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Biology LibreTexts

2.2A: Water’s Polarity

  • Page ID
    12667
  • Water’s polarity is responsible for many of its properties including its attractiveness to other molecules.

    LEARNING OBJECTIVES

    Describe the actions that occur due to water’s polarity

    KEY TAKEAWAYS

    Key Points

    • The difference in electronegativities between oxygen and hydrogen atoms creates partial negative and positive charges, respectively, on the atoms.
    • Water molecules attract or are attracted to other polar molecules.
    • Molecules that do not dissolve in water are known as hydrophobic (water fearing) molecules.

    Key Terms

    • hydrophilic: having an affinity for water; able to absorb, or be wetted by water
    • hydrophobic: lacking an affinity for water; unable to absorb, or be wetted by water
    • polarity: The intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule.

    One of water’s important properties is that it is composed of polar molecules. The two hydrogen atoms and one oxygen atom within water molecules (H2O) form polar covalent bonds. While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing to water’s properties of attraction. Water’s charges are generated because oxygen is more electronegative, or electron loving, than hydrogen. Thus, it is more likely that a shared electron would be found near the oxygen nucleus than the hydrogen nucleus. Since water is a nonlinear, or bent, molecule, the difference in electronegativities between the oxygen and hydrogen atoms generates the partial negative charge near the oxygen and partial positive charges near both hydrogens.

    image

    Nonpolar Molecules: Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound.

    As a result of water’s polarity, each water molecule attracts other water molecules because of the opposite charges between them, forming hydrogen bonds. Water also attracts, or is attracted to, other polar molecules and ions, including many biomolecules, such as sugars, nucleic acids, and some amino acids. A polar substance that interacts readily with or dissolves in water is referred to as hydrophilic (hydro- = “water”; -philic = “loving”). In contrast, nonpolar molecules, such as oils and fats, do not interact well with water, as shown in. These molecules separate from it rather than dissolve in it, as we see in salad dressings containing oil and vinegar (an acidic water solution). These nonpolar compounds are called hydrophobic (hydro- = “water”; -phobic = “fearing”).

    Hydrogen bonds: This interactive shows the interaction of the hydrogen bonds among water molecules.