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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: cliverlong on February 12, 2009, 01:46:27 PM

Title: Action of heat on hydroxides
Post by: cliverlong on February 12, 2009, 01:46:27 PM

Hi,

This one has me stumped. I have googled and looked in various textbooks and I can't find an answer to this question from a CIE IGCSE chemistry paper.

What is the action of heat on

1) Zinc Hydroxide
2) Sodium Hydroxide

If someone could point me in the direction of a resource that describes or discusses this topic I would be most grateful


Thanks

Clive

Title: Re: Action of heat on hydroxides
Post by: Donaldson Tan on February 13, 2009, 03:15:32 AM

Zinc Hydroxide changes from colourless to yellow upon heated.  Heat causes the compound to rearrange its structure, thus giving it the yellow colour. Zinc Hydroxide revert to colourless when it cools down.

No colour change happens to Sodium Hydroxide upon heating.

Title: Re: Action of heat on hydroxides
Post by: Arkcon on February 13, 2009, 06:17:37 AM

Well, that's kinda a funny question.  What happens when you heat something.  How much do you heat it?  Nothing ever happens to NaOH when you heat it?  It never converts to NaO?  It never melts?  How about NaOH vapor? Yeah, that takes a lot of heat, but they didn't ask cliverlong: to do it, they just asked what happens.  I got a similar "Huh?" moment with this question:  http://www.chemicalforums.com/index.php?topic=31171.0;topicseen

Title: Re: Action of heat on hydroxides
Post by: Astrokel on February 13, 2009, 11:59:01 AM

Quote

Zinc Hydroxide changes from colourless to yellow upon heated.  Heat causes the compound to rearrange its structure, thus giving it the yellow colour. Zinc Hydroxide revert to colourless when it cools down.

Ah, this sounds familiar to me. I remember reading this few years back when i was in grade 10. I can't remember what was the reason though. Perhaps zinc peroxide is formed at high temperature? Correct me if i'm wrong.

Title: Re: Action of heat on hydroxides
Post by: cliverlong on February 13, 2009, 03:05:31 PM

Quote from: geodome on February 13, 2009, 03:15:32 AM

Zinc Hydroxide changes from colourless to yellow upon heated.  Heat causes the compound to rearrange its structure, thus giving it the yellow colour. Zinc Hydroxide revert to colourless when it cools down.

No colour change happens to Sodium Hydroxide upon heating.

Geodome,

Can you tell me from where you got your information?


Thanks

Clive

Title: Re: Action of heat on hydroxides
Post by: Vidya on February 14, 2009, 02:22:44 AM

unstable hydroxides on heating lose a water molecule and get converted into oxides.Now we can see that NaOH is a stable hydroxide and  zinc hydroxide is less stable.
so now you can predict who is going to lose water .
Zinc compounds are white in colour when cold.Zinc oxides are yellow when hot and white when cold.
 I hope this much information is enough to undersatand this question.

Title: Re: Action of heat on hydroxides
Post by: cliverlong on February 14, 2009, 12:02:05 PM

Quote from: vidya kaushik on February 14, 2009, 02:22:44 AM

unstable hydroxides on heating lose a water molecule and get converted into oxides.Now we can see that NaOH is a stable hydroxide and  zinc hydroxide is less stable.
so now you can predict who is going to lose water .
Zinc compounds are white in colour when cold.Zinc oxides are yellow when hot and white when cold.
 I hope this much information is enough to undersatand this question.

Yes, thank you for the additional information. My follow on question was where is that information documented so I can reference it to support this explanation? None of the sources I have searched describe this behaviour of hydroxides.

Clive