The chemical concept we are going to cover on is Hydrogen Bonding in Water. First of all, what is hydrogen bonding?

Hydrogen bonding is the electrostatic attraction that occurs between a hydrogen atom attached to an electronegative atom of one molecule, and another electronegative atom that can come from the same or another molecule. Thus, hydrogen bonding can occur within the same molecule (intramolecular hydrogen bonding) or between molecules (intermolecular hydrogen bonding). Hydrogen bonds are stronger than van der waals forces but weaker than ionic bonds or covalent bonds (Encyclopaedia Britannica, 2016).  

In the case of water, the water molecule consists one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom is the donor that is covalently bonded to a hydrogen atom (forming -OH).  

The higher relative electronegativity of oxygen to hydrogen makes it pull the shared electrons closer to itself. This gives the oxygen atom a slightly negative charge as compared to the hydrogen atoms. This is called a dipole, which causes the water molecule to have a positive and negative side. (Sciencing, 2017)


Figure 4: The Partial Charges on the Atoms in a Water Molecule (Callaway’s Chemistry, 2017)

Water molecules form hydrogen bonds, aligning in a way that the (positive) hydrogen on one molecule will face the (negative) oxygen on another molecule. This gives water a greater viscosity and also allows water to dissolve other molecules that have either a slightly positive or negative charge. (Sciencing, 2017)

Figure 5: Hydrogen Bonding in Water Molecules. The hydrogen (positive) on one molecule faces the oxygen (negative) on another molecule. (Allana, 2014)

Both in liquid and solid state of water, the formation of the hydrogen bonds between the water molecules holds the molecular mass together. Intermolecular hydrogen bonding is responsible for the high boiling point of water as more energy is needed to break the bonds before the water can boil. (Evans, 2017)

Figure 6: Water Molecules bonded together by Hydrogen Bonds (Awesomealexs, 2017)

Hydrogen bonding forces water molecules to form crystals when it freezes. The lattice of ice crystal isn’t as compact as the liquid form, this is because the positive and negative ends of the water molecules must arrange themselves in a way that the positive ends attract the negative ends of the molecules. This is the reason why ice is able to float on water. (Evans, 2017)