# What rules govern the solubility of ionic substances?

**URL:** <https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189>\
**Category:** Factual Questions\
**Created:** [April 2, 2014, 6:59pm UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189 "2014-04-02T18:59:46Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\
**Post date:** [April 2, 2014, 6:59pm UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189/1 "2014-04-02T18:59:46Z")

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As a teacher, I struggle explaining why NaCl is soluble but AgCl is not. I’ve read it must have something to do with the charge mass ratio of the ions but this makes no sense for AgCl.  
Can anyone explain?

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**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\
**Post date:** [April 3, 2014, 5:01am UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189/2 "2014-04-03T05:01:09Z")

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> **[Silver chloride](https://en.wikipedia.org/wiki/Silver_chloride)**
>
> insoluble in alcohol, dilute acids.
> Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+) and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver (and chlorine), which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as a mineral chlorargyrite.
> Silver chlorid...

The fundamental idea is that the rate of deposition must be higher than the rate of dissolving.  
What strikes me as the cause of that is the way it makes a tight “face centre cubic” (FCC) crystal. The crystal is not particularly polar. There’s Ag+ and CL- in nearly equal amounts all over the surface, and so water has no grip on it to destroy it. Its slightly more ‘Ag+’ at the surface, which is repulsive to H+, but its dense and OH- is too big to get close to it. So H2O ,H+ and OH- are not dissolving it.

Meanwhile Cl- can get close and join the crystal , so then Ag+ can join… then CL-, Ag+ , Cl- … a high rate of deposition.

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**Author:** ![the\_diego](https://avatars.discourse-cdn.com/v4/letter/t/7bcc69/32.png) [@the\_diego](https://boards.straightdope.com/u/the_diego)\
**Post date:** [April 3, 2014, 5:26am UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189/3 "2014-04-03T05:26:53Z")

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Ok, I’m starting to remember. For ionic compounds in water, it’s the solubility product Ksp.

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**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\
**Post date:** [April 3, 2014, 6:41am UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189/4 "2014-04-03T06:41:59Z")

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Isn’t it because the difference in electronegativities of the 2 elements, silver and chlorine, isn’t that great, resulting in the bonding in silver chloride having a high degree of covalent character, and thus not being soluble by the adage, “like dissolves like”?

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**Author:** ![buddha\_david](https://avatars.discourse-cdn.com/v4/letter/b/ee7513/32.png) [@buddha\_david](https://boards.straightdope.com/u/buddha_david)\
**Post date:** [April 3, 2014, 6:45am UTC](https://boards.straightdope.com/t/what-rules-govern-the-solubility-of-ionic-substances/685189/5 "2014-04-03T06:45:34Z")

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I’m just happy this thread isn’t about that damn Alanis Morissette song.