Oil is primarily a mixture of hydrocarbons organic compounds composed of only carbon and hydrogen atoms. Because of its composition, oil does not dissolve in water.
As a result, much of the Gulf of Mexico was contaminated, as was a great deal of shoreline in the affected area. Nonpolar compounds do not dissolve in water. The attractive forces that operate between the particles in a nonpolar compound are weak dispersion forces. However, the nonpolar molecules are more attracted to themselves than they are to the polar water molecules. When a nonpolar liquid such as oil is mixed with water, two separate layers form because the liquids will not dissolve into each other Figure below.
When another polar liquid such as ethanol is mixed with water, they completely blend and dissolve into one another. Liquids that dissolve in one another in all proportions are said to be miscible. Liquids that do not dissolve in one another are called immiscible. In this process, water-water and ethanol-ethanol attractions are broken and ethanol-water attractions are formed.
The attractions that form between the ethanol and water molecules are also hydrogen bonds Figure below. Because the attractions between the particles are so similar, the freedom of movement of the ethanol molecules in the water solution is about the same as their freedom of movement in the pure ethanol. The same can be said for the water.
Because of this freedom of movement, both liquids will spread out to fill the total volume of the combined liquids. In this way, they will shift to the most probable, most dispersed state available, the state of being completely mixed. There are many more possible arrangements for this system when the ethanol and water molecules are dispersed throughout a solution than when they are restricted to separate layers. Figure below. We can now explain why automobile radiator coolants dissolve in water.
These substances mix easily with water for the same reason that ethanol mixes easily with water. The attractions broken on mixing are hydrogen bonds, and the attractions formed are also hydrogen bonds. There is no reason why the particles of each liquid cannot move somewhat freely from one liquid to another, and so they shift toward the most probable most dispersed , mixed state. We have a different situation when we try to mix hexane, C 6 H 14 , and water. If we add hexane to water, the hexane will float on the top of the water with no apparent mixing.
The reasons why hexane and water do not mix are complex, but the following gives you a glimpse at why hexane is insoluble in water. There actually is a very slight mixing of hexane and water molecules. The natural tendency toward dispersal does lead some hexane molecules to move into the water and some water molecules to move into the hexane.
When a hexane molecule moves into the water, London forces between hexane molecules and hydrogen bonds between water molecules are broken. New attractions between hexane and water molecules do form, but because the new attractions are very different from the attractions that are broken, they introduce significant changes in the structure of the water.
It is believed that the water molecules adjust to compensate for the loss of some hydrogen bonds and the formation of the weaker hexane-water attractions by forming new hydrogen bonds and acquiring a new arrangement. Overall, the attractions in the system after hexane and other hydrocarbon molecules move into the water are approximately equivalent in strength to the attractions in the separate substances.
For this reason, little energy is absorbed or evolved when a small amount of a hydrocarbon is dissolved in water. To explain why only very small amounts of hydrocarbons such as hexane dissolve in water, therefore, we must look at the change in the entropy of the system.
It is not obvious, but when hexane molecules move into the water layer, the particles in the new arrangement created are actually less dispersed lower entropy than the separate liquids. The natural tendency toward greater dispersal favors the separate hexane and water and keeps them from mixing.
This helps explain why gasoline and water do not mix. Gasoline is a mixture of hydrocarbons, including hexane. Gasoline and water do not mix because the nonpolar hydrocarbon molecules would disrupt the water in such a way as to produce a structure that was actually lower entropy ; therefore, the mixture is less likely to exist than the separate liquids. We can apply what we know about the mixing of ethanol and water to the mixing of two hydrocarbons, such as hexane, C 6 H 14 , and pentane, C 5 H When the nonpolar pentane molecules move into the nonpolar hexane, London forces are disrupted between the hexane molecules, but new London forces are formed between hexane and pentane molecules.
Because the molecules are so similar, the structure of the solution and the strengths of the attractions between the particles are very similar to the structure and attractions found in the separate liquids. When these properties are not significantly different in the solution than in the separate liquids, we can assume that the solution has higher entropy than the separate liquids.
Molecular compounds are further divided into polar and non-polar. Metals are generally not very soluble in the common solvents but non-metals, molecular compounds and ionic compounds are all soluble in at least one common solvent. Liquid solvents are normally either polar or non-polar but, at high temperatures, liquid salts and metals can act as solvents.
We will limit ourselves to polar or non-polar solvents, marked in red in the above table, as they are the most common solvents. Dipoles Molecules are made of atoms that have bonded together in set patterns. Depending on how the atoms are arranged in the molecule, the molecule itself can sometimes have different charges on each end of the molecule thus forming a dipole. Such molecules are said to be "polar", while molecules with no or little charge separation are called "non-polar" molecules.
Polar molecules are aggressively attracted to other polar molecules, or even free ions, and form strong bonds between themselves. They feel little attraction to non-polar molecules and tend to ignore them in their rush to find other polar molecules or ions. Non-polar molecules tend to group together, because they are pushed out of the way by polar molecules, forming weak bonds between themselves. In this way, polar molecules aggressively seek each other out, excluding non-polar molecules from mixing with them and forming a solution.
While non-polar molecules form much weaker attractions for each other, they will mix and form solutions.
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