Water is a fundamental substance on Earth. It exhibits unique characteristics, some counterintuitive. For instance, ice floats on its liquid counterpart, unlike most other substances. Water also has an unusually high boiling point and a notable capacity to absorb and store heat.
The Polar Nature of Water
Water’s unique behavior stems from its molecular structure and polarity. A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is more electronegative than hydrogen, attracting shared electrons more strongly. This uneven sharing creates partial charges across the molecule.
The oxygen atom develops a slight negative charge (δ-), while the hydrogen atoms acquire slight positive charges (δ+). The water molecule’s bent shape prevents these partial charges from canceling out, making water a polar molecule with positive and negative ends.
Hydrogen Bonds
The polarity of water molecules leads to intermolecular attractions called hydrogen bonds. The slight positive charge on a hydrogen atom of one water molecule attracts the slight negative charge on the oxygen atom of an adjacent molecule. These attractions are weaker than covalent bonds within a water molecule, but strong enough to significantly influence water’s behavior.
Each water molecule can form up to four hydrogen bonds. While individual bonds are transient, their collective strength creates a cohesive network. This network of intermolecular forces dictates many of water’s physical and chemical properties.
Water’s Unique Properties
Hydrogen bonding gives water several unique properties. Water exhibits high cohesion, the attraction of water molecules to one another, allowing them to stick together. It also exhibits adhesion, where its molecules are attracted to other polar or charged surfaces. The combined effect enables capillary action, where water moves upwards through narrow tubes.
Water possesses a high specific heat capacity, meaning it can absorb or release a large amount of heat with only a small change in its own temperature. This is because much of the added energy is used to break numerous hydrogen bonds before the molecules gain enough kinetic energy to increase temperature. Liquid water at room temperature has a specific heat capacity of approximately 4.184 Joules per gram per degree Celsius (J/g°C), or 1 calorie/g°C, which is notably high compared to many other substances.
Water also has a high heat of vaporization, requiring substantial energy to convert from liquid to gas. To evaporate, water molecules must overcome the strong network of hydrogen bonds holding them in the liquid state. The heat of vaporization for water is around 540 calories per gram (cal/g) at 100°C, or 40.65 kilojoules per mole (kJ/mol), a considerable amount for a small molecule.
Another unique property is water’s density anomaly. Unlike most substances that become denser upon freezing, water reaches its maximum density at approximately 4°C, where it is about 1 gram per cubic centimeter (g/cm³). As water cools below 4°C and freezes, hydrogen bonds force molecules into a more open, crystalline lattice in ice. This arrangement spreads the molecules further apart, making ice about 9% less dense than liquid water at 0°C. The density of ice at 0°C is approximately 0.9167-0.9168 g/cm³, while liquid water at the same temperature is about 0.9998-0.999863 g/cm³.
Water’s polarity also makes it an excellent solvent, often called the “universal solvent.” Its charged ends can effectively interact with and surround other charged or polar molecules, pulling them apart and dispersing them in solution. When ionic compounds, like table salt, are added to water, water molecules form “hydration shells” around ions, dissolving the substance.
The Significance of Water’s Polarity
Water’s properties, all stemming from its polarity, have significant implications for life and Earth’s systems. Its high specific heat capacity helps moderate global temperatures, preventing extreme fluctuations detrimental to most organisms. Large bodies of water absorb vast solar energy during the day and release it at night, stabilizing regional climates.
The high heat of vaporization is important for cooling organisms through evaporation, such as sweating in humans, which removes excess heat from the body. The density anomaly of water is important for aquatic ecosystems. Floating ice forms an insulating layer on lakes and ponds, preventing the entire body of water from freezing solid and allowing aquatic life to survive. Water’s effective solvent capabilities enable nutrient and waste transport within organisms and facilitate biochemical reactions necessary for life.