Module 7: Water and Solubility

What is the Lewis structure of water (H₂O)?

• Central atom: Oxygen

• Oxygen forms two single bonds with two hydrogens

• Oxygen has two lone pairs
  Structure:
  H — O — H
  (with two lone pairs on O)

How do you use a Lewis structure to predict molecular shape?

• Count bonded atoms + lone pairs on the central atom.

• Use VSEPR theory: electron groups repel each other.

• Example: H₂O has 2 bonds + 2 lone pairs → bent shape.

What is electronegativity?

The ability of an atom to attract shared electrons in a chemical bond.

What makes a bond polar?

A difference in electronegativity between two atoms causes uneven electron sharing, creating partial charges (δ⁺ and δ⁻).

How do you use electronegativity values to decide if a bond is polar?

Check the electronegativity difference:

• Small difference → nonpolar

• Larger difference (≥ ~0.5) → polar

• Very large difference → ionic

What is an ion?

A molecule or atom with a net charge because the number of electrons ≠ number of protons.

How do you find the number of electrons in an ion?

• Start with electrons of the neutral atom/molecule.

• Add electrons equal to the negative charge.

• Subtract electrons equal to the positive charge.
  Example: NO₃⁻ has 32 protons total; charge –1 → 1 extra electron.

Nitrate

NO₃⁻

Nitrite

NO₂⁻

What are nitrate and nitrite?

• Nitrate: NO₃⁻

• Nitrite: NO₂⁻
  Common ions in soils and water, usually formed through bacterial processes in the nitrogen cycle.

Solvent

substance doing the dissolving (often water).

Solute

substance being dissolved.

Aqueous solution

solute dissolved in water.

Homogeneous solution

uniform composition.

Heterogeneous mixture

uneven composition; visible phases.

How do you predict solubility using “like dissolves like”?

Polar solvents dissolve polar or ionic compounds.

Nonpolar solvents dissolve nonpolar substances.

Why can contaminants move quickly into groundwater in Winona’s blufflands?

The limestone bedrock is porous and karst, allowing water and pollutants to travel rapidly from the surface into groundwater.