At the end of Wizard of Oz, Dorothy chucks a bucket of water on the wicked Witch of the West, who proclaims that she is "melting" before disappearing into a puddle on the floor. But "melting" is a heating process, so unless the witch's natural body temperature is below room temperature, it is much more likely that she is, in fact, dissolving--which let's face it is a far less dramatic thing to yell at a group of strangers who have just defeated you with water.
"I'm dissolving! I'm dissolving!"
But why would she dissolve? Why are some things water soluble, and others aren't? Simply put, "like dissolves like," and in this case the "likeness" has to do with the polarity of the solids (solutes) and liquids (solvents) at play. Polarity, in turn, is partially determined by the electronegativity of the elements in the molecules.
If something is highly "electronegative" it means it has extra non-bound electrons. If something is less electronegative it means it doesn't have these electrons. Each element is rated on a scale of 0 to 4, with 4 being very electronegative and zero being not electronegative.
As you move from the bottom left towards the top right of the periodic table, electronegativity increases. This means that chlorine, for instance, has a higher electronegativity than iodine or sodium; iodine is closer to the bottom of the periodic table than chlorine, while sodium is farther to the left.
Carbon (2.55) and hydrogen (2.20) are in the middle of this range, and are considered neither electro- positive or negative, while oxygen (3.44) is said to be electronegative.
Polarity is determined by looking at a molecule's shape, electronegative parts, and distribution of electronegativity. For starters, let's take a look at water. The molecule is not a straight line, but is instead slightly bent. This is because the oxygen in the center has four un-bonded electrons on top of it which take up space. This causes the two bonded hydrogens to be pushed downwards, creating a shallow V shape.
If we were to draw two parallel lines, and label one positive and one negative, we could then draw our water molecule in the center, and align it such that the negatives (oxygen) were pointing toward the negative "pole," and the positives (hydrogens) were pointing toward the positive "pole." Because the molecule can be aligned thus between our two "poles" water is called a polar molecule.
However, unlike water's bent V shape, carbon dioxide, which contains two electronegative oxygens and one neutral carbon, is drawn in a straight line.
Even though the molecule has electronegative parts, it cannot be drawn to align with the poles because the electronegative parts are on opposite sides of the molecule. It is therefore called a non-polar molecule.
Polar solvents (like acetone) can dissolve polar solutes (like nail polish).
Non-polar solvents (like pentane) can dissolve non-polar solutes (like oils).
But generally speaking, polar solvents can't dissolve non-polar solutes, and non-polar solvents can't dissolve polar solutes. This is why you cannot easily wash oils off of your hands using only water--the oils are non-polar while the water is polar.
Based on polarity alone, we'd have to assume that if the Wicked Witch of the West easily dissolves in water she was probably made of a polar substance, like table sugar...or maybe salt. She was anything but sweet.
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