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What is Water and Polarity?

Updated on August 30, 2016
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Below are some notes on water and polarity that can be used for an understanding or refresher that is helpful and useful for the sciences; specifically biology, chemistry, and biochemistry.

The Importance of Water

Water is an important molecule for life. In fact, some would argue that water is life. Or similarly, that without water, there is no life.

All of these arguments are valid and based on truth. I don't go into the nitty-gritty, but here is some information that may be useful with respect to water and polarity.

Key Terms and Ideas

Hydrophilic= “water loving” in Greek; tending to dissolve in water.

Hydrophobic= “water hating” in Greek; does not dissolve in water.

Hydrophobic interactions= interactions between nonpolar molecules.

Hydrophobic bonds= bonds between nonpolar molecules.

Amphipathic=substances or molecules that have both hydrophilic (polar) and hydrophobic (nonpolar) parts.

Hydrocarbons= compounds that contain only carbon and hydrogen; nonpolar.


More on Hydrocarbons

-Hydrocarbons tend to not dissolve in water and so are not favorable for solubility of ionic and polar compounds.

-It is less thermodynamically favorable for water molecules to be involved with nonpolar molecules than it is for them to be involved with other water molecules.


Properties of Water

-Can act as both an acid and a base.

-Is a polar molecule. As a polar solute it can be both a donor or an acceptor of hydrogen bonds meaning that it can form hydrogen bonds with other water molecules or it can be involved with non-specific dipole-dipole interactions.

-Will have favorable ion-dipole and dipole-dipole interactions with ionic and polar compounds; meaning that ionic compounds with full charges and polar compounds with partial charges will dissolve in water.

-Has weak interactions with non-polar molecules; these interactions are weaker than dipole-dipole interactions.

-Each water molecule is involved in four hydrogen bonds. It is a donor in two of the bonds that it forms and an acceptor in the other two bonds.

-The geometric arrangement of hydrogen-bonded water molecules makes a tetrahedral arrangement and the bonds in this arrangement are constantly breaking and forming new bonds in liquid water.

-Hydrogen bonds are weaker than normal covalent bonds and so require less energy to break and form new bonds with.

-Water has a high melting point and boiling point for its molecular size.

-Ice has a lower density than liquid water.

-Liquid water has less hydrogen bonds and is denser than ice.

-Water expands when it freezes.

-Polar compounds that dissolve easily in water are small organic molecules that contain one or more electronegative atom. This includes alcohols, amines, and carboxylic acids.

-Attraction forces between the dipoles of organic molecules (alcohols, amines, carboxylic acids) and water make them tend to dissolve.

-These organic molecules (alcohols, amines, carboxylic acids) are hydrophilic.


Importance of Hydrogen Bonds

-Hydrogen bonds stabilize the three-dimensional structure of biologically important molecules. These molecules include: DNA, RNA, and proteins.

-Hydrogen bonds between complementary bases in DNA are one of the most striking characteristics of DNA's double-helix structure.

-Transfer RNA also has a three-dimensional structure that is defined by its hydrogen-bonded regions.

-Hydrogen bonding gives the alpha-helix and beta-pleated sheet conformations their structures and both alpha and beta conformations are found in proteins.

Bonds Involving Polarity

Polar Bonds = Bonds in which there is a difference in electronegativity between two bonded atoms such that one of the atoms has a partial positive charge and one of the atoms.

Non-Polar Bonds = Bonds in which there is little to no difference in electronegativity between two bonded atoms such that the sharing of electrons is very nearly equal.


Covalent Bonds (Strong Bonds)

Type of Bond
Energy (kJ Mol ^-1)
Energy (kcal mol ^-1)
O--H
460
110
H--H
416
100
C--H
413
105
Based off of Table 2.3 "Some Bond Energies" in "Biochemistry" by Mary K. Campbell and Shawn O. Farrel; 7th edition.

Noncovalent Bonds (Weaker Bonds)

Type of Bond
Energy (kJ mol ^-1)
Energy (jcal mol ^-1)
Hydrogen Bond
20
5
Ion-Dipole Interaction
20
5
Hydrophobic Interaction
4-12
1-3
Based off of Table 2.3 "Some Bond Energies" in "Biochemistry" by Mary K. Campbell and Shawn O. Farrel; 7th edition.

Four Types of Noncovalent ("Weak") Interactions among Biomolecules in Aqueous Solvent

Hydrogen Bonds
Ionic Interactions
Hydrophobic Interactions
van der Waals Interactions
Between neutral groups
Attraction
Between polar and nonpolar molecules
Any two atoms in close proximity
Between peptide bonds
Repulsion
--
--
Based off of Table 2-5 in "Biochemistry" by Mary K. Campbell and Shawn O. Farrel, 7th edition. Note: Each of these interactions are weak by themselves but are significant in force when they are together.

Types of Bonds found in Biologically Important Molecules

Where The Bond Takes Place
H bond formed in H20
Bonding of water to other molecules
Important in protein and nucleic acid structures

Source Information

The information used for this hub was taken from the following sources:

"Biochemistry" by Mary K. Campbell and Shawn O. Farrel; 7th edition.

My biochemistry lectures at school.

Knowledge and notes taken from previous courses in chemistry and biology.

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