Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
Density question?
Can someone please explain what happens when Say an oil has about the same density of H2O? Say +/- whithin its density and what occures?
I'm currious as to if what is happening is surface tension because they are similar in density? Just speculating. But there is oil below the water and
oil pooling at the top, its thick enough that you can paint in it and swirl it around. Very fascinating.
|
|
|
Artemus Gordon
Hazard to Others
Posts: 178
Registered: 1-8-2013
Member Is Offline
Mood: No Mood
|
|
If the oil and water had EXACTLY the same density, you would have blobs of oil all through the water (or vice versa if you have more oil and less
water). But it is far more likely there is a slight density difference, so they would separate into two layers, with the least dense on the top.
This is the way a lava lamp works. Colored wax is placed in a liquid that is only slightly less dense than the wax. The heat of the lamp makes the wax
a bit less dense so it slowly floats to the top, where it cools and start to sink again.
Or, perhaps you were really wanting to know why oil and water don't mix?
[Edited on 7-10-2014 by Artemus Gordon]
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
what about measuring density? is it as simple as adding a graduated cylinder to a scale, tarring, and then weighing water then oil or vice versa?
Or is there more science to it? Some of the info found online seems to just add way too much crap that takes away from the point and simplifying it.
Now will dissolved solids effect the S.G. just like light fractions in oils will change the overall density of a sample even if there are heavy
fractions contained herein?
|
|
|
Oscilllator
National Hazard
Posts: 659
Registered: 8-10-2012
Location: The aqueous layer
Member Is Offline
Mood: No Mood
|
|
That is a perfectly valid way to measure density, yes. And dissolved solids will affect the density of water or indeed any other solvent - this is why
seawater is more dense than fresh water.
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
Quote: Originally posted by Oscilllator  | | That is a perfectly valid way to measure density, yes. And dissolved solids will affect the density of water or indeed any other solvent - this is why
seawater is more dense than fresh water. |
Thanks Osc.
I suspect that even store bought distilled water can have alot of dissolved solids unless reverse Osmosis which is pretty expensive to do on a
commercial scale. could be wrong?
Would Deionized water be better? and is there infact a good way to calibrate your Cylinder? I know that the top has T.C. (to contain) and T.D (to
deliver) and at 22c. but if density is relevant to T.D.S. than the measurements are not absolute, so how infact to measure accurately?
Thanks.
-Ac-
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
If you want determine density very accurately (4 significant digits for example) you need to use a pycnometer:
https://www.google.co.uk/search?q=pycnometer&sa=X&rl...
But the principle behind pycnometry is the same as your measuring cylinder method.
For measuring the density of solid objects use this little known trick:
http://oxfordchemserve.com/density-of-solid-objects-archimed...
[Edited on 7-10-2014 by blogfast25]
| Quote: Originally posted by Actinium | I suspect that even store bought distilled water can have alot of dissolved solids unless reverse Osmosis which is pretty expensive to do on a
commercial scale. could be wrong?
|
No, store bought distilled water or deionised water contains almost no dissolved solids.
[Edited on 7-10-2014 by blogfast25]
|
|
|
Artemus Gordon
Hazard to Others
Posts: 178
Registered: 1-8-2013
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by Actinium | what about measuring density? is it as simple as adding a graduated cylinder to a scale, tarring, and then weighing water then oil or vice versa?
Or is there more science to it? |
Yes, that will work. Density is simply how much mass a substance has divided by the volume. But a <a
href="http://www.elementalscientific.net/store/scripts/prodView.asp?idproduct=942">volumetric flask</a> gives a more accurate volume than a
grad. cyl. does.
If you need to make many density measurements, you can use a <a
href="http://www.elementalscientific.net/store/scripts/prodView.asp?idproduct=1247">hydrometer</a>. You float it in the liquid and how high
or low it floats gives the density reading.
|
|
|
Artemus Gordon
Hazard to Others
Posts: 178
Registered: 1-8-2013
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by Actinium |
Would Deionized water be better? and is there infact a good way to calibrate your Cylinder? I know that the top has T.C. (to contain) and T.D (to
deliver) and at 22c. but if density is relevant to T.D.S. than the measurements are not absolute, so how infact to measure accurately?
Thanks.
-Ac- |
I always use steam-distilled water rather than DI water, mostly because I am more familiar with the process, but for home lab use I don't really think
it makes any difference. As Blogfast25 said, the dissolved solids are very, very low for both types.
Yes, you can calibrate your volumetric vessels. Place the (clean and dry) vessel on your balance (hopefully 0.01 or 0.001 gram sensitivity, and
properly calibrated itself), and tare. Using a brand-new bottle of distilled or DI water (opened bottles can allow CO2 to be absorbed by
the water), carefully fill to the mark, keeping your eye level to the mark. The meniscus should be at the mark. Note the mass of the added water. Use
a water density vs. temperature table, such as <a
href="http://jupiter.plymouth.edu/~jsduncan/courses/2012_Spring/Techniques/Exams/DensityOfWater-vs-Temp.pdf">this one</a> to calculate the
actual volume of that mass of water. (Obviously, you need to measure the temperature of the water as carefully as possible.)
Volumetric flasks have only 1 mark, so record that volume on the flask and you're done. Graduated cylinders have many marks, and they all could be off
by random amounts, but if you have a good cylinder, the marks should be proportionate to each other. It's kind of a question how anal retentive you
want to be, but you should take measurements at least at 100% 75% 50% and 25% capacity, or as close to those as you can. Then it's a question of, can
you fit the delta of those measurements vs. the declared values marked on the cylinder onto a straight-line graph? If you are lucky, it is a
horizontal line and you only need to add or subtract a constant from your marked value. If it is a tilted line, then your measurement error changes at
each mark but in a way you can calculate. If it is not close to a straight line, maybe you need to buy a better grad. cyl.
[Edited on 7-10-2014 by Artemus Gordon]
|
|
|
Eddygp
National Hazard
Posts: 858
Registered: 31-3-2012
Location: University of York, UK
Member Is Offline
Mood: Organometallic
|
|
Hmmm... what if the solid you are working with is a small block of rubidium?
there may be bugs in gfind
[ˌɛdidʒiˈpiː] IPA pronunciation for my Username |
|
|
chornedsnorkack
National Hazard
Posts: 563
Registered: 16-2-2012
Member Is Offline
Mood: No Mood
|
|
The classical Plateau experiment.
Oil is immiscible in either ethanol or water.
Oil is denser than ethanol, but less dense than water.
Ethanol and water are miscible.
If oil is placed in ethanol, where it sinks, and the ethanol then slowly diluted, then the density of ethanol-water mixture becomes equal to oil. The
surface tension is not zero, and the oil would float as a spherical drop. Of course the drop can be disturbed and broken into pieces; when the drops
meet, they should merge back.
Another classical example for it is water and aniline. The mutual solubility is limited; at 80 degrees 100 g aniline dissolves 7 g water and 100 g
water 4,5 g aniline.
At low temperatures, aniline is slightly denser than water, but has bigger thermal expansion; at 70 degrees, the densities of aniline-rich and
water-rich phase are equal, with nonzero surface tension.
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
thank you guys for all of your impute. Chemistry is such a hard and demanding hobby, very expensive but very rewarding. I appreciate all the knowledge
you are sharing and going to soak some of this in and report back.
What about using a Refractometer for home brewing? I know that its used to measure optical refraction and you can get specific gravity this way. Beer
and wine makers use it all the time to measure there wort content. Would this work for general purpose or does a chemistry specific device need be
used?
I still need/want to learn to do things without the use of such devices but its always nice to just be sure.
-Ac-
|
|
|
Artemus Gordon
Hazard to Others
Posts: 178
Registered: 1-8-2013
Member Is Offline
Mood: No Mood
|
|
I've never used a refractometer, except once in a gemstone identification class I took one time. As I understand it, using the index of refraction to
determine the density of a liquid entails knowing what the liquid is. So winemakers use them to determine the level of sugar in grape juice because
grape juice is the only thing they ever need to study. Also, I think they have a limited range of measurement. In a general chemistry lab, I think
they are probably not too useful.
|
|
|
Actinium
Hazard to Self
Posts: 67
Registered: 2-9-2014
Member Is Offline
Mood: synthetic
|
|
thank you. I'm currently running it under a vacuum at moderate temp to remove the water to get an accurate reading. Now that I understand what a
meniscus is it will make allot more sense to do a Sp. Gravity test.
Next stop, TLC express.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
