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How to determine the boiling point of a solution?





likeabreeze
In my mind, the highest temperature of a solution is the boiling point of it,
So, just boils the solution, and put a thermometer in, and the highest record is what I want.
Is this method accurate?
Bikerman
Not really. BP is the highest temperature of that 'phase' of a material.
Consider water - H2O. The liquid phase is from 0-100, but once you enter a new phase - gaseous - then the steam phase can go much higher until it eventually breaks at a molecular level....
likeabreeze
Bikerman wrote:
Not really. BP is the highest temperature of that 'phase' of a material.
Consider water - H2O. The liquid phase is from 0-100, but once you enter a new phase - gaseous - then the steam phase can go much higher until it eventually breaks at a molecular level....

Yes, you are right.
But in this experiment, the thermometer is put into the solution, the steam dont touch the thermometer, so the result is the temperature of the liquid phase, and that's why I think this method is accurate..
Bikerman
Well, next you have to consider pressure.
Pressure*volume/temp is constant. If you work out what that means, the BP will change with pressure.
likeabreeze
Bikerman wrote:
Well, next you have to consider pressure.
Pressure*volume/temp is constant. If you work out what that means, the BP will change with pressure.

BP changes with pressure, that's right.
Pressure*Volume/Temp=[constant], that's wrong, are u referring to PV=nRT? this equation is only for gas.
mmm, so, you think this BP determination method is accurate if the pressure is considered?
Bikerman
P1V1/T1 = P2V2/T2 for a gas - ie PV/T IS constant.
If the pressure is recorded then yep, you have all the variables covered - pressure temp
likeabreeze
Bikerman wrote:
P1V1/T1 = P2V2/T2 for a gas - ie PV/T IS constant.
If the pressure is recorded then yep, you have all the variables covered - pressure temp

So, here comes the question:
the method mentioned above is not only simple but accurate, why so many people use Thiele tube to do this experiment?
click here to see how this experiment is conducted.
the method showed in the pdf seems to be complex and not that accurate.
your opinion?
Bikerman
The method has another variable that I didn't think about - quantity and distribution of thermal energy 'in'.
The Thiele is designed to produce an even warming of the sample - buson burners over a flask will produce hot-spots and fluctuating convection currently.
likeabreeze
Bikerman wrote:
The method has another variable that I didn't think about - quantity and distribution of thermal energy 'in'.
The Thiele is designed to produce an even warming of the sample - buson burners over a flask will produce hot-spots and fluctuating convection currently.

That sounds reasonable..
many thx
Dennise
Depending on where you live, you can do a simple experiment that proves the above posts. Consider the boiling point of fresh water is 100 deg. C. at sea level and standard atmospheric pressure.

Now take that same water up to a reasonable elevation increase .... say 2,000 meters, then boil the water again. You will see a lower boiling temperate because there is less atmospheric PRESSURE at 2,000 meters. If you can get away with it, try the experiment in a commercial airplane at cruise altitude where the cabin pressure is maintained at an equivalent altitude of about 2,400 meters. There you will measure the boiling point at an even lower temperature.

In space - if your water container were suddenly open to the vacuum of space - the water would boil before you could even get around to measuring it, at a MUCH lower temperature. This is one of an astronaut's worse fears should his space suit be compromised outside his craft.
likeabreeze
Dennise wrote:
Depending on where you live, you can do a simple experiment that proves the above posts. Consider the boiling point of fresh water is 100 deg. C. at sea level and standard atmospheric pressure.
Now take that same water up to a reasonable elevation increase .... say 2,000 meters, then boil the water again. You will see a lower boiling temperate because there is less atmospheric PRESSURE at 2,000 meters. If you can get away with it, try the experiment in a commercial airplane at cruise altitude where the cabin pressure is maintained at an equivalent altitude of about 2,400 meters. There you will measure the boiling point at an even lower temperature.

your method is time-consuming,expensive and inaccurate.
but a good science experiment for kids. Wink
thx for your reply.
Bikerman
It is also the reason why one should never order a cup of tea on a plane - at least not if you know what good tea should taste like. Because of the lower BP, you simply cannot make a good brew since, as any tea drinker will know, the water has to be boiling - ie pretty much smack-on 100C. The best you will manage on a plane at cruising altitude of 20,000ft plus is about 90C.
Dennise
Quote:
your method is time-consuming,expensive and inaccurate.
but a good science experiment for kids. Wink
thx for your reply.


Time-consuming, expensive and inaccurate = good science experiment for kids ????

Please explain the inaccuracy.
Bikerman
Yes, I wondered about that myself. It won't be incredibly accurate unless you have very accurate sensors for pressure, but it will be accurate enough for most purposes. As for being expensive...how much does it cost to climb a hill? Smile
likeabreeze
very accurate sensors for pressure=expensive..
and, the atmospheric pressure isn't stable,actually, which makes it difficult to conduct such an experiment.
I say it's a good science experiment for kids because this experiment is conducted outside of the room, seem to be a lot fun.
THX for your reply.
cabenqc
Once solution boiling is the boiling point at that condition.

Avoid the solution to boil for long period of time.
Once concentration of that solution (if it's a mixture) changed, boiling point will change.

Use water and alcohol mixed solution as an example.
If you keep that solution boiling,
Plot boiling point against time,
you will see boiling point increase according to time after boiling.
jamesparker
I dont't think so, There are special instruments for measuring the boiling point of solutions
lightworker88
likeabreeze wrote:

the method mentioned above is not only simple but accurate, why so many people use Thiele tube to do this experiment?
click here to see how this experiment is conducted.
the method showed in the pdf seems to be complex and not that accurate.
your opinion?


The above document says the vapor will boil at the equilibrium vapour pressure.

Theoretically, if the gaseous pressure is higher than this, wouldn't it return to a liquid state, such as if the container space became smaller?
zarawatsonn
Yes, your method is absolutely right, But remember that your thermometer should be in room temperature before put it in the solution, for accurate results.
pjohnthomas
There is one potential issue with sticking a thermometer in a boiling liquid like water. During the phase change, nearly all available heat is used to convert the liquid to vapour. This means the thermometer will not register the correct boiling point. Hence text books say that you can't calibrate thermometers using boiling points.
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