Water chemical formula is H₂O. A covalent bond forms between one oxygen atom and two hydrogen atoms. Oxygen has a higher electronegativity to attract the shared electron cloud and carrying a partial negative charge (𝛿−). The electron cloud in hydrogen atoms is stripped by oxygen, so it exhibits partial positive charge (𝛿+). The sp³ hybridization in oxygen atom results in a V-shaped structure that prevent uneven distributed charges from canceling each other out. Consequently, neutral molecule presents polarity. The H-O-H bond angle is approximately 105°, slightly smaller than ideal 109.5° angle in sp³ hybrid orbitals. When considering their lone pairs, the entire molecule resembles a tetrahedron.
Hydrogen atom in another water molecule is strongly attracted to lone pairs in oxygen. Although it is belong to a van der Waals force, it is much stronger than typical dipole-dipole forces and weaker than covalent bonds. Therefore, we often discuss it separately and refer to it as a hydrogen bond. Each water forms hydrogen bonds with four other water molecules. These bonds are in a dynamic equilibrium of formation and dissociation. Its average lifespan is only a few femtoseconds.
Water's unique physical and chemical properties are attributed to hydrogen bonds, the interactions between ordinary molecular forces and covalent bonds.
Surface Tension
This arises from different interactions externally and internally. Molecules on the surface are attracted by liquid below which tries to pull them back inside. Inside the liquid, each molecule is surrounded by others and experiences zero net force. A tight "film" appears on liquid surface. Hydrogen bonds give them stronger attraction, thus water's surface tension is very large. This is the basis for many unique physical phenomena. Small insects skate on ponds; you observe droplets climbing up capillaries against gravity. This is because surface tension and attraction of tube walls are greater than water itself weight. Capillary action transports water from roots to plant tips.
High Specific Heat Capacity of Water
Hydrogen bonds must be broken before acceleration, so extra thermal energy is consumed, and their kinetic energy doesn’t increase obviously. More heat is absorbed by water than other liquids to raise the same temperature. Maintaining moderate temperatures is crucial for life. For example, oceans absorb heat from sun, and their temperature rises only a few degrees during the day; at night, oceans release heat gained during the daytime to prevent rapid temperature drop. This inhibits drastic temperature changes on Earth (greenhouse effect is another reason).
When water moves from liquid to air, additional kinetic energy is consumed to overcome hydrogen bonds. In summer or during intense exercise, a lot of heat is taken away by sweat evaporation, so that their body temperature decreases to normal levels rapidly.
Ice Floats on Rivers, Lakes, Sea
A solid of the same substance is denser than its liquid. However, water density is greater than ice, because its thermal motion is more intense, and molecules overcome hydrogen bonds to stay closer. When temperature is below freezing point, the adjacent water molecules are pushed apart by hydrogen bond to form a regular hexagon, since its thermal motion is slowed down. This is why solid ice is lighter and float on river surface. This characteristic is vital for life. If ice sank to bottom, it would freeze benthic organisms and disrupt ecosystems. The sunlight is blocked by liquid above, and the ice at the bottom might not completely melt in hot summer. Over time, the ice in rivers, lakes, and oceans would increase. Eventually our earth isn’t suitable for living things.
It is a Good Solvent
Ions are easily stripped by electrostatic attraction from the partially charged polar water molecules, so ionic compounds like NaCl and MgCl₂ dissolve easily in aqueous media. Although saccharides are not composed of ionic bonds, they have hydrophilic groups -OH, C=O or -CHO whose uneven charge distribution attracts water molecules to make them very soluble. Even some large molecules, like proteins, are soluble if they have hydrophilic or ionic regions. Because it easily dissolves polar molecules and ions, it is an ideal medium for various chemical reactions in life. Cellular nutrients and waste are also transported by it.