Why does glass get elastic when immersed in HF; like this video?

In summary, the conversation discusses the effects of immersing glass in HF and the resulting elasticity of the glass. Some speculate that the HF etches away surface imperfections and stress concentrations, making the glass more elastic, while others believe that it actually makes the glass more brittle by introducing defects. There is also a mention of the importance of safety when handling HF. The conversation ends with the realization that their theories and explanations are supported by multiple sources in books.
  • #1
Panthera Leo
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0
Why does glass get elastic when immersed in HF; like this video?

Hi,

I was surfing YouTube and came across a very interesting video... Check it out yourself;

http://youtu.be/gzUDjHDj0fQ"

Why did the glass become elastic?

I was thinking it might be due to destruction of some bonds on SiO2 but not all causing mobility of the molecules such that they slide over one and other? But its just a guess, I have no idea.

Thanks in advance.
 
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  • #2


The elasticity of the glass didn't change, but the HF smoothed the glass by etching it for a short time. The etching removed surface imperfections that produce high stress concentrations and make glass the brittle material that we're used to.

Incidentally, I can't approve of someone pouring undiluted HF over their head, with little personal safety gear, and outside of a fume hood. This person sets a bad example.
 
  • #3


Mapes said:
The elasticity of the glass didn't change, but the HF smoothed the glass by etching it for a short time. The etching removed surface imperfections that produce high stress concentrations and make glass the brittle material that we're used to.

You could be right but I have a different take on it. I think that HF etches glass and would introduce lots of defects making it more brittle. What I think is going on is that the glass rod isn't annealed and the glass near the surface that already under stress is preferentially etched away in the HF. All the glass rods I have ever used are flame polished and don't have any defects like micropits and so forth. None of the ones that I have ever used are ever annealed after flame polishing so they have significant surface stresses frozen into them. The untreated rod breaks early because the additional stress of the mass added to the inherent stress frozen into the glass rod during manufacture is too much. A rupture occurs near the surface.
 
  • #6
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  • #7


chemisttree said:
I think it's just amazing that there are at least two books crammed full of our correct answers!

That's what makes it so nice for us to kick back in our easy chairs and speculate. :smile:
 

1. Why does glass react with hydrofluoric acid (HF) differently than other acids?

HF is a weak acid, meaning it does not fully dissociate into ions in solution. This allows it to easily penetrate the surface of glass and react with the silicon dioxide molecules, leading to the elastic behavior seen in the video.

2. How does the elastic behavior of glass in HF differ from its behavior in other solutions?

In other solutions, the glass may experience corrosion or dissolution, but not the same elastic deformation seen in HF. This is due to the unique ability of HF to penetrate the glass surface and react with the silica molecules.

3. Is there a specific concentration of HF that is needed to observe this elastic behavior in glass?

Yes, the concentration of HF needs to be high enough to allow for a strong enough reaction with the glass, but not too high that it causes the glass to dissolve completely. A concentration of 49% is commonly used to achieve this elastic behavior.

4. Can this elastic behavior be reversed once the glass is removed from the HF solution?

Yes, once the glass is removed from the HF solution, it will return to its original shape. This is because the elastic deformation is caused by the chemical reaction between HF and the glass, rather than a permanent physical change.

5. Are there any potential safety concerns when working with HF and glass?

Yes, HF is a highly corrosive and toxic substance. It can cause severe burns and damage to the respiratory system if proper precautions are not taken. It is important to handle it with caution and follow all safety protocols when working with it in a laboratory setting.

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