Water Properties and Facts You Should Know

Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

Water is the most abundant molecule on the Earth's surface and one of the most important molecules to study in chemistry. The facts of water chemistry reveal why this is such an incredible molecule.

What Is Water?

Water is a chemical compound. Each molecule of water, H₂O or HOH, consists of two atoms of hydrogen bonded to one atom of oxygen.

Properties of Water

There are several important properties of water that distinguish it from other molecules and make it the key compound for life:

• Cohesion is a key property of water. Because of the polarity of the molecules, water molecules are attracted to each other. Hydrogen bonds form between neighboring molecules. Because of its cohesiveness, water remains a liquid at normal temperatures rather than vaporizing into a gas. Cohesiveness also leads to high surface tension. An example of the surface tension is seen by beading of water on surfaces and by the ability of insects to walk on liquid water without sinking.
• Adhesion is another property of water. Adhesiveness is a measure of water's ability to attract other types of molecules. Water is adhesive to molecules capable of forming hydrogen bonds with it. Adhesion and cohesion lead to capillary action, which is seen when the water rises up a narrow glass tube or within the stems of plants.
• The high specific heat and high heat of vaporization mean a lot of energy is needed to break hydrogen bonds between water molecules. Because of this, water resists extreme temperature changes. This is important for weather and also for species survival. The high heat of vaporization means evaporating water has a significant cooling effect. Many animals use perspiration to keep cool, taking advantage of this effect.
• Water is a polar molecule. Each molecule is bent, with the negatively charged oxygen on one side and the pair of positive-charged hydrogen molecules on the other side of the molecule.
• Water is the only common compound that exists in solid, liquid, and gas phase under ordinary, natural conditions.
• Water is amphoteric, which means it can act as both an acid and a base. Self-ionization of water produces H + and OH - ions.
• Ice is less dense than liquid water. For most materials, the solid phase is denser than the liquid phase. Hydrogen bonds between water molecules are responsible for the lower density of ice. An important consequence is that lakes and rivers freeze from the top down, with ice floating on water.
• Pure liquid water at room temperature is odorless, tasteless, and nearly colorless. Water has a faint blue color, which becomes more apparent in large volumes of water.
• Water has the second highest specific enthalpy of fusion of all substances (after ammonia). The specific enthalpy of fusion of water is 333.55 kJ·kg−1 at 0 °C.
• Water has the second highest specific heat capacity of all known substances. Ammonia has the highest specific heat. Water also has a high heat of vaporization (40.65 kJ·mol−1). The high specific heat and heat of vaporization result from the high degree of hydrogen bonding between water molecules. One consequence of this is that water is not subject to rapid temperature fluctuations. On Earth, this helps to prevent dramatic climate changes.
• Water may be called the universal solvent because it is able to dissolve many different substances.

Interesting Water Facts

• Other names for water are dihydrogen monoxide, oxidane, hydroxylic acid, and hydrogen hydroxide.
• The molecular formula of water is H₂O
• Molar mass: 18.01528(33) g/mol
• Density: 1000 kg/m 3 , liquid (4 °C) or 917 kg/m 3 , solid
• Melting point: 0 °C, 32 °F (273.15 K)
• Boiling point: 100 °C, 212 °F (373.15 K)
• Acidity (pKa): 15.74
• Basicity (pKb): 15.74
• Refractive index: (nD) 1.3330
• Viscosity: 0.001 Pa s at 20 °C
• Crystal structure: hexagonal
• Molecular shape: bent