Water and pH

              

           Why is it important to study water and its properties?

The answer is simple.

Water accounts for 60% to 90% of the mass of cells making it the most abundant molecule. Macromolecules of the cell (proteins, polysaccharides, nucleic acids and lipids) assume their characteristic shapes and structures in response to water. 

The physical properties of water allow it to act as a universal solvent for polar and ionic substance. The chemical properties allow it to form weak bonds with other molecules including other water molecules.

Structure of Water Molecule

The Water Molecule is Polar

• A water molecule (H2O) is V-shaped and the angle between the two covalent (O—H) bonds is 104.5° due to the strong repulsion exerted by the lone pairs of electrons on the oxygen atom.
• The uneven distribution of charge that occurs within each O—H bond of the water molecule leads to the appearance of a partial negative charge (δ-)on O and a partial positive charge (δ+) on the hydrogen. This uneven distribution of charge within a bond is known as a dipole and the bond is said to be polar.
• The angled arrangement of the polar O—H bonds of water creates a permanent dipole for the molecule. Thus the polarity of a molecule depends both on the polarity of its covalent bonds and its geometry.

•         Not all molecules are polar.Carbon dioxide contains polar covalent bonds but the bonds are oppositely oriented so the polarities cancel each other and the entire molecule has no net dipole, thus it is nonpolar.

Hydrogen Bonding in Water

Attraction forces between the slightly positive hydrogen atoms of one water molecule and the slightly negative electron pairs of the oxygen atom of another water molecule produce a hydrogen bond. 

In effect, the hydrogen atom is being shared (unequally) between the two oxygen atoms. The distance from the hydrogen atom to the acceptor oxygen atom is about twice the length of the covalent bond.



Water molecules are unusual because they can form four aligned hydrogen bonds with up to four other water molecules.  In liquid water, each H₂O molecule forms, on average, 3.4 H-bonds and thus a high amount of energy is needed to break the H-bonds. This is why water has a high boiling point. In ice, each H₂O forms 4 H-bonds causing ice density to decreases as the number of bonds increases. This is why less dense ice floats in liquid water, and why water expands when frozen. In addition the strength of these 4 H-bonds gives ice a unusually high melting point because a large amount of energy will be needed to disrupt the lattice of ice.

The three states of water and the degree of hydrogen bonding between water molecules are explained in a funny way in the figures below. In the liquid sates, each molecules has some space to move and form h-bond with other molecules interchangeably. The more H-bonds  are formed, the harder it becomes to move as there will be less free space (Ice). The less the attraction and bonds, the more free space to move freely (Gas)

Water Is an Excellent Solvent

Water molecules are polar, have low intrinsic viscosity, are small compared to other solvents, and can associate with solute particles and make them more soluble. Water can interact with and dissolve other polar compounds that ionize. Ionization is associated with the gain or loss of an electron, or an H⁺ ion, giving rise to an atom or a molecule that carries a net charge. Molecules that can dissociate to form ions are called electrolytes. Substances that readily dissolve in water are said to be hydrophilic, polar or water loving. The shell of water molecules that surrounds each ion or molecule is called a solvation sphere. A molecule or ion  surrounded by solvent molecules is said to be solvated. When the solvent is water, such molecules or ions are said to be hydrated.

     

      Nonpolar molecules are said to be hydrophobic, or water fearing and tend to hide from surrounding water molecules. This phenomenon of exclusion of nonpolar substances by water is called the hydrophobic effect. The hydrophobic effect is critical for the folding of proteins and the self-assembly of biological membranes.

  Detergents, sometimes called surfactants, are molecules that are both hydrophilic and hydrophobic. They usually have a hydrophobic chain at least 12 carbon atoms long and an ionic or polar end. Such molecules are said to be amphipathic.

Typical Noncovalent Interactions in Biomolecules

Ionization of Water

• Water has a slight tendency to ionize. Pure water contains a low concentration of hydronium ions (H₃O⁺) and an equal concentration of hydroxide ions (OH^-).  The hydronium and hydroxide ions are formed by a nucleophilic attack of oxygen on one of the protons in an adjacent water molecule. 
• Hydronium (H₃O⁺) ions  act as acids;  are capable of donating a proton to another ion. To simplify chemical equations we often represent the hydronium ion as simply H⁺
•  Hydroxide ions OH^- can accept a proton and be converted back into water molecules and thus act as bases
• The ionization of water can thus be depicted as a simple dissociation of one proton from a single water molecule and can be analyzed quantitatively.

The pH Scale

•      pH is defined as the negative logarithm of the concentration of H+. 

     In pure water [H⁺]  [OH^- ] = 1.0 × 10^–7 M.

•      Pure water is said to be “neutral” with respect to total ionic charge since the concentrations of the positively charged hydrogen ions and the negatively charged hydroxide ions are equal.

•      Neutral solutions have a pH value of 7.0 (the negative value of log 10^–7 is 7.0).

If you want further information go to https://www.youtube.com/watch?v=i1LyWQ_lPik