Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: mana on March 26, 2019, 02:39:28 AM
-
hi all
I have dissolved sugar in water with heating, and I thought if I cool this saturated solution the sugar will participate, but It didn't happen ???
can anyone help?
-
Hi Mana, you might double-check if the dissolved amount exceeds the known solubility of sucrose. If this is certain, consider supersaturated solutions. Precipitation doesn't always happen immediately and spontaneously. That's similar to phase change.
Be careful with the Wiki article
https://en.wikipedia.org/wiki/Supersaturation
as the author puts many personal interpretations as if they were facts.
-
Sugar is highly soluble and difficult to crystallise. To crystallise successfully, keep adding sugar until no more is dissolving at an elevated temperature while stirring. Then filter the liquid into a clean container and allow to cool. It might take a long time due to high viscosity and long induction times.
-
seed crystal?
-
thanks for all replies, actually I always thought when I dissolve sth with heating, so cooling it must lead to participation. with out seed crystal or even scratching it :P but now it is very confusing for me why participation didn't happen.
and one more question, does this phase change occur for especial chemical substances?
-
hi all
I have dissolved sugar in water with heating, and I thought if I cool this saturated solution the sugar will participate, but It didn't happen ???
can anyone help?
At room temperature, you may dissolve 200 g of sugar in 100 g of water.
Sugar creates stable supersaturated solutions, sometimes you have to wait even months for crystallization. It is worth looking at different species of honey. Some crystallize after a few months, others even after a few years. In honey, there is glucose and fructose, but only glucose crystallizes.
There are also substances (although there are very few) that reduce the solubility during heating.
-
Your solution may be undersaturated or have low supersaturation. If you leave it long enough, it will crystallise, due to evaporation. Sugar is problematic because of high viscosity, slow molecules making it less likely to form critical nuclei necessary for crystal growth. This slows everything down. Try my suggestion above and you may speed things up, but generally in crystallisation, the faster you force something to crystallise, the smaller the crystals.