is 1.08 kg/m3, and only decreased by about 4%.

 

If your water reaches temperatures of 80 deg C or more, then this becomes a different story altogether. Note that detergents also act differently at different temperatures.