1. Nonpolar Molecules:
* Hydrocarbons: Chains or rings of carbon and hydrogen atoms (e.g., methane, propane, oil, fats). These molecules have very little difference in electronegativity between their atoms, making them nonpolar.
* Aromatic Compounds: Cyclic hydrocarbons with alternating double bonds (e.g., benzene, naphthalene).
* Gases: Most gases like nitrogen, oxygen, and carbon dioxide are nonpolar and insoluble in water.
2. Large Molecules with Mostly Nonpolar Groups:
* Waxes: Long chains of fatty acids linked to long-chain alcohols.
* Steroids: Complex ring structures like cholesterol and hormones.
* Proteins: While proteins have polar regions, they can also have large nonpolar sections that make them insoluble in water.
3. Molecules with Strong Intermolecular Forces that Overpower Water's Attraction:
* Some Salts: While many salts dissolve in water, some, like silver chloride (AgCl) and barium sulfate (BaSO4), have very strong ionic bonds that prevent them from dissolving.
Key Principles:
* "Like dissolves like": Polar molecules dissolve well in polar solvents like water, while nonpolar molecules dissolve well in nonpolar solvents like oil.
* Hydrogen Bonding: Water's strong hydrogen bonding makes it a good solvent for polar molecules that can also form hydrogen bonds.
* Hydrophobic Interactions: Nonpolar molecules tend to clump together in water to minimize their contact with the polar water molecules, leading to their insolubility.
Important Note: The concept of "insoluble" is relative. Even seemingly "insoluble" substances can dissolve in water to a tiny extent. It's more accurate to say that they are *poorly soluble* or have very low solubility.
