Water's ability to form hydrogen bonds, due to its polar nature, is the primary reason it excels as a solvent, particularly for other polar molecules and ionic compounds.
Here's a breakdown of why this is the case:
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Polarity of Water: Water molecules (H₂O) are polar because oxygen is more electronegative than hydrogen. This means that oxygen attracts electrons more strongly, resulting in a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms.
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Hydrogen Bond Formation: The partial positive charge on the hydrogen atoms of one water molecule can form a hydrogen bond with the partial negative charge on the oxygen atom of another water molecule. This intermolecular force is relatively strong compared to other intermolecular forces like van der Waals forces.
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Solvation Process: When an ionic compound (like NaCl) or a polar molecule (like ethanol) is introduced into water, the following happens:
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Ions: Water molecules surround the individual ions (Na+ and Cl- in the case of NaCl). The partially negative oxygen atoms of water are attracted to the positive Na+ ions, while the partially positive hydrogen atoms are attracted to the negative Cl- ions. This process is called hydration. The energy released during hydration can overcome the ionic bonds holding the NaCl crystal together, causing it to dissolve.
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Polar Molecules: Polar molecules, such as ethanol (CH₃CH₂OH), have regions with partial charges. These regions can form hydrogen bonds with water molecules. The more hydrogen bonds a solute can form with water, the more soluble it typically is. This interaction disrupts the solute-solute interactions and allows the solute molecules to disperse amongst the water molecules.
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"Like Dissolves Like": Water, being polar, is a good solvent for other polar and ionic substances. Nonpolar substances, which lack significant partial charges and cannot form hydrogen bonds with water, do not dissolve well in water. Think of trying to mix oil (nonpolar) and water – they separate.
In summary, water's polarity and ability to form hydrogen bonds allow it to effectively surround and separate ions and polar molecules, disrupting their intermolecular attractions and enabling them to dissolve.
