science in simple english

kelly124

can someone explain to me in simple english why the solubility of sodium carbonate decreases with temperature?????

Boeing777

My guess is that sodium carbonate requires a certain temperature to fully dissolve in water. Therefore, as you reduce the temperature it doesn't dissolve as quickly or as easily.
So, if sodium carbonate requires X temperature to dissolve and you keep lowering the temperature it's not going to dissolve very well for you. Does that make sense?

kelly124

thats great thank you but im wondering why does it only reach max solubility at 36 degrees usually as temperture increases so does solubility but this defies that theory

Boeing777

kelly124 wrote: »

thats great thank you but im wondering why does it only reach max solubility at 36 degrees usually as temperture increases so does solubility but this defies that theory

Again, my guess would be that 36 degrees is it's optimum (or most ideal) temperature that it likes to dissolve in so at this temperature it is most happy to dissolve fully. You're right in saying that as temperature increases so does solubility, but in the case of sodium bicarbonate it may be most happy to fully dissolve at 36 degrees whereas something else might only dissolve at 80 or 90 degrees. I really hope I'm not further confusing you. I'm doing Chemistry in college but sometimes the easiest of things are the hardest to explain.

whizbang

Basically, its not just dissolving, Its a chemical reaction, which generates more heat. This creates different forms of the solid, some of these forms are much harder to dissolve.

SOL

whizbang wrote: »

Basically, its not just dissolving, Its a chemical reaction, which generates more heat. This creates different forms of the solid, some of these forms are much harder to dissolve.

Eh? Unless you mean something I don't think you mean what you have said isn't true...



But to answer the question... in a manner which is not entirely accurate but should help you on the way...

consider what happens when a substance dissolves - bonds are broken and made - that is to say the bonds holding the solid together are replaced with bonding between the liquid and the now dissolved solid - in the case of an ionic solid in water the ionic bonds are broken and the ions are solvated by the water.

There will come a point when the level of solvation offered by the water is affected by the presence of other ions - they compete for solvation and ultimately only a limited number or molecules can actually dissolve

This is then controlled by two factors
- the enthalpy of the bonds which are made or destroyed (the difference in the energy of the bonds destroyed or made)

-The entropy of the change in state of the materials

i.e. ΔG = ΔH - TΔS

The entropy contribution is temperature dependent - so depending on whether the solvation is entropically favoured or not will say whether it has some optimum temperature or not...

whizbang

But to answer the question... in a manner which is not entirely accurate bu

Criteria:

in simple english

SOL

touché

- though your explanation even if it is simple doesn't make any sense because that isn't what happens


- I'll try to rephrase mine to make it simpler

SOL

The picture below shows what happens when a molecule is dissolved compared to when it is a solid.




If you think about this - it goes from ionic bonds between the + and - ions to bonds between the water and the ions. - this is an energy change and will govern in part whether it happens or not.

As you can see, at some point there will be so many ions dissolved that there aren't enough water molecules to dissolve more solid ones, if there aren't enough waters to dissolve the ions then they will have to pay the cost of breaking there solid state bonds without any pay back so no more will dissolve.


Now, here is the tricky part. Entropy, I'm going to try and explain this by analogy which probably wont be perfect, and if anyone has any improvements I'd love to hear them...

Imagine a box of sand containing tiny magnets. All the magnets are stuck together. If we were to stir the sand we would break apart the magnets and they would mix in with the sand because if the mixture is moving around then the magnets are going to un stick (if they are weak enough) and the sand will stop them sticking back together. Even though this costs energy, it is going to happen because there are many more ways to arrange the magnets where they are broken up than if they are all stuck together so the process - which is increasing entropy (can be said to be entropically favoured) will happen even though it has to take in energy from the surrounding particles. Now imagine what happens if we increase the speed of the sand particles (or heat up the salt and water) - if the process is increasing entropy it will be even easier - more particles will dissolve.


But - in out model we can see that we go from a solid to a dissolved state that it is possible that our system fill become more ordered because our ions arrange the water molecules around them - so if we heated that up and moved them around faster, they would likely fall apart and revert back to being a solid.


Thus increasing temperature is not always going to increase how well our substance dissolves...


I hope that makes some sense?