Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Andrew K Fletcher on 30/06/2008 17:51:47
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Comments welcomed.
Video
What You Need:
2 litres of water 2 kilos of granulated sugar, large saucepan and supervision is a must because boiling anything on a cooker is dangerous and boiling syrup will cause severe burns if it comes in contact with the skin.
Scale down to 1 k of sugar and 1 litre of water in a smaller pan.
Add water and all of the sugar to the saucepan and begin to heat it.
Observe closely how the denser fluid at the bottom of the pan behaves as the heat begins to motivate the syrup. At the same time observe the vapour bubbles and the rapidly agitating syrup below the surface.
Adding heat to the water and sugar crystals accelerates the dissolving of the sugar creating a very dense solution. The surface of the syrup does not boil, yet below the surface about half way down the saucepan is clearly boiling and if you look very close you can see lots of large and small gas bubbles forming and rising as you would expect them to do. However if you study what is happening you will see that the surface of the syrup remains unbroken and shows little if any motion while below the surface it is completely different and actively bubbling and boiling.
So what do you think is happening?
I suspect that a flow and return circulation is operating in the lower active level of the syrup where the heat is causing the fluid to form gas and rise but in doing so is generating a return flow from the cooler water causing the rotation of the syrup rather than it reaching the surface and disrupting the stagnant state. The dense syrup is acted upon by gravity and the heat at the base of the pan changes the density of the syrup causing it to rise, where it meets the lower part of the cooler surface less dense syrup and returns back to the base of the pan taking with it the vapour bubbles and preventing them from reaching the surface of the liquid.
Before all of the sugar has turned into clear liquid stir the solution with a wooden spoon and let it return back to the un-agitated state and you should see the lower level behave as before and the surface layer remain once again still.
Eventually the surface syrup heats up and the liquid boils as one would expect a liquid to boil. Yet when another Kilo of sugar is added to the now boiling syrup the same low surface flow happens again and the surface of the liquid stagnates until all the sugar has dissolved and the liquid is boiling in the normal manor. The sugar looks like clouds viewed from an aircraft for a while
This is a fascinating experiment that requires supervision as boiling syrup is very dangerous.
What does it tell us?
Having been working on a density flow theory in plants, trees, animals and humans that generates circulation by density changes occurring in the fluids due to evaporation, the boiling syrup experiment shows how powerful this gravity driven flow really is. It also shows how density changes at the surface of the ocean due to evaporation and the resulting increases in density of surface water generate an underwater river that drives the Atlantic Conveyor system, a river thought to be larger than all of the combined rivers in the world that powers the world’s weather.
But does it also tell us something about the nature of gravity?
You may also be interested in my other density videos on You Tube and please remember to leave a comment and rate them :)
Andrew K Fletcher
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AF
Nice experiment, next time I make fudge I will give it a try.
Comment we can thank god/nature that water does not react like this andonly water and bismuth float on the surface when frozen.
If water did not have this property there simply would be no life as we know it on earth
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Andrew, from the video I couldn't see vapour bubbles; are you sure there are? I can only see a fluid's movement (and in this case the fact the surface is unaffected is quite obvious).
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LightArrow,
Yes there are lots of vapour bubbles forming and then collapsing half way up the pan and they dont disturb the surface. Water boiling without the sugar forms small bubbles of vapour to begin with and thes rise right to the surface. On the video these are more prominent if you look at the side of the pan.
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Thanks Alan
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LightArrow,
Yes there are lots of vapour bubbles forming and then collapsing half way up the pan and they dont disturb the surface. Water boiling without the sugar forms small bubbles of vapour to begin with and thes rise right to the surface. On the video these are more prominent if you look at the side of the pan.
Ok. Can this effect come from the higher vertical gradient of temperature (with respect to pure water) which cools the vapour bubbles when they goes up enough to find cooler water and so becomes liquid again?
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I can see where you are coming from but we still have to realise that heat rises in both liquid and gas so how come this hot liquid from the bottom of the pan rises and then falls back down to the bottom of the pan back to the heat source without making it past the half way mark on the pan?
Clearly gravity is pulling this denser syrup back down and preventing it from reaching the surface and in doing so is collapsing the gas bubbles back into the syrup.
A bag of sugar is around 87p so a smaller pan and a litre of water and you can see this happening for yourself.
In Florida there is a density problem with the ocean, the water at the ocean floor is warmer than on the surface.
Thinking of a way to obtain temperatures of the syrup at different levels so need a thermometer that will tolerate these temperatures. If anyone has such a thermometer could you please repeat the experiment and determine the temperature at the bottom middle and surface of the syrup?
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The results of the experiments are interesting but mostly show nothing special or unexpected if you think about the properties of dense splutions and diffusion. The ability of the water column to go to such a great height before the hydrostatic tension causes the liquid column to break is quite interesting though but you have to consider that the plastic pipes were probably collapsed by the tension and so the water channel was quite narrow and liquids in narrow cappilliaries behave quite differently from those in larger pipes and that probably accounts for the effect.
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Soul Surfer
The plastic tubes do not collapse they are very tough nylon and can withstand a considerable vacuum Capillary does not play any part in the experiment at Brixham. The diameter of the tube remains at 6.5 mm at 24 metres and more.
Thanks for checking out the other video's and look forward to more questions :)
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Was the water very pure, de ionised, and had it been heated to expel disolved gases?
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Im glad you asked that because the water was tap water and yes it was heated to expel disolved gas. The system still works with water that has not been previously degased but tends to fail quicker and at lower heights than 24 metres.
The water in the syrup boiling exp was also tap water and a further 1 k of sugar was added after the syrup had boiled and all of the previously added 2k of sugar dissolved and the same two tier boiling took place again. I have that on video also if it's of use
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No there is nothing unusual about the sugar solution this is just what you would expect from a layered system with a heavier and higher boiling point fraction underneath even if the liquids are miscible. You can demonstrate the same effect by half filling a glass with a coloured sugar solution and topping it up gently with water and just leaving it on the shelf. The layer will be stable for years before diffusion spreads the colour through the mass of the liquid.
You say fail quicker. does the column eventually break if you leave it at say 12-15 metres for a long time?
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yes the column fails because the water inside is not circulating when the solutes rest at the bottom. But this experiment is designed to show how water copes with tension. In a tree there is an outer skin / bark that provides a reduction in the tension inside the tree. Picture several tubes inside a larger tube instead of tubes exposed to atmospheric pressure at the bottom. This would provide the tree with an advantage over the simple tubular model.
The sugar solution is unusual because it is being heated at the bottom on a fully opened gas ring yet does not rise to the surface but clearly boils and rises fast to the half way mark and the returns back to the base of the can. Your analogy of two liquids without heat does not apply here.
Diffusion spreads a little faster than your years of stability predicts.
However, since you have mentioned this the other video showing a U tube spirit level with solute in one side does appear to be stable for many weeks even though there is a difference in water levels on each side of the tube by several Cm's
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Anyone thought about the connection with gravity yet?
The molten core and density changes in the molten rock and metal might be worth considering as this video shows that the lower liquid becomes molten and agitated yet the surface liquid remains relatively motionless.
It must follow that density properties at the molten part of the core should be affected similarly to this simple sugar experiment also.
What you think?
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I see nothing special about what you are suggesting and the sugar experiment is not a desperately good example.
It is generally accepted that the earth like other large planets is differentiated in the sense that denser material in its composition (iron) has mostly sunk to the core with intermediate material (Dense rocks) in the mantle and lighter material and water in the crust. this process probably started with the heat from the gravitational energy released during accretion but has continued for longer because of the energy released by radioactive elements.
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Exactly my point. Down at the core the denser particles can be melting violently and the upper layer of the molten core may be relatively stable and less dense as in the sugar experiment. I have pondered on this intriguing two tier boiling observation and can see how it might apply down at the molten core of the planet. Furthermore, just like heating crude oil at different temperatures we get different densities during separation. Maybe the same density isolation applies to how denser materials that have been formed at the core are produced?