• Question: When you turn the sugar in water in a particular direction, it dissolves the sugar, why not back when flipping in the other direction.

    Asked by funnymangohead to Cesar, Emily, Jamie, Kate, Philippa on 22 Jun 2011.
    • Photo: Jamie Gallagher

      Jamie Gallagher answered on 22 Jun 2011:

      Hey, another chemistry question! yay!

      dissolving sugar (which is little crystals!) in sugar. You can stir in any direction and it will still dissolve. The sugar and the water doesnt actually know what way it is going. To dissolve the sugar we need the water particles to hit the side of the sugar and tear a little piece off. If we stir the water then we make more of the particles of water hit the surface of the sugar crystals and it will all dissapear quickly.

      Once something is dissolved it can be a bit more difficult to get it back. For sugar the easiest way would be to get rid of all the water. If you heat the water slowly and gently the water will evaporate but the heavy sugar willl stay in the container. Eventually when all the water is gone only the sugar will be left.

      Hope that answers your question, if not- just give me a shout!

    • Photo: Cesar Lopez-Monsalvo

      Cesar Lopez-Monsalvo answered on 22 Jun 2011:


      There has not been many questions around. I will give a different view from Jamie’s answer. It has to do with the flow of time.

      So, say you are given two videos to watch in two screens (left and right). One stars with unstirred water with sugar at the bottom, and the other one with something that seems plain water (maybe sweet). Then you play both videos and observe what happens. In the first one, someone stirs the fluid, say clockwise, until the sugar is completely dissolved. In the other video you see someone stirring the water in the opposite direction (anti-clockwise) and suddenly you notice than some crystals start forming until they condensed at the bottom. Suppose both videos have the same duration. You see the last frame and notice that it looks just as before you started playing just that they have swapped screens.

      It wont take long before you discover that you saw the same video played in both screens, just that one was going forward in time, while the other one was played backwards….but how do you know? What makes you believe such thing? If everything you have are ALL the theories of physics to explain why do you know…well, you will find yourself in trouble.

      All our theories of physics happen to be “symmetric in time”. That means that if we change the sign of the time variable in every equation we have, what we get is physically allowed! …You may say, what about thermodynamics and the second law, is it not the case that entropy should increase? That is a partial answer…but the second law of thermodynamics is kind of an additional hypothesis to our physical theories, not a theory itself.

      So the way around to your question is, when we dissolve sugar in water, no matter which direction we stir, we go from an “ordered” state (you can “see” the crystals in the water) to a highly disordered state (the sugar is there but scrambled all around). The second law of thermodynamics tells us that it is basically impossible to go back to the previous stage “for free”. As Jamie pointed out, you can still recover the sugar content, but at the expense of heating the water and separating it from the sugar…and that “costs” much more than just stirring.

      The actual answer to your question, however, lies deeply in the roots of physics that we still do not understand. That is, what is the origin of the second law of thermodynamics – the thing that does not allow to recover the sugar just by stirring in the opposite direction (as in the thought video example)…what is the origin of the arrow of time?