Lecture on Solutions and Solvent Properties of Water

Definitions

• Solution: Composed of two types of molecules or compounds:
  • Solvent: Substance that dissolves other substances.
  • Solute: Substance that is dissolved.
  • Example: Water (solvent) dissolves salt (NaCl, solute).

Properties of Water as a Solvent

• Universal Solvent: Water can dissolve a wide variety of substances due to:
  • Polarity
  • Hydrogen Bonds

Polarity

• Water molecules are polar due to unequal sharing of electrons in covalent bonds.
  • Oxygen has a greater electronegativity, pulling electrons closer.
  • Results in a slight negative charge on oxygen and a slight positive charge on hydrogens.
• Polarity leads to the formation of hydrogen bonds among water molecules.

Hydrogen Bonds

• Formation: Occurs between fluorine, oxygen, and nitrogen atoms with hydrogen.
• Strength: Individually weak, but collectively strong in large numbers.
• Facilitate the breakdown of solute molecules like NaCl.

Solubility Principles

• "Like Dissolves Like"
  • Polar solvents dissolve polar solutes.
  • Non-polar solvents dissolve non-polar solutes.
  • Water, being polar, effectively dissolves polar substances like NaCl.

Polar vs. Non-Polar

• Polar Dissolving Polar

  • Example: Water dissolves NaCl by forming ions.
  • Oxygen clumps around positive sodium ions, and hydrogen clumps around negative chlorine ions.

• Non-Polar Dissolving Non-Polar

  • Example: Mixing of hydrocarbons like pentane.
  • Bonds in non-polar solutes do not break; they disperse among the solvent.
  • Non-polar solute and solvent mix without breaking bonds.

Summary

• Identify polarity of compounds to predict solubility.
• Polar solute and solvent dissolve each other; same goes for non-polar.
• Mismatch in polarity means substances won't dissolve.

Next Lecture

• Discussion on solution concentrations and equilibrium.

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End of Lecture.