Re: general question on salts
- From: Martin Brown <|||newspam|||@nezumi.demon.co.uk>
- Date: Thu, 06 Oct 2005 08:59:21 +0100
stuart_noble@xxxxxxxxxxxx wrote:
The volume of what's left seems to be larger,
Yeah, you've added more liquid.
The difference I'm talking about is in the solid material left after the reaction i.e. when the water has evaporated. Sodium citrate from the sodium carbonate reaction gives you more powder than from the sodium hydroxide reaction, at least I think it does. Maybe I'd better check this out again...:-)
You would be much better off weighing them! And that might well explain the differences. Also your sodium hydroxide may well already be partially carbonate / bicarbonate due to absorbing CO2 from the air.
There is a possibility that the impurities in your reagents may cause slightly different crystalline forms. If one packs better than the other you could have different volumes occupied by the same mass of compound.
On the face of it your observation should be wrong: NaOH contains 23g of Na in every 41g (56%) Na2CO3 contains 46g or Na in every 106g (43%)
In practice you may well have hydrated salts rather than anhydrous reagents. NaOH is extremely deliquessant and likely to be dripping wet unless properly stored. If you describe exactly how you did the experiment it may be possible to explain your results.
Historically certain African countries purchase(d) common tablesalt by volume, and a UK chemical company made a small fortune by finding a crystal habit modifier that made salt crystals grow as long interlocking fluffy radial needles. The resulting "new improved" salt product was a significantly larger volume than the normal cubic crystal form and just by the addition of a few tiny impurities.
Moral of story: in chemistry mass is conserved, but volumes can be misleading. eg Try adding 1 part of meths to 1 part of water in a measuring jug.
Regards, Martin Brown .
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