| The Solvent of Life |
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Water is a good solvent due to its polarity. The partial charge that develops across the water molecule helps make it an excellent solvent. When an ionic or polar compound enters water, the water molecules surround it. Water dissolves many substances by surrounding charged particles and "pulling" them into solution. The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends. For example, common table salt, sodium chloride, is an ionic substance that contains alternating sodium and chlorine ions. When salt is added to water, NaCl, separates into Na+ cations and Cl- anions, each being surrounded by water molecules. The negative oxygen ends of water molecules will surround the positive sodium ions; the positive hydrogen ends will surround the negative chlorine ions. The sphere of water molecules around each dissolved ion is called a hydration shell. Working inward from the surface of the salt crystal, water eventually dissolves all the ions.
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Crystal of table salt dissolved in water. A sphere of water molecules, called a hydration shell, surrounds each solute ion |
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In a similar fashion, any substance that carries a net electrical charge, including both ionic compounds and polar covalent molecules (those that have a dipole), can dissolve in water. Ionic and polar substances such as acids, alcohols, and salts are relatively soluble in water, and non-polar substances such as fats and oils are not. Because there is no net electrical charge across an oil molecule, it is not attracted to water molecules and therefore does not dissolve in water. Non-polar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with non-polar molecules.
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