1. ## A Science Puzzle

Imagine that you are a chemist just starting in the polyurethane industry. You have developed a two component systems comprised of an isocyanate and a resin. The system is "water-blown" (water is added to the resin which is converted to carbon dioxide upon reaction with some of the isocyanate) and the material expands into a foam. The cells of the foam contain the carbon dioxide formed during the reaction.

To test this material, you prepare a rectangular panel (5 cm x 20 cm x 200 cm) having a weight of 0.65 kg. To prepare this material required 0.68 kg of starting material (0.31 kg of resin and 0.37 kg of isocyanate).

Upon seeing the results you realize that you "lost" 0.03 kg of material. This bothers you a little since mass is conserved in all reactions.You know that the balances have just been calibrated and can be considered accurate and precise. You investigate and discover that none of the carbon dioxide is lost during the foaming, it is all contained in the cells of the foam. You also know that there are no other volatile materials which could be lost during the foaming process.

Upon contemplation of the facts and remembering some basic principles of science, you realize where the 0.03 kg are and you proceed with your development.

What happened to the 0.03 kg?

Steve

2. ## Re: A Science Puzzle

<span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">has the foam displaced 0.03Kg of air?</span hide>

StuartR

3. ## Re: A Science Puzzle

<span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">It has been converted to energy (warmth) in the chemical reaction.</span hide>

4. ## Re: A Science Puzzle

You went nuclear without realizing it... although I fail to see how - with 2,700,000,000 MJ of mass-energy released into the room, it must have been awfully uncomfortable.

More seriously <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">what material was the mold made from?</span hide>

Alan

5. ## Re: A Science Puzzle

Steve

Not a chemist but a guess

<span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">There is a volatile reaction where a lot of heat is produced. It appears you have created a closed system so, as the reaction becomes warmer the density of the closed sytem becomes less and therefore appears lighter. Is there not a preservation of mass law?

So in short, the density has changed</span hide>

6. ## Re: A Science Puzzle

<img src=/S/clapping.gif border=0 alt=clapping width=19 height=23> That is it exaclty.

<span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">We are trying to determine the mass of something in a fluid (ie air). We must take bouyancy of the material into account. The masses will balance, unfortunately the weights may not and we are weighing, not determing the mass (unless corrections for air are done).

The panel has volume of 0.02 m

7. ## Re: A Science Puzzle

Molds are aluminum

Steve

8. ## Re: A Science Puzzle

Mold is at 120

9. ## Re: A Science Puzzle

Ah yes, the ol' <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">buoyancy</span hide> correction... forgot about that one. I was thinking about the funny way in which a wooden mold can significantly absorb some organics, following the "usual" absorption behaviour at first, but then uping the pace considerably. I think it's to do with the organic opening up the structure of the matrix that binds the wood fibres. I guess to account (very precisely) for the <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">buoyancy</span hide> effect, you'd have to take into account the orientation of the block when it was weighed too. <img src=/S/grin.gif border=0 alt=grin width=15 height=15>

Alan

10. ## Re: A Science Puzzle

i don't think the orientation effects it at all. <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">buoyancy</span hide> is based only on properties of the solid... <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">B = density * gravity * volume, meaning buoyance = the weight of the displaced fluild</span hide> unless i misunderstood your reply.

11. ## Re: A Science Puzzle

You are correct. the oprientation is immaterial. <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">It is only the volume of the fluid (air in this example) displaced and what the mass of that volume of fluid. The volume displaced is the same whether it is horizontal or veritical.</span hide>

Steve

12. ## Re: A Science Puzzle

I did use the term "very precisely" for a specific reason.
Hint: <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">What is the value of "gravity" in your equation?</span hide>

Alan

13. ## Re: A Science Puzzle

It doesn't matter. You would get the same results on the moon if the <span style="background-color: #FFFF00; color: #FFFF00; font-weight: bold">atmosphere was the same (it the atmosphere, the fluid, not the gravity)

They are being "weighed" on a balance, not a scale. (Labs uses balances, not scales). Both sides use the same "gravity" since they are being measured at the same time.

One way to look at this is to start with an "empty" (no foam, just ambient air) refrigerator cabinet. It is just the metal sides and the plastic liners. If the cabinet is tared you get the weight of the cabinet plus the weight of the air within the cabinet (both the air in the freezer and the fresh food sections) and the air within the walls (between the metal and plastic liner)

If you fill the cab with foam to insulate it (between the metal sides and liner) the air that was within the wasll is displaced. So when you weigh it to see how much foam was added and you subtract the tare, you are determining the foam wt - the wt of the volume of air:

Tare: Cab + Air in FZ section + air in FFsection + air in walls
After foaming: Cab + Air in FZ section + air in FFsection + foam in walls

Difference = Foam in walls - air in walls.

To get the true "mass" you need to add the mass of the air in the walls to "compensate" for this "discrepancy".

Even in a panel (without the cab), a panel displaces more air than another mass (of higher density) than it is balanced against so the side of lower density (even with an identical mass) will seem lighter.

If you try to balance a 1 kg (mass) lead block and a 1 kg (mass) bag of feathers you will find that the lead block will seem "heavier" and the balance will be lower on the lead side. To "balance" the 2 objects (no matter what the gravity) will require extra mass on the feather side to compensate for difference in the displaced air between the 2 objects. Since much more air is displaced by the feathers, it will seem that much "lighter".</span hide>

Steve

14. ## Re: A Science Puzzle

your value of gravity depends on 1-the planet & postion: longitude, latitude & elevation, 2- your system (metrics or crappy US), so ~9.81m/s^2 or ~32.2ft/s^2 for earth, and finally 3- your significant digits, generally 2-3 decimal places. if you want to get more precise than that, start counting your atoms <img src=/S/laugh.gif border=0 alt=laugh width=15 height=15>

15. ## Re: A Science Puzzle

Steve, if you balance a deflated ballon compared to an inflated balloon, you will find that the inflated balloon is "heavier" - however thats different from the feathers because the balloon is compressing the air inside of it making it more dense - the feathers, of course, are not. I always found that interesting.

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