How does the polar nature of water molecules affect the ability of substances to dissolve in water?

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Water is very polar. Polar substances are covalently bonded substances that contain partially positive and negative charges. The partial charges within a polar substance are the result of electronegativity differences between the atoms that share the bond. Electronegativity is the likelihood that an atom will attract a pair of bonded electrons. Amongst all of the elements on the periodic table, electronegativity ranges between approximately 0.7 to 4.0. Some periodic tables contain electronegativity values of each atom. The electronegativity difference between two atoms forming a bond can be used to determine the type of bond that will be formed between the two atoms, as identified below.

  • Electronegativity differences of less than 0.5 = nonpolar covalent
  • Electronegativity of 0.5 – 1.6 = polar covalent
  • Electronegativity difference of 2.0 or more = ionic bond

Hydrogen has an electronegativity of 2.1 and oxygen has an electronegativity of 3.5. Therefore, the electronegativity difference is 1.4. Thus, water is very polar.

This means that water has partial positive and partial negative charges within each of its molecules. Because the oxygen is more electronegative than hydrogen, the oxygen atoms within water molecules obtain a partial negative charge and the hydrogen atoms obtain partial positive charges.

If another polar substance is placed in water, then the partial charges within that substance will attract the oppositely charged partial charges within water. As a result, water molecules will surround and dissociate the particles of the polar substance. As a result, the polar substance will dissolve in the water.

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Polar and ionic substances dissolve in water due to its polarity. A polar molecule has charge separation such that there are slightly positive and slightly negative regions. This is called a dipole. The two oxygen-hydrogen bonds in a water molelecule are polar because of the electronegativity difference between oxygen and hydrogen. In addition to oxygen attracting more of the bonding electron density, it has two lone electron pairs that contribute to the molecule's polarity. Water is a bent molecule, with the hydrogen ends being slightly positive and the oxygen end being slightly negative (see diagram). 

A polar solute dissolves in water because its positive end is attracted to the negative region of the water molecule and its negative end is attracted to the positive region. The solute becomes evenly dispersed in water as dipole-dipole interactions form.

An ionic solute dissolves in water because the positive ions are attracted to the negative region of the water molecule, and the negative ions are attracted to the positive region. This results in the ions in the crystal lattice separating from each other as ion-dipole attractions form with water molecules.

Non-polar substances don't dissolve well in water because they have a weaker type of intermolecular attraction. Water molecules are more attracted to other water molecules than to non-polar molecules.

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