Overview
This lecture explains how polarity determines solubility in water, using examples of salt and oil, and introduces key terms like hydrophilic and hydrophobic.
Polarity and Molecules
- Polarity refers to the presence of charge or partial charges in a molecule.
- Polar molecules have partial positive and negative charges; nonpolar molecules have no charge.
- Water (H₂O) is a polar molecule and a dipole, with partial positive charges on hydrogen and partial negative on oxygen.
Solubility and Water as a Solvent
- Solubility is the ability of a substance (solute) to dissolve in a solvent.
- Water is a key biological solvent due to its polarity.
Polarity’s Effect on Solubility
- Polar substances (like salt) dissolve in water because their charged parts interact with water’s partial charges.
- Sodium chloride (salt) dissociates in water: Na⁺ interacts with water’s negative side; Cl⁻ with water’s positive side.
- This process allows salt to dissolve and spread throughout the water.
Hydrophilic and Hydrophobic Interactions
- Hydrophilic ("water-loving") substances are polar and dissolve in water (e.g., salt).
- Nonpolar substances (like oil) do not dissolve in water because they lack charge and cannot interact with water molecules.
- Water molecules form hydrogen bonds with each other, pushing nonpolar molecules away (hydrophobic effect).
- Oil is less dense than water, so it forms a separate layer on top.
Examples
- Polar, water-soluble: sugars, amino acids, salts (like sodium chloride).
- Nonpolar, water-insoluble: oils, fats, and some polysaccharides (like starch).
- Some carbohydrates (e.g., alpha glucose) are soluble, but others (like starch/amylose) are not.
Key Terms & Definitions
- Polarity — Presence of partial electrical charges in a molecule.
- Polar Molecule — Molecule with partial positive and negative charges.
- Nonpolar Molecule — Molecule with no electrical charge.
- Dipole — Molecule with both positive and negative poles (e.g., water).
- Solubility — Ability of a substance to dissolve in a solvent.
- Hydrophilic — Water-loving; describes polar substances that dissolve in water.
- Hydrophobic — Water-fearing; describes nonpolar substances that do not dissolve in water.
Action Items / Next Steps
- Review previous videos on hydrogen bonding and polysaccharides for additional context.
- Be prepared to explain the difference between hydrophilic and hydrophobic substances with examples.