Solutions

The Water Molecule: Key Points

Water Molecule Structure
- H2O is a tri-atomic molecule.
- Each O-H bond is highly polar due to oxygen's high electronegativity.
- Bond angle is 105°.
- The polarities of the O-H bonds do not cancel out, so water is a polar molecule.
Charge Distribution
- Symbol δ (delta) indicates partial charges:
- δ- for partial negative charge (oxygen).
- δ+ for partial positive charge (hydrogens).
- Water is considered polar due to the difference in charge distribution.
Hydrogen Bonding
- Water molecules attract each other through dipole interactions (hydrogen bonding).
- Key properties resulting from hydrogen bonding include:
- High surface tension
- Low vapor pressure

Concept
- Liquid water behaves as if it has a "skin" due to hydrogen bonding.
- For example, water can bulge over the edge of a glass.
- Water forms beads when sprayed on greasy surfaces.
- Insects can walk on water due to surface tension.
Factors Affecting Surface Tension
- Surface tension results from hydrogen bonding between water molecules.
- Interactions also occur with other polar substances, like glass.
Meniscus Formation
- Water curves up along the sides of glass creating a meniscus in graduated cylinders.
- Plastics, being non-polar, do not attract water, so no meniscus forms.
Decreasing Surface Tension
- Surface tension can be decreased by adding surfactants (surface-active agents) such as soaps and detergents.

Low Vapor Pressure Explained
- Hydrogen bonding keeps water molecules held together, preventing easy evaporation.
- Lakes and oceans would evaporate rapidly without this property.

Density Changes
- Most liquids become denser as they cool, but water expands and becomes less dense when it freezes.
- Ice floats due to its lower density (10% less) compared to liquid water.
- Water reaches maximum density at 4°C before it starts to expand when cooled further.
Structure of Ice
- Ice forms a honeycomb structure due to hydrogen bonding, which is less dense than liquid water.
- This structure acts as an insulator on water bodies in winter.
Energy Involvement in Phase Changes
- Melting water requires significant energy (334 J for 1 g) to convert from solid to liquid.

Definitions
- A Solution is a homogenous mixture of solute and solvent.
- A Solvent is the dissolving medium, and a Solute is the dissolved substance.
- Aqueous solutions use water as the solvent; particle size is less than 1 nm.
Concentration
- Concentrated solutions contain a lot of solute.
- Dilute solutions have little solute.
Dissolving Process
- Requires agitation, surface area, and temperature (higher temperature often increases the solubility of solids).

Suspensions versus Colloids:
- Suspensions separate on standing; particles > 1000 nm can be filtered.
- Colloids have particle sizes between 1-1000 nm; they scatter light (Tyndall effect) but do not settle.
Solutions vs. Colloids:
- Solutions: particles 0.1-1 nm, homogenous, no light scattering.
- Colloids: 1-1000 nm, exhibits Tyndall effect.

Key Colligative Properties: Depend on the number of dissolved particles, not their nature:
- Vapor Pressure Lowering: Addition of solute reduces the surface area available for evaporation.
- Boiling Point Elevation: Requires higher temperature for boiling due to lower vapor pressure.
- Freezing Point Depression: Dissolved solutes disrupt the formation of orderly patterns in solids, thus lowering the freezing point.

Electrolytes: Conduct electricity in solution due to the presence of ions (e.g., NaCl dissociating into Na+ and Cl-).
- Strong electrolytes dissociate nearly 100% into ions; weak electrolytes do so only partially.
Nonelectrolytes: Do not conduct electricity in solution (e.g., molecular compounds like methanol).