1.Ionic compounds such as sodium chloride or other water-soluble salts.
1. Ionic compounds such as sodium chloride or other water-soluble salts.Table salt (NaCl) is a crystal, held together by strong ionic bonds. An ionic bond is formed from the electrostatic attraction between oppositely charged ions. In ions, electrons of one atom are transferred to another atom. The atom that loses the electrons becomes a positively charged ion, or a cation, while the one that gains electrons becomes a negatively charged ion, or an anion.
When we try to dissolve an ionic compound by stirring it in water, the positive ends of the water molecules are attracted to the anions while their negative ends are attracted to the cations, so the polar water molecules "pull" at the substance. As a result of this interaction, the ionic bonds weaken and eventually break and ions become surrounded by water molecules. This process is called "hydration." The hydration of its ions causes a salt to break apart (dissolve) in the water. Therefore, when table salt (NaCl) is dissolved, two ions appear in the water: a positively cation Na+ and a negative charged anion Cl-.
As a result of hydration, a number of water molecules become strongly associated with the charged particles, creating watery shells around them. These watery shells prevent the ions from getting together again into a solid. Hydrated ions jostles around the solution, dispersing evenly in the solution and causing the particles to disappear from view as they are spreading within the solution.
Different ions can attract different number of water molecules into the "watery shell" because they can have different strength (density) of their charge. The density of the charge depends on the ratio of charge to surface area of an ion; the larger the ration, the larger the hydration number will be.[There are limits to what can enter solution. Eventually particles can be stripped off a solid crystal only as fast as they attach to another crystal. This is called equilibrium, and it has much in common with the equilibium reached by diffusing molecules. In both cases, equilibrium means that about the same activity is going on everywhere.].
2. Polar substances (e.g. sugar, alcohol or phenol)
Compared to ions, polar molecules have weaker charges, which are distributed throughout a molecule. It happens when atoms are linked with partially polarized covalent bonds (a shared pair of electrons gets "shifted" toward one of the atoms). For example, an oxygen atom on the OH group, has a localized (and strong) negative charge. The hydrogen, from which an electron was shifted to Oxygen, becomes positively charged. Such polar groups are typical for ethanol [See pix] C2H5-OH.As a result, ethanol is dissolved in water in any proportion. (It isn't even clear which dissolved which!). Sugars, like glucose or fructose, have several OH groups and therefore several localized charges on every molecule; they are therefore attracted to each other very strongly, even looking like crystals. In water, sugar dissolves as easily as table salt because of those multiple localized charges.
3. Partially charged large molecules such as fatty acids and phospholipids Things gets tricky when large organic molecules such as fatty acids have a highly polar group of atoms at their head and long, neutral hydrophobic (water-fearing) tails. So part of this molecule will be welcomed by water molecules that attempt to make a hydrated shell around them. But the long "water-fearing tail" of the molecule will be pulled away from the water. As the result of this dual action of water, such molecules will create a film on the water surface with their polar heads attracted into the water and hydrophobic tails sticking out as a microscopic lawn. (If you mow such a lawn, all the polar heads will sink in the water, as the compound will become water-soluble!). If such molecular system is well shaken, for a short period of time these long molecules may organize themselves into tiny spheres or gathering their "water-fearing tails" toward each other and "sticking" the polar charged "heads" out, toward polar water molecules. Such a process may have been important in the formation of early cells in the prehistoric ocean. Modern cells have in their membranes a two-layer phospholipids structure, called the lipid bilayer, with the polar heads facing out interacting with water, and the ""water-fearing tails" driven pointing toward one another [pic. xx]
4. Completely neutral substances, such as animal fat.
Substances such as animal fat or candle's paraffin do not dissolve in water. They are electrically neutral We call them "hydrophobic" because polar water molecules, being attracted to each other, push the neutral molecules away, usually to the surface.