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Why isn't solution state a true physical state ?

  1. Dec 6, 2014 #1
    why isn't solution state a true physical state but a pseudo state?
     
  2. jcsd
  3. Dec 6, 2014 #2
    A solution is a combination of substances, it's either:
    a solid in a liquid,
    a liquid in a liquid,
    a gas in a liquid
    etc.
    When a substance is dissolved in a another substance, for instance a solid in a liquid, the substances don't change state, the liquid is still a liquid, and the solid is still a solid. Furthermore a substance can undergo a state change, this is done adding or removing 'heat', dissolving a substance is not a change of state, because no 'heat' is added or removed, therefore solution can't be a state. When you add or remove 'heat', you are actually adding or removing energy, the more energy a substance has, the warmer it is, and the more the molecules in the substance move. So one could define state change as adding or removing energy, when you dissolve a substance, no energy is added or removed, and therefore the state doesn't change
    I hope this answered your question
     
  4. Dec 6, 2014 #3

    TeethWhitener

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    Not all of this is true. For example, if you dissolve sugar in water, the sugar is no longer solid. The solid dissociates into individual molecules, and you can pass the solution through essentially any size filter without separating the solid sugar from the liquid water. The two substances (sugar and water) combine to form one homogeneous phase (in this case, a liquid). It might not be a liquid of a single pure substance, but it's a liquid nonetheless. It conforms to its container, and it can be boiled and frozen just as a liquid can. Of course, it's still possible to recover the sugar as a solid, but you have to add energy (usually in the form of heat) to the system to do this. Just because a solution is made of two different substances doesn't mean you can't identify it as a liquid/solid/gas. It just means you can't identify it as a pure liquid/solid/gas.

    In addition, dissolving substances does cause an exchange of heat. Compare the dissolution of lithium chloride, sodium chloride, and potassium chloride in water. KCl causes the water to cool significantly while LiCl causes the water to heat significantly (NaCl doesn't cause much of a temperature change). Here's a video of NH4Cl dissolving in water endothermically:

    https://www.youtube.com/watch?v=0GjJ3LiD-zU

    This dissolution process is associated with an enthalpy change (heat of solution), just as vaporization or freezing (fusion) are associated with enthalpy changes.

    To answer your question, we typically don't call a solution a state of matter because a solution can be a solid, liquid, or gas (which are states of matter). As for the term "pseudo state," I've never heard of this term, and a quick google search brings up no real instances of it in the literature. Maybe it's something specific to your textbook.
     
  5. Dec 7, 2014 #4
    Thanks for your reply TeethWhitener, I always enjoy a good discussion. :)

    The sugar IS solid, because the individual sugar molecules still have the same amount of energy as they had before dissolving, therefore there is no change of state. yes you can pass the solution through essentially any size filter without separating the solid sugar from the liquid water, but this is because the sugar molecules are separated. Normally sugar molecules group together, and therefore won't pass through the filter, but when dissolved, this structure breaks, and therefore sugar can pass through the filter, the sugar is still solid though.

    True, but this is a chemical reaction, not a change of state. It all comes down to how one defines dissolving, I would define this as a reaction, not as a dissolution, because the substances before the reaction aren't the same as after the reaction, when dissolving the substances before the reaction are the same as after the reaction. After dissolving sugar, the solution still contains sugar molecules. Salts is a different story, it falls apart in ions when added to water, and therefore is not the same as before.

    It all comes down to how one defines stuff, I define solution/dissolving as above, but I respect it if you define it differently. It's not really important, as long as one can explain what one means to another, there is no real problem.
     
  6. Dec 7, 2014 #5

    Ygggdrasil

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    This is incorrect. The energy associated with a solid, the lattice energy, is the potential energy associated with interaction between sugar molecules (so it is a property of the bulk material and not the individual molecules). When dissolving the sugar in water, these intermolecular interactions must be broken and instead replaced by sugar-water interactions. The difference in energy between these interactions is the heat of solvation. Indeed, if you look up various substances in thermodynamic tables, you will see that the solid forms and aqueous forms have different enthalpies of formation.
     
  7. Dec 7, 2014 #6

    TeethWhitener

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    No, it's not. There is a heat of solution associated with any solvation process, regardless of whether ionic dissociation occurs or not. Look at Ygggdrasil's post about lattice energy. For another clear example (one which very clearly involves no chemical change), consider the dissolution of eicosane in toluene. The toluene gets quite a bit colder in this case. (Interestingly, in this case, it's not so much an enthalpy contribution as it is an entropy contribution--from an increase in internal degrees of freedom of the alkane--which causes the cooling.) But regardless, there is always energy exchange associated with rearranging molecules to form stable solvation shells.
     
  8. Dec 7, 2014 #7
    I did not know about lattice energy, thank you for correcting me, now I learned something too.
     
  9. Dec 7, 2014 #8

    DrDu

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    I never heard of this. How do you define a state an in which context?
     
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