Does Calcium Hydroxide deteriorate?

[mazcheck]

I have tried making calcium hydroxide solution from "The Laboratory" book however it hasn't been going milky in experiments. Does anyone have any ideas as I need "workable" solutions for a yr 10 prac test.

[lada]

Our Ca(OH)2 must be at least 20yrs old and I still use it.
Make sure, you have enough undissolved powder on the bottom of container. Maybe leave it for a couple of days and shake it regularly and than filter the clear solution. I never had that trouble, so not sure what to advise you except make sure your bulk solution is OVER saturated. Than decant and filter on the day you need it.
Good luck
Lada

[dime]

I agree with Lada. Ours is as old, but works. I have about 2cm of the powder covering the bottom of the winchester and filled with water. This would be at least 10 years old and still works.

[Graham Kemp]

I have tried making calcium hydroxide solution from "The Laboratory" book however it hasn't been going milky in experiments. Does anyone have any ideas as I need "workable" solutions for a yr 10 prac test.

The solid should not decompose as long as it is kept reasonably dry. Dry Calcium Hydroxide's shelf life is practically infinite.

Preparation of limewater is easy, but not necessarily quick. Just stir in excess of 1.5g Ca(OH)2 into 1 L of deionised water. Allow stock to stand a few days until the solution clears, and decant carefully on demand (so as not to stir up the precipitate). However, if needed immediately (or next day), just filter off the insoluable suspension. Do ensure you have sufficient time to pass a litre through the filterpaper; this step does take a while.

A tightly sealed stock solution should remain viable for quite some time. Leaving some excess calcium hydroxide on the bottom of the stock bottle will replenish any removed from solution by carbon dioxide in the air (just don't shake before use).

Another long term storage tip I've found is to use a clear plastic water bottle, sealed by a two-hold bung with long and short glass tubes, with tubing conected to each, a sirnge connected to the short tube so that when the plunger is depressed, limewater is expelled though the long tube's tubing. A clamp or plug on the end of the expelling tubing may be needed to prevent leakage. But that's going an extra mile than may be needed.

However you dispense the stock solution, be careful not to avoid mixing in any precipitate. The working solution needs to be clear as clean water.


To test your solution's viability: pour a sample into a test tube, take a drinking straw, and carefully blow bubbles through the solution. (Glasses on; it's an alkaline solution!) After a minute or five the clear solution should start to turn cloudy from the CO2 in your breath; a milky-white Calcium Carbonate suspension precipitates. This is probably close to your experiment.

Ca(OH)2(aq) + CO2(g) = CaCO3(s) + H2O(l)

However, if you leave this to stand for a little while, the suspension will mostly settle to the bottom. It will not remain milky over time.


( Naturally, you're wearing safety glasses, angling the tube away from yourself, and not breathing through it too hard in order to preven splashing. And, of course, breathing out through the straw, but in through the nose. Be aware of how many year 10 students will not think of this unless told. ... Seriously. )

[mazcheck]

Thanks for your quick replies. I added more powder to the bulk solution I had, shook it throughout the day then filtered a small amount to test with a straw and it worked. I'm so happy!

[lada]

Well done,mazcheck,

I LOVE chemtalk, really helpful.
Lada

[Lyn]

My bottle has been going for thirty years. I add extra calcium hydroxide every so often to give it a lift you might say. Shake the bottle by upending backwards and forwards several times and then let it stand overnight or longer. I siphon off the clear liquid using a length of plastic tubing. So far I have never needed to filter the solution. You just have to make sure that the sediment is not disturbed while siphoning the liquid from the bottle. It always works turning a nice milky shade when you blow into it with a straw. Another little trick is to continue blowing into the milky solution until it goes clear again.

[malook]

Thanks Graham, I just tried your two hole bung with tubing and syringe for a teacher in a far flung lab who couldn't find me when her limewater had gone off. It works a treat. She can now help herself from a stock bottle. Malook