Why weight change when I put effervescence tablet ?

[Gh778]

Hi,

I measure weight of : column of water + water + effervescence tablet + cover (hermetic)

When I put effervescence tablet (give a lot of small bubbles) in water and close very quickly the cover, the weight decrease of 0.2g with a precision of 0.01g for the weight-balance. The weight decrease more and more until -0.2g. When I open the cover I can ear pshiiitttt, this could say no gas has move out. I tried with big column (1 liter) and others smaller. Could you explain where is my error when I do experimentation ?

 

[Simon Bridge]

You need to be more detailed about the setup and how you are determining that the weight has decreased.
Weight decrease could be due to bad calibration of the scales, systematic error due to shifting load (water sloshes around while the tablet is gassing), a slightly leaky seal... lots of possibilities. We do not imagine that matter is vanishing from a sealed container right?

Note: just because there is a "pshiiitttt" does not mean that zero gas escaped - just that some gas was trapped.

I imagine there is a sharp weight discrepancy at the start, then it slowly increases? Is that correct?

Did you try rigging something so the tablet drops after the cover is in place? i.e. stick to the underside of the cover with gum so it is held out of the water, then, with the cover in place, turn the whole flask upside down. Wait until the entire tablet has dissolved, then, carefully, place the flask on the scales. Don't weigh the flask while water is moving about.

 

[Gh778]

it's only home experimentation, I see weight decrease but maybe not 0.2g like I said before maybe 0.03g but I'm not sure maybe it's gas escape.

 

[Simon Bridge]

it's only home experimentation, I see weight decrease but maybe not 0.2g like I said before maybe 0.03g but I'm not sure maybe it's gas escape.

You've repeated the experiment several times with different volumes of water and you are not sure of the order of magnitude of the weight discrepancy? Did you not make a record of each attempt?

You need to check your data before reporting a problem.
Try it again.

 

[Gh778]

yeah, I do 17 measures, and each time the weight decrease about 0.03g (accuracy 0.01g), different volumes, different containers, different type of cover. Long time before weight and I'm waiting long time for watch if weight move.

Datas (g):

1/ Only 1 tablet

170.35 -> 170.32
690.55 -> 690.52
122.89 -> 122.87
440.81 -> 440.79
190.63 -> 190.60

682.76 -> 682.73
520.30 -> 520.28
135.96 -> 135.94
269.95 -> 269.92
220.30 -> 220.27

489.36 -> 489.33
190.80 -> 190.78
164.32 -> 164.30
612.58 -> 612.55
338.22 -> 338.19

266.32 -> 266.29
694.29 -> 694.26

2/ 2 tablets:

655.32 -> 655.25
298.56 -> 298.50

Weight decrease more and more with time after the tablet is in water. It's not a lost of weight when I manipulate. Maybe it's the gas escaping but with different covers it's always the same and even I let one hour column on the weight-balance, the weight don't move.

Even I press the plastic column (when it's plastic), the weight don't change after 15 minutes, this could say if it is an escape of gas my pressure will escape more gas, but not.

 

[Simon Bridge]

How long is "a long time"?
Is this the same time in each case?
Which measurements had which covers etc?

However - it looks to me like your covers are not airtight.
If you tip the container, with cover in place, upside down - does it leak?

Here's an experiment for you:
Pick a container and a cover -
part 1.
put water in container, put lid on, put tablet on the lid (i.e. NOT in the water), weigh the ensemble like that.
Make a note of the weight. This is M0

part 2.
remove from scales, lift lid, drop tablet while looking at a clock, quickly replace the cover, make a note of the time. This is T0. To be clever, wait until the clock reads an easy time before dropping the tablet.

Wait until you are sure the tablet is completely dissolved.
Make a note of the time. This is T1.

Carefully weigh the whole thing. This is M1.

Work out the interval T=T1-T0, and the mass discrepancy M=M0-M1 - write them down.

Repeat until you have at least 5 sets of data - be sure to use the same interval T each time - record the data in a table.

Compare the mass discrepancies M between results.

Repeat with different covers.

 

[Gh778]

long time => in one minute the weight decrease, but I let 15 minutes or more
Yes, the same each time
Plastic and glass container, cover plastic and steal with gasket

Yes with upside down, it's the same weight decrease,this would say there is no gas escape

I will test your experiments and give you result

 

[Simon Bridge]

It was the same each time but you cannot say if it was 15 mins or more than 15 mins?

You tried it upside down?
Did any water leak when you did?
(It's a simple seal test for improvised seals.)
... what did you make your covers out of? What sort of plastic? How cut?
Are these specially manufactured to be airtight?
What sort of containers?

What did you do to make sure the seals were airtight?

If you are getting a mass loss while the covers are in place - then the seal is leaking gas.
When you lift the cover, you must get a substantial weight discrepancy since now you know that gas has escaped right?

You can use this to find out how much gas is produced by each tablet - then you can see if the small weight drop while the cover is on added to the big weight loss when the cover is removed, is the same as the weight of gas produced by the tablet.

 

[Gh778]

I done 5 measures with 2 tablets

difficult to measure alone and have good time especially at start but all measures are approx the same

T=2'40s with M=-0.06 g
T=2'50s with M=-0.05 g
T=2'35s with M=-0.06 g
T=2'45s with M=-0.05 / -0.06 g
T=2'30s with M=-0.05g (after T=10' => M=-0.06g)

Plastic bottle with plastic cover only, but with upside down like that I don't think gas escape, and if liquid escape the weight will decrease again and again (the last measure I'm waiting 10 minutes)

 

[Gh778]

What did you do to make sure the seals were airtight?

I'm not sure cover are perfect, but:

1/ upside is down, no water escape (and if water espace, it would be on balance because there is a curve)
2/ I press for add pressure for see if weight decrease and the weight is the same after 3'
3/ I use bottles of plastic and glass/steal/gasket, I think plastic bottle is good because when I open, the pressure is big enough for to be dangerous for eyes

I suppose a problem with my balance but if I take several weight and wait 1,3 or more time, the weight is always the same. The only thing I do careffuly is to wait when I'm approaching to the weight-balance because vibration change the weight due to the floor.

The mass of gas for a tablet is approx 0.28g

 

[Simon Bridge]

You using something like a PET bottle?
Like you get coke in?

So the plastic+gasket is the bottletop?

You realize that plastic bottles are not gas-tight right?
Gasses can diffuse through the plastic itself.

Have you tried using a metal flask?

 

[Simon Bridge]

If you get 0.28g gas per tablet
and you get a gradual weight drop of 0.06g (say) with the lid on.
and you get a further 0.22g loss after the lid is removed
then you have support for the idea that gas escapes the bottle even with the lid on.

What I mostly want to get across to you is control of variables, and careful record keeping - as well as careful reporting.

BTW: following my method for post #9, M should be positive - you have reported a weight-gain ;)
But do you see how that extra care made things much clearer?

Off that data - the stats are not great but:
You lost (54±3)mg

 

[Gh778]

yes, I have tried with metal flask, it's the same, with glass bottle, this don't change the weight...

But do you see how that extra care made things much clearer?

the care of manipulations ? even I take time and precautions the lost of weight is the same

following my method for post #9, M should be positive - you have reported a weight-gain

I give negative because weight is lost, but with your method, the weight is positive ok, this don't change the result

After a long time, there is no bubble in the water, but pressure is always there. Water must transmit pressure like gas...

Why 3 tablets => -0.09g

I tried with a small bottle with small volume for gas, the bottle is under great pressure, and it's the same with low pressure when there is big volume for gas

Edit: I added a sheet of aluminium around tablets for delay effect, and this increase time for weight to reduce, but at final it's the same loss

I changed the room for measures, after let a moment the balance for temperature. It's the same weight decrease.

Maybe someone else can test for compare results ? I hope someone can do this experimentation in a laboratory, it's very easy and fast.

 

[Simon Bridge]

yes, I have tried with metal flask, it's the same, with glass bottle, this don't change the weight...

What is the difference between the metal and the glass? Perhaps the glass is thick and the metal is thin?

Anyway - I suspect there is gas loss due to diffusion - it depends on the difference in pressure between the inside and outside of the bottle.

Note: bubbles displace water - so they cannot form unless there is someplace for the water to go.
This also means that there could be a gas-permiable seal on the cover - this will allow gas diffusion even when the container is upside down.

Did you check to see if the total weight lost is equal to the total weight of gas released by the tablets?

 

[Gh778]

If you get 0.28g gas per tablet

I'm not sure of that, it's not write on the paper, it's just the lost of weight I suppose it has...I done what I lost + what I lost after and I suppose it's the weight of gas.

The tickness of glass is 3 mm and the thickness of metal is 1 mm. With a big volume (1,5 l) the pressure is only a pshiiit like you move a bottle with CO2 inside.

No the total lost is not the all gas give by tablet, I lost 0.06g but if I open the bottle I lost 0.2 g more.

With 2'40 it's not possible to incriminate temperature but maybe balance or diffusion or ...

Edit: I tested with another balance today, it's the same result approx, the weight is not the same, I use glass+steel cover. And I have add oil to gasket.

 

[berkeman]

Hi,

I measure weight of : column of water + water + effervescence tablet + cover (hermetic)

When I put effervescence tablet (give a lot of small bubbles) in water and close very quickly the cover, the weight decrease of 0.2g with a precision of 0.01g for the weight-balance. The weight decrease more and more until -0.2g. When I open the cover I can ear pshiiitttt, this could say no gas has move out. I tried with big column (1 liter) and others smaller. Could you explain where is my error when I do experimentation ?

Do not edit your posts after you have received help here at the PF. We have restored your original post.

 

[Gh778]

Do not edit your posts after you have received help here at the PF.

sorry, but when I edited, I don't have a reply.

 

[berkeman]

sorry, but when I edited, I don't have a reply.

Interesting. The reply was maybe being composed as you edited, because it sure looked like he understood your question. Okay, thanks for the clarification.

 

[Simon Bridge]

Oh OK - so the tablet provides 80mg of gas to the air normally without the top being screwed on - leaving bubbly water behind?

Note: 1mm is very thick for a metal flask - what kind of flask are you using?

That's rather annoying actually - you could wait for the water to go flat, plotting weight against time at equal intervals to see what figure it tends to.

It would take quite a while for me to duplicate the experiment.
My experiment is certain to be different from yours anyway, since you have not given a complete writeup.

Since the gas is probably CO2 - I suppose you could immerse the bottle in lime-water?
I'm not sure if such small quantities would show up though.

 

[Gh778]

I use a flask for food, the precise thickness for cover is 0.8 mm due to a 0.2 mm of plastic (not gasket).

I'm waiting a long time (1 hour) and the weight never moved after 4', lost 0.07g maybe one time 0.08g. With plastic or glass it's the same.

What you need for duplicate experiment ? Great if you can do this type of experimentation :)

Yes, gas is CO2 and true I can use lime-water for what ?

Edit:

1/ with bottle opened I lost 0.17g (with 1 tablet)
2/ with bottle closed I lost 0.04g, and after if I open the bottle the total lost is 0.17g

When I measure: with or without bubbles (on the walls) the weight move very few, so each time I shake the botlle a little for escape bubbles on the walls.

 

[256bits]

Simon, one thing I thought is that he has a bouyancy effect due to the container expanding due to the pressure and affecting the weight recorded, but I don't see a discrepancy between containers ( plastic, glass, metal ) as the op says it is always 0.02 g per tablet, or multiples therof per tablet, which seems odd.

 

[Gh778]

For take Archimede's law in account: Density of air is 0.0013 g/cm3, so the volume is 0.03/0.0013 = 69 cm3. For 0.03 g lost, with a 78 cm² for surface's cover, the deformation of steal cover must be like 0.88 cm. And for 0.09 g the deformation must be like 2.64 cm. I'm right ?

 

[Gh778]

Maybe I found: when bubbles move up in water, they have gas in it (CO2), this gas move up with a speed, so there is a quantity of movement, the weight losses is the sum of mass of bubbles multiply by speed. Like quantity of movement is conserved, the weight don't change after the loss of 0.03 g/tablet. What do think about that ?

 

[jbriggs444]

Maybe I found: when bubbles move up in water, they have gas in it (CO2), this gas move up with a speed, so there is a quantity of movement, the weight losses is the sum of mass of bubbles multiply by speed. Like quantity of movement is conserved, the weight don't change after the loss of 0.03 g/tablet. What do think about that ?

Steady-state movement does not change apparent weight. You've indicated that you waited for one hour.

This theory is perilously close to the classic story of the man banging on the sides of a truck full of chickens to reduce the scale reading at the weigh station.

 

[Simon Bridge]

@256bits:
I had thought of the buoyancy effect too ...
Considering the different materials involved, I don't think the volume expansion was big enough but it's easy to check - simplifying the 1l of water is a cube 10cm on each side that weights 1kg. Water is nice like that. How much would the volume have to increase to cause a weight decrease of the order of 50mg? Ergo: how much do the sides increase? Would you notice?

The fact that there is no measureable difference in weight-loss for the other materials is problematic, but there is not a good enough control available to see if it is real. The trouble with home experiments is that you seldom get the kind of rigor needed to draw decent conclusions.

@Gh778:
As the bubbles rise, an equal volume of water must fall...

It's a bit like the sealed container of birds sitting on a sensitive electronic balance - and all the birds start flying around. What happens to the reading on the balance?

Another imagining: you sit in a sealed container with a cylinder of helium and a stack of balloons. You inflate the balloons and let them float up to hit the top of the container.

Note:
... lime water (a saturated solution of calcium hydroxide) turns cloudy when carbon dioxide bubbles through it.

There's all sorts of problems concerning the quantities over the time involved.
You could do the experiment under water to see if bubbles form around the seals and over the surface of the container. Presence of bubbles confirms gas is escaping. Lack of bubbles is no information because the escaping gas is so small and slow that it could be dissolving into the water instead of forming bubbles. Similarly, doing the experiment under lime water - the concentration of CO2 may be too low for any clouding to be visible.

You could ramp up the experiment with an insane number of tablets so outgassing will be significant... but that does not confirm outgassing for the low-pressure case unless you are very careful.

There is probably something else I'm forgetting.
It's really not that difficult though:

Things that cause scales to read low are:
1. reducing the amount of matter present on the scales (outgassing suspected)
2. adding an extra gravity-opposing force from outside (buoyancy)
3. faulty scales - perhaps the scales lose 50-60mg over that time anyway?
4. acceleration of gravity changes

 

[Simon Bridge]

I still favor outgassing as most likely.
See: de Segovia, J. L. Physics of Outgassing p99 s2 "Permeation of gasses through matter"
... The vacuum systems de Segovia is talking about only need to oppose ~100 kPa @ ~300K.

You seem to think that the pressure in a can of soda ("pshhhht") is low. People like Pepsi and Coca-Cola bottlers pressurize the cans to 40PSI gauge @294K ... that's about 276kPa over atmospheric. Soda goes flat in the bottles if left for too long even if the bottles are unopened... PET iirc Coca Cola Co bottles go flat in 3-4 months, and cans in 6-9 months.

Of course it depends on what you are comparing it to.
40psi is a reasonable pressure to inflate car tyres to - is that "low"? It's probably higher than many people would imagine inside a soda can. OTOH: My LPG bottle is about 300psi (3mm steel-plate walls with a weld around the middle) and hospital O2 often comes like 2000psi or so.

Listening is not a good way to tell pressure anyway - you have to measure it.

 

[Simon Bridge]

@256bits:
I had thought of the buoyancy effect too ...
Considering the different materials involved, I don't think the volume expansion was big enough but it's easy to check - simplifying the 1l of water is a cube 10cm on each side that weights 1kg. Water is nice like that. How much would the volume have to increase to cause a weight decrease of the order of 50mg? Ergo: how much do the sides increase? Would you notice?

... crunching the numbers back-of-envelope ... the change in volume needed is about 0.5ml ... or half a cubic centimeter. Gives a change in dimensions of 0.002cm on each side ... about 1/50th of a millimeter.
So no.. you wouldn't notice.

Buoyancy is a contender!

 

[Simon Bridge]

Steady-state movement does not change apparent weight.

No, but the transitions could - when the momentum changes. If you drop stones onto the pan of a balance (which subsequently bounce off the pan) you get a weight reading.

So - in principle, a bubble forming on the wall of the container and detaching would require a change in momentum that could, in principle, affect the scale reading.

The trouble here is that the system is presumed to be closed.

You've indicated that you waited for one hour.

After a long time I'd expect the rate bubbles form to taper off and some weight-loss (by that mechanism) to be recovered. That has not been reported... which counts against the proposed mechanism being significant to the observed readings.


I see this sort of discussion a lot.
The answers are not very obscure - only the details of the process.
Because of this, when we say "well it must be due to effect A", the experimenter responds "but it couldn't be because I have done x and y and z to account for it"... which just shows that their r efforts have just been in vain. They are mistaken about having accounted for effect A. We end up trying to say why effect A is still plausible ... especially with the Holmes precept[*]. But this is seldom convincing and the experimenter insists that we have to repeat the experiment.

-----------------------

[*] Doyle, A.C. (1890) The Sign of Four ch6. (Doubleday p111)

 

[Gh778]

I'm just wake up, so I need time to read all messages...

As the bubbles rise, an equal volume of water must fall...

I'm agree, water give a mass*velocity, idem for gas. But when water move down and gas move up, nothing happens, it's only when gas has reached the surface it can give extra velocity to gas in the bottle. If velocity of gas is conserved, the speed is around 1cm/s, the velocity of CO2 at 20°C is 427 m/s, so the difference of temperature is around 427²-427.01² => 2.9°C.

I'm freezing the bottle with water (-12°C) and the weight come back near to normal: +0.03g. Another problem to understand. Like one tablet give sometimes -0.04g (I saw -0.05g after one hour) maybe it's not good enough to tell something (and it's only one bottle, and the external temperature where is the balance has changed). Maybe this come from the junction liquid/solid. In a solid, there is no vapor pressure like in liquid, the vapor pressure can take less molecule to the solid than it can do with a liquid. When a molecule is ripped away an extra force is added to up due to the rip away, it's just an idea. In the contrary, the pressure of gas don't change result, I test with small volume for gas and this don't change the result: -0.03 g (but this confirm there is no escape for gas). But vapor pressure change a lot with the external pressure or not ?

And for one bottle I pasted with glue (special glue) the cover and it's the same -0.03g and magnet is used for move down the tablet with external system, I waited 2 hours before to move down tablet.

... crunching the numbers back-of-envelope ... the change in volume needed is about 0.5ml ... or half a cubic centimeter. Gives a change in dimensions of 0.002cm on each side ... about 1/50th of a millimeter.
So no.. you wouldn't notice.

Buoyancy is a contender!

Are you sure, for me lost -0.03g with Archimede's law need more. Air is 0.0013 g/cm3, for lost 0.03g you need: 0.03/0.0013 = 23 cm3 and for 0.09g you need to expanse 69 cm3, no ?

 

[Simon Bridge]

I'm freezing the bottle with water (-12°C) and the weight come back near to normal

... don't know what this means - you cooled it in a water bath?

for buoyancy - I just did the proportionality and compared volumes - I may have missed a constant.