What You Need: 2L Of water 2 kg of sugar, large saucepan & supervision is a must because boiling anything on a cooker is dangerous and boiling syrup coming into contact with your skin will cause severe burns. Object of Experiment to Observe how solute density alters the way water boils when heated. 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.
This is a fascinating experiment that requires supervision as boiling syrup is very dangerous. The sugar looks like clouds viewed from an aircraft for a while. 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?