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How to tell if something is soluble in water?

Discussion in 'MCAT Study Question Q&A' started by junkct, 12.20.08.

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  1. junkct

    junkct 5+ Year Member

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    A couple questions here:

    1. How do you tell if something is soluble in water?
    2. How do you tell if it is polar?
    3. Are there any rules for determining if something is strongly/weakly acidic (I know all the strong acids, but when it comes down to differentiating between weak acids, I get stuck)?

    4. Also--esters, ethers, amines... are these soluble in water? are they polar or nonpolar? agh these ones get me every single time!

    Thanks!
     
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  3. MedicineAHP

    MedicineAHP No easy way out 5+ Year Member

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    Hi, in regards to the first two questions, a polar molecule has an uneven distribution of electrons(unsymmetric) so instead of being evenly distributed (following is the case of h20) on hydrogen and oxygen, the electrons are on oxygen. If you draw out h20, and draw arrows pointing to the most electronegative atom in each bond(oxygen in this case), you can see that one end of the molecule is positive(H side) and the other negative(oxygen). This is depicting a polar molecule.(knowing your molecule's geometry helps-VSEPR) And in the example of CH4(methane), do the same thing and you will see that this is not the case, the ends of the molecule are positive(no polarity, so non polar). You can tell if something dissolves in h20 by following the rule "like dissolves like", so polar dissolves polar. Hope this helps
     
  4. junkct

    junkct 5+ Year Member

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    cool thanks.. so does this mean that ions will dissolve in water? also, I've seen many times that carboxylic acids don't dissolve in water, although they are polar too (same goes for esters). What does this mean?
     
  5. MedicineAHP

    MedicineAHP No easy way out 5+ Year Member

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    No problem, Ions in the form of electrolytes(specifically strong electrolytes, which are composed of a positively and negatively charged species/ions, like NaCl) will be soluble/dissolve in water. Strong acids are also soluble in water, and there are 7 notable ones, like HCl, HBr, HI, HNO3, HClO4, HClO3, and H2SO4. For example, CO2 is not on this list so it will not be soluble in water, since it is not a strong electrolyte(it doesn't break down in water into ions). Carboxylic acids in the form RCO2H will lose the hydrogen off of the oxygen first, which will be the soluble portion, assuming the R-group is a saturated(all carbons and hydrogens and no double bonds) chain. It is known as an "acidic hydrogen"--which correlated to the relationship with acids and how they (like HCl-remember a strong electrolyte) lose their hydrogens in aqueous(water) solutions. Hope this helps.
     
    Last edited: 12.21.08
  6. G1SG2

    G1SG2 5+ Year Member

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    It is easier for short chain carboxylic acids to dissolve as opposed to longer chain carboxylic acids. The longer the hydrophobic alkyl group, the less important the polar -OH region and the less water soluble the carboxylic acid is in water.
     
  7. TooMuchResearch

    TooMuchResearch i'm goin' to Kathmandu... Lifetime Donor 7+ Year Member

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    Carbon dioxide is soluble in water. It forms carbonic acid with the water molecules.
     
  8. MedicineAHP

    MedicineAHP No easy way out 5+ Year Member

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    I was speaking in terms of "strong electrolytes". CO2 will hydrogen bond with the water molecules, as do water molecules with one another. And as I said it won't break down into ions!!!:) You are referring to:

    CO2 + H2O ⇌ H2CO3

    not to electrolytes and how they are related to dissolving in h20. The seven listed items will break down into ions. Hope that clears up any ambiguity..
     
    Last edited: 12.28.08

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