November 24, 2020, 08:33:39 PM
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Topic: Why is (aq) put at the end of compounds that are said to be insoluble?  (Read 254 times)

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Offline EA

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This has always confused me about soluble and insoluble compounds. My teacher JUST told me that (aq) means it dissolves in water, but then turns around and tacks it onto a compound that she just said was insoluble. Did she define it wrong?? Does (aq) just mean the compound is IN water? Someone help.

Offline Meter

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Re: Why is (aq) put at the end of compounds that are said to be insoluble?
« Reply #1 on: October 06, 2020, 02:37:19 PM »
This has always confused me about soluble and insoluble compounds. My teacher JUST told me that (aq) means it dissolves in water, but then turns around and tacks it onto a compound that she just said was insoluble. Did she define it wrong?? Does (aq) just mean the compound is IN water? Someone help.
I think solubility in water depends on a compound's ability to more or less evenly distribute in the water. If it forms a layer at the bottom or at the top (over time), then it can't be said to be very soluble (it might have some solubility). Remember that it doesn't actually have to dissociate to be dissolved. Also, she might just have misspoken. My gen chem professor has a ph.d and constantly makes silly mistakes. Nobody is perfect.

Offline jeffmoonchop

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Re: Why is (aq) put at the end of compounds that are said to be insoluble?
« Reply #2 on: October 06, 2020, 04:42:24 PM »
No, solubility is when a compound is fully dissolved. If there is any separation from the solution, it isn't dissolved. This is the case if the separation are solid particles (sol colloids), liquid particles (emulsion), etc. Look up colloids. If a compound forms micelles it is still separated from solution, even if the solution looks clear. So to find the true solubility, you need to separate the micelles from the saturated monomer solution. As to your question, I don't know the proper use of Aq, because since graduating from my chemistry degree 8 years ago, I've never written out a chemical equation. But I'm assuming that Aq should mean a fully dissolved solute. Precipitation should be (s), but ALL compounds have solubility, so you will always have some still dissolved at the solubility of the solute at the given solvent composition and temperature. Normally you try to cool to crash more product out so you can increase yield. This happens because you reduce solubility by cooling, so less can dissolve. It may be the difference between a 70% yield and 90% yield.

Offline Babcock_Hall

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Re: Why is (aq) put at the end of compounds that are said to be insoluble?
« Reply #3 on: October 06, 2020, 04:58:39 PM »
Suppose that I had a salt of formula AM and only a little of it dissolved in water to form a cation and an anion (I would say that it was sparingly soluble).  I would still write an equation along the lines of AM (s)  :lequil: A+(aq) + M-(aq).  To me, it just differentiates the solid material from what is in solution.Caveat: perhaps my interpretation is not the standard one.
« Last Edit: October 06, 2020, 05:09:16 PM by Babcock_Hall »

Offline Meter

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Re: Why is (aq) put at the end of compounds that are said to be insoluble?
« Reply #4 on: October 06, 2020, 06:18:36 PM »
Suppose that I had a salt of formula AM and only a little of it dissolved in water to form a cation and an anion (I would say that it was sparingly soluble).  I would still write an equation along the lines of AM (s)  :lequil: A+(aq) + M-(aq).  To me, it just differentiates the solid material from what is in solution.Caveat: perhaps my interpretation is not the standard one.
Some people would argue that a salt can be dissolved, theoretically speaking, without being ionized. But it might not be for very long anyway. I would just be vary, as in UG you will get questions like "assume [salt] does not ionize in water, what is the van't hoff factor...". Some people will incorrectly assume it is 0 because it is said it doesn't dissociate, but it's 1 as it is still dissolved, just not ionized.

Have to remember that there is a long way from first year UG to the real world. Non-ideal behavior can be a bit much to take on if you are just taking gen chem.

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