The Naked Scientists

The Naked Scientists Forum

Author Topic: Mystery material....almost unknown  (Read 3944 times)

Offline lola

  • First timers
  • *
  • Posts: 2
    • View Profile
Mystery material....almost unknown
« on: 08/07/2005 18:06:00 »
I found this on a website :

This is an awesome discrepant event for your most advanced 'density' students! When this solid white object is placed in water, it initially sinks. Wait about 60 seconds, and it mysteriously floats to the surface. When removed and placed in different water, it continues to float initially and, in about 60 seconds, mysteriously sinks. Why? How can this be? Great for demonstrating how temperature can affect an object's density! In the experiment described above, the first beaker contained hot water from the tap; the second beaker contained ice water.

Does anybody knows which material is used ?

I know that it's a plastic but which one ???

LOLA.


French, blond but loves science !


 

Offline anthony

  • Full Member
  • ***
  • Posts: 65
    • View Profile
Re: Mystery material....almost unknown
« Reply #1 on: 09/07/2005 09:43:46 »
Sounds a bit like the "Cartesean Diver", correct spelling I hope. Imagine a balloon filled with air but also wieghted. It is initally cold in warm water, it sinks then floats to the surface as the gas expands. When placed in cold water it floats until the gas has contracted again and sinks.

I can't imagine which plastic has the correct density and expansion characteristics. Though there are many interesting ways, like what I describe above that you could "cheat" the experiment.
 

Offline lola

  • First timers
  • *
  • Posts: 2
    • View Profile
Re: Mystery material....almost unknown
« Reply #2 on: 09/07/2005 10:01:40 »
Tks antony for your suggestion of diver.

Unfortunately, this is about density and I really need to find this exact material...

I can't beleive that nobody knows about this ;)

But who has the clue ?

French, blond but loves science !
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5341
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Re: Mystery material....almost unknown
« Reply #3 on: 09/07/2005 10:49:55 »
thanks for a great question lola, and thanks anthony for a great answer !

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx
 

Offline anthony

  • Full Member
  • ***
  • Posts: 65
    • View Profile
Re: Mystery material....almost unknown
« Reply #4 on: 11/07/2005 04:10:28 »
You must also consider that the density of the water is also changing. I believe Galilean thermometers work on this principle, the oil in the tube expands with increasing temperature. This type of thermometer is now quite common in that certain type of shop.

Naming an individual material is no easy task. A rubber is your best bet, as this contracts as the temperature rises. The contraction is caused because the polymer chains gain entropy as they coil up more, and this is more dominant at higher temperatures. DeltaG = DeltaH - Temp * DeltaS
« Last Edit: 12/07/2005 03:59:29 by anthony »
 

Offline Andrew K Fletcher

  • Neilep Level Member
  • ******
  • Posts: 2331
  • KIS Keep It Simple
    • View Profile
Re: Mystery material....almost unknown
« Reply #5 on: 11/07/2005 10:54:36 »
could it be an effervescent type reaction with the water that cause tiny gas bubbles to adhear to it, like an egg in heated water. Does it work in pre-boiled water? Or is there a metalic reaction as in when sodium is introduced to water, but to a lesser degree.

Intrigued

"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
 

Offline gsmollin

  • Hero Member
  • *****
  • Posts: 749
    • View Profile
Re: Mystery material....almost unknown
« Reply #6 on: 12/07/2005 03:30:24 »
What was the shape of the white plastic?
How deep in the water did it float, when it was floating?
Do you know the temperature of the hot water? I guess the ice water was about 2 C.

I could be Nylon-12, with a density of 1.01 g/cc.
HDPE has a range of densities spanning 1 g/cc, and most plastic's densities can be altered by adding glass beads or fibers. The plastic expands faster than water, so it would become bouyant in hot water, and sink in cold.
« Last Edit: 12/07/2005 03:59:51 by gsmollin »
 

Offline anthony

  • Full Member
  • ***
  • Posts: 65
    • View Profile
Re: Mystery material....almost unknown
« Reply #7 on: 12/07/2005 04:01:43 »
Of course my rubber suggestion works the opposite way to the sample you describe. I hadn't paid much attention to the details you originaly posted, sorry. Interestingly there are two metal hooks on either side of the lump of plastic, which suggests that the plastic itself is less dense than water.
 

Offline anthony

  • Full Member
  • ***
  • Posts: 65
    • View Profile
Re: Mystery material....almost unknown
« Reply #8 on: 12/07/2005 08:53:51 »
Plastics are composed of rubber-like (molecules like heaps of spagetti) and crystaline domains (ordered strands of molecules tightly packed). It is these crystaline domains that expand with increasing temperature, the suggestion of HDPE is a good one since this plastic contains a large proportion of crystaline domains, making it more likely to expand with increasing temperature. It seems like a very good suggestion.

You can either try and find it's expansion behaviour in books or try it by experiment.
« Last Edit: 12/07/2005 08:54:30 by anthony »
 

The Naked Scientists Forum

Re: Mystery material....almost unknown
« Reply #8 on: 12/07/2005 08:53:51 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums