MadSci Network: Chemistry

Re: Why did all the liquids mix when we poured in rubbing alcohol?

Date: Mon May 14 12:43:07 2001
Posted By: Tracy Cheatham, Faculty, Chemistry, Central Carolina Community college
Area of science: Chemistry
ID: 988064654.Ch

Hi Mindy:

I would like to answer your question in two parts.  First what SHOULD have 
happened, and then what PROBABLY happened.

Everyone has heard the saying "like dissolves like".  It refers to the 
miscibility or mixing ability of liquids.  Liquids can be separated into 
two groups, polar and nonpolar.  Polar liquids like water, kool-aid, and 
milk will completely mix with one another. "like dissolves like".  
Nonpolar liquids like gasoline and oil will mix together, again "like 
dissolves like".  However, polar and nonpolar liquids will not mix.  So if 
you mix oil with water, no matter how hard you shake it or how hard you 
stir it, they will separate and form two distinct layers.

First, what SHOULD have happened:  The polar maple syrup goes right to the 
bottom because it is very heavy, or dense.  When the oil, which is 
nonpolar, and will not mix with maple syrup was added, it became the top 
layer because it is less dense.  Next, the red-dyed water is added.  Water 
is polar, so it won't mix with the oil.  It is more dense than the oil, 
but less dense than the maple syrup, so it should have settled between the 
two.  The layers should now be maple syrup on the bottom, red-dyed water 
above it, and the oil on top.  Now, we add the blue-dyed alcohol.  Alcohol 
is unique because it has both polar and non-polar parts.  Slowly adding 
the alcohol to the top, it should have become the very top layer because 
it has a polar part that doesn't want to mix with the oil and it is less 
dense than the oil, so it should remain on top.  Final view, maple syrup 
on bottom, red-dyed water next, oil next, and on top the blue-dyed alcohol.

Next, what PROBABLY happened.  The first mixings went as planned.  We have 
maple syrup on bottom, red-dyed water next, and oil on top.  If the 
alcohol was introduced in such a way that the oil was mixed with it (ie 
rapidly pouring it in, or shaking the container), the oil could dissolve 
in the alcohol.  The alcohol will mix with the water because they are both 
polar.  We no longer have separation of layers because we dissolved our 
only non-polar liquid into a polar one and now they are all polar and can 
all mix.  The maple syrup probably stayed on the bottom unless it was 
stirred.  Of course when the dyes mixed, a darker color (blue/purple)was 
formed.  Coincidentally, did you notice that you can only dye the polar 
liquids (water and alcohol) and not the nonpolar one (oil)?  This is  
because the dye itself is polar and won't dissolve in a nonpolar liquid.  
Hope this helps!!

Chemistry: The Central Science, Brown, LeMay and Bursten, Prentice Hall, 
1997, pp463-465.

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