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Homework Help: Why does sucrose have a higher density than water

  1. Sep 2, 2012 #1
    Hello there

    I did an experiment a while ago about mixing various weights of sucrose and water together to form a solution. However, I cannot fathom why dissolving sucrose in solution would make the overall density of the solution higher. Because water itself is a very dense substance where the molecules are already close together, and sucrose is a seemingly complex an organised molecule. Wouldn't the awkward shape of the sucrose molecule make it harder for the individual molecules to get closer together and fill up space?

    I understand that in the crystallized form, sucrose opens up and creates large interstices, but I don't know what happens in solution now that there are no solid bonds.
  2. jcsd
  3. Sep 2, 2012 #2

    Simon Bridge

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    Because there is more mass per unit volume.
    Clearly the shape of the sucrose molecule does not hinder it being packed tighter in solution.

    Dissolving two things is not just a matter of mixing them - try it with oil in water.
    It exploits polar aspects of molecules ... the sucrose molecules are free to assume any orientation in solution while in a crystal they are restricted in orientation. Water is not just H2O but also H+ and OH- and these mixed in can help the sucrose get closer together too.
  4. Sep 2, 2012 #3
    So it's simply the sheer size of the molecule?

    Because I looked at interactions between ethanol-water and sucrose-water, and even though ethanol leaves smaller gaps because of its size, sucrose makes the solution more dense?

    Also, in solution, isn't ethanol free to assume any orientation because the exposed hydrogen will on the ethanol, will hydrogen bond with the oxygens in water and conversely: the hydrogens on water, will hydrogen bond with other the oxygen in the hydroxyl group?
  5. Sep 2, 2012 #4

    Simon Bridge

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    "bond" is too strong a word to describe how the polar parts interact in a liquid.

    However - you realize that if you have a volume of water and an equal volume of lead+water, not a solution, the lead+water mix would be denser. Nothing to do with closeness of molecules in solution.

    I bring this up because you seem to be asking about two things.
  6. Sep 2, 2012 #5
    Well, I'm assuming that when water interacts with ethanol/sucrose hydrogen bonds are formed.

    I don't understand how lead and water relates to this, because I thought that density was just a measure of mass per unit volume. If the average "closeness" (I can't think of a word) was high in a solution, doesn't that make it more dense?
  7. Sep 2, 2012 #6

    Simon Bridge

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    What leads you to assume that hydrogen bonds form in solution?

    Where there is lead, there is no water - since the lead is more dense than the water, adding lead (keeping the volume the same) you get higher overall density.

    Where there is a sucrose molecule, there cannot be a water molecule - the more massive sucrose is displacing water: it's a bigger molecule so each sucrose can displace more than one water. In the same volume, that makes sucrose+water more massive than water+water without having to decrease the mean inter-molecular spacing.

    For that matter - being slower moving (more massive, more bonds, same temperature) means that each sucrose is going to occupy less space than each water molecule (beyond it's own "rest" volume). So the mean intermolecular spacing is also decreased.
  8. Sep 2, 2012 #7
    I figured because sucrose has many exposed hydrogen and oxygen atoms on the outside, it is able to be connected to water molecules. Just like how hydrogen bonds are formed in pure water, and pure ethanol.

    On the massive note, ethanol is more massive than water? yet it has a lower density? it also has more bonds for waters to attach?
  9. Sep 2, 2012 #8

    Simon Bridge

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    Ethanol molecule is more massive.

    There are several things going on here all at once.
    You started off asking about sucrose ...

    Certainly when you dissolve sucrose the overall volume does not increase all that much.
    But if it did (by displacing water) then it is easy to see that the density would increase. The question reverts to why sucrose dissolves instead of just displacing water.

    I found a collection of animations that claim to show the process of different things dissolving in water.
  10. Sep 2, 2012 #9


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    Because, ethanol has lower degree of Hydrogen bonding. A water molecule is capable of forming H-Bond with 4 other molecules, while ethanol isn't, if you want to explain with H-Bond argument.

    H-Bonding is not a massive role player in liquid phase in determining the density, because there are only very few H-Bonds present. Compare it with Ice, where it has a definite crystal structure, thanks to H-Bond.

    In solution phase, you can successfully explain Boiling Points with H-Bonding. But I don't think that you can explain density solely on H-Bonding.
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