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Author Topic: Why does more sugar dissolve in hot water, but less oxygen?  (Read 5195 times)

Offline chris

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Animals can grow to enormous sizes in the Antarctica because the water there contains much higher dissolved oxygen concentrations.

So why does lots of oxygen dissolve in cold water, but other solutes - such as sugar - dissolve better when the water is hotter?



 

Offline lightarrow

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Why does more sugar dissolve in hot water, but less oxygen?
« Reply #1 on: 29/11/2007 08:53:32 »
Animals can grow to enormous sizes in the Antarctica because the water there contains much higher dissolved oxygen concentrations.

So why does lots of oxygen dissolve in cold water, but other solutes - such as sugar - dissolve better when the water is hotter?

ΔG = ΔH - TΔS

For O2 dissolving in water ΔS < 0
For sugar dissolving in water ΔS > 0

So, in the first case, increasing the temperature increases ΔG so the reaction is less spontaneous; in the second case is the opposite.
 

Offline Bored chemist

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Why does more sugar dissolve in hot water, but less oxygen?
« Reply #2 on: 29/11/2007 19:56:04 »
The reason for the differing sign of the entropy change ΔS is that a gas coming out of solution is totally disorganised whereas sugar coming out of solution forms neat, organised crystals. Entropy is a measure of the disorder of things.
There's a potential problem with this explanation. Some crystaline solids are more soluble in cold water than hot. The sulphates of the lanthanides are the example usually given. The reason here is that the lanthanide ionsin solution are surrounded by tightly bound water molecules. This ordering is enough to offset the effect of the ordering of the crystal.
 

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Why does more sugar dissolve in hot water, but less oxygen?
« Reply #2 on: 29/11/2007 19:56:04 »

 

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