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Why water can't mix with oil or cooking oil (both saturated and unsaturated)?

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2 Answers 2

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The basic answer is that water molecules attract each other and so clump together forcing almost all of the oil out of the clump.

The attraction exists because water is a polar molecule. That is, it has a positive end and an negative end. Thus the molecules tend to clump together just as a bag of small magnets tend to clump together. Oils, on the other hand, are not polar.

This principle can be expanded. In general polar materials tend not to dissolve in oils or, if they are liquids, dissolve in them. The rule in chemistry is "like dissolves like".

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The textbook answer for why water can't mix with oil is to use the rule 'like dissolves like'. But a rule in science is none other than summarizing a few collected facts. Answering a question with a rule is like pointing a single fact - why water can't mix with oil - to a collection of facts instead. This easily invites a follow-up question, why like dissolves like. Besides, this rule is vague and superficial; it does not address the facts that

  • ice is not dissolved in the same matter, i.e., water at 0°C,
  • cooking oil has a polarity comparable to that of water (as measured in Debye units),
  • oil molecules do not clump together as strong as water, then how could they prevent water molecules, which are attracted more to oil than oil itself, from coming in? or
  • stuff like calcium fluoride is very polar but insoluble.

So, IMO, it's better explained in terms of energy. When water clumps with water and oil does the same, the total energy is lower than when they are mixed. The system is more stable if unmixed. But watch for a delicate point in this argument. Energy can be lowered when water molecule is moved into oil molecules. How do we account for it? The lowered energy is not enough to compensate the energy needed to pull that water molecule out of its pool.

However, the difference in energy is not the only factor of insolubility. Temperature plays a role too. When temperature is raised, the difference between dissolved and undissolved states becomes less steep, relatively speaking. When thermal energy chips in, the two substances are more miscible.

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From a thermodynamic perspective, saying that a water/oil mixture is more stable when unmixed is going against the fact that this state has lower entropy. Perhaps if you focused on explaining why the total energy is lower it could improve your answer. –  Leonardo Dec 10 '12 at 6:56
    
Entropy changes little in liquid mixing except at both ends where concentrations are very small. The entropy plot against concentration is shaped like an inverted U. –  Chin Yeh Dec 10 '12 at 15:57
    
I mean to say that when attempting to explain why like dissolves like, it requires us to quantify something that is somewhat difficult to quantify. I feel like the answer you have given brings more questions while both give the same answer. –  Leonardo Dec 11 '12 at 1:56
    
I agree with Leonardo. This simple question is hard to crack. Comparing with 'like dissolves like', the approach from energies is more understandable but so far has no quantitative result to show. Perhaps Paul can enlighten us on it. –  Chin Yeh Dec 11 '12 at 17:07
    
@Leonardo While the facts may perhaps be in dispute, I think that Chin's answer does contribute additional/complementary information that may turn out to be useful to the OP. –  jonsca Dec 11 '12 at 23:05

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