Need help with my chemistry report

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Vegitto

Diamond Member
May 3, 2005
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Okay, so, last week, we had to do a bunch of little experiments with water and soap, to understand 'micels' better. For some reason, I can't understand (or actually, explain) two of the most simplest experiments. Can anyone help, please? :)

These are the experiments:

1:
Take a large petri dish and shake some fine ground pepper in. Add a drop of soap, and the pepper forms a star-like body, moving away from the soap. Explain why, and how.

2:
Take a large, clear glass, and add a layer of tap water. Carefully add an equal layer of oil. Stir vigorously. The water and the oil will not mix. Add soap, and mix again. The oil and the water will mix, now. Explain.

EDIT: I've already Google d, there are plenty of sites telling me WHAT will happen, but I can't find any that tell me WHY it happens.
 
P

PurdueRy

Lifer
Nov 12, 2004
13,837
4
0
Originally posted by: Vegitto
Okay, so, last week, we had to do a bunch of little experiments with water and soap, to understand 'micels' better. For some reason, I can't understand (or actually, explain) two of the most simplest experiments. Can anyone help, please? :)

These are the experiments:

1:
Take a large petri dish and shake some fine ground pepper in. Add a drop of soap, and the pepper forms a star-like body, moving away from the soap. Explain why, and how.

2:
Take a large, clear glass, and add a layer of tap water. Carefully add an equal layer of oil. Stir vigorously. The water and the oil will not mix. Add soap, and mix again. The oil and the water will mix, now. Explain.
Click to expand...

for #2 soap is water soluable so it mixes with the water and makes the water less dense. The water and oil are then close enough in density to mix.

I am not sure if that's the correct answer but it makes sense in my head ;)
 
V

Vegitto

Diamond Member
May 3, 2005
5,234
1
0
Originally posted by: PurdueRy
Originally posted by: Vegitto
Okay, so, last week, we had to do a bunch of little experiments with water and soap, to understand 'micels' better. For some reason, I can't understand (or actually, explain) two of the most simplest experiments. Can anyone help, please? :)

These are the experiments:

1:
Take a large petri dish and shake some fine ground pepper in. Add a drop of soap, and the pepper forms a star-like body, moving away from the soap. Explain why, and how.

2:
Take a large, clear glass, and add a layer of tap water. Carefully add an equal layer of oil. Stir vigorously. The water and the oil will not mix. Add soap, and mix again. The oil and the water will mix, now. Explain.
Click to expand...

for #2 soap is water soluable so it mixes with the water and makes the water less dense. The water and oil are then close enough in density to mix.

I am not sure if that's the correct answer but it makes sense in my head ;)
Click to expand...

It makes sense to me, too :). Thanks, PurdueRy! Anyone know the answer to the first one? :)
 
T

TheChort

Diamond Member
May 20, 2003
4,203
0
76
PurdueRy's answer isn't quite correct
Soap has a special property that neither water or oil has.
While oil is only non-polar (hydrophobic), and water is only polar (obviously hydrophilic) they can't mix with each other. But an individual particle of soap has a polar head and non-polar tail. So soap can make micelles (micro-bubbles) with a polar outside and a non-polar inside. This allows the oil to fit inside the micele, and the micele itself goes inside the water.
Badabing! Badaboom! Water and Oil mix! :D

EDIT: btw, alcohol and water don't have the same density, but they still mix. In fact, alcohol is LESS dense than oil
 
V

Vegitto

Diamond Member
May 3, 2005
5,234
1
0
I think I've got it. The micels have 'heads' with a positive charge. Pepper, if I recall correctly, is also positively charged. Is this correct?
 
V

Vegitto

Diamond Member
May 3, 2005
5,234
1
0
Originally posted by: TheChort
PurdueRy's answer isn't quite correct
Soap has a special property that neither water or oil has.
While oil is only non-polar (hydrophobic), and water is only polar (obviously hydrophilic) they can't mix with each other. But an individual particle of soap has a polar head and non-polar tail. So soap can make micelles (micro-bubbles) with a polar outside and a non-polar inside. This allows the oil to fit inside the micele, and the micele itself goes inside the water.
Badabing! Badaboom! Water and Oil mix! :D

EDIT: btw, alcohol and water don't have the same density, but they still mix. In fact, alcohol is LESS dense than oil
Click to expand...

:heart:
 
P

PurdueRy

Lifer
Nov 12, 2004
13,837
4
0
Originally posted by: TheChort
PurdueRy's answer isn't quite correct
Soap has a special property that neither water or oil has.
While oil is only non-polar (hydrophobic), and water is only polar (obviously hydrophilic) they can't mix with each other. But an individual particle of soap has a polar head and non-polar tail. So soap can make micelles (micro-bubbles) with a polar outside and a non-polar inside. This allows the oil to fit inside the micele, and the micele itself goes inside the water.
Badabing! Badaboom! Water and Oil mix! :D

EDIT: btw, alcohol and water don't have the same density, but they still mix. In fact, alcohol is LESS dense than oil
Click to expand...

:Q Good to know :)
 
T

TheChort

Diamond Member
May 20, 2003
4,203
0
76
I've been trying to find some info on the composition of pepper, and nothing helpful is coming up
If you can link me to one the sites you found describing the pepper experiment, maybe I'll be able to understand it based on that
 
T

TheChort

Diamond Member
May 20, 2003
4,203
0
76
OK, I found the answer
but for now, my clue to you is that pepper has nothing to do with the answer. The same results would be obtained with flour, or anything else floating on the water.
The key is the surface tension of the water, and what the soap/detergent/oil will do to that surface tension
 
T

TheChort

Diamond Member
May 20, 2003
4,203
0
76
It DOES break the surface tension, but it DOESN'T matter if it makes a ripple or not
Think of surface tension like a million rubber bands pulling the pepper in every single direction at the same time. That's why, at first, the pepper stays in place. But when you add the soap ... well basically you're slingshotting the pepper in the other direction ;)

short answer
longer version
 
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