Properties of Water
Written by tutor Sarah V.
Water is found all throughout nature, and is a necessary component in nearly all organic reactions. It is the most abundant compound on the surface of the planet. The majority of water on Earth is sea water, and water is also present as solid ice and invisible gaseous clouds. All life on Earth requires water to survive, and it is speculated that extraterrestrial water may be a sign of life (or at least a precursor to life) elsewhere in the universe. Along with enabling life, water has an effect on non-living things, such as its weathering and erosion of rocks and soil. Water also acts as a heat sink, absorbing excess heat and buffering the Earth's temperature from extreme changes. Water is a vital part of any living system, from entire habitats down to individual cells. So what is water?
Chemical Properties of Water
Water (also known as H2O) is a polar chemical compound containing one oxygen (O) atom and two hydrogen (H) atoms (as seen in the first figure.) These three atoms are bound together with covalent bonds, meaning they share their pairs of valence or outer electrons together. Each hydrogen atom is separately bound to the central oxygen atom and the oxygen also has two pairs of unshared electrons, giving one water molecule a total of four shared and four unshared valence electrons. Water molecules have a bent shape, with a 104.45° angle between the two hydrogens.
Despite sharing their electrons, water molecules are polar because the valence electrons are shared unequally between the oxygen and hydrogen atoms. Oxygen has a greater electronegativity than hydrogen does, meaning it is highly attracted to electrons, so it pulls more strongly on the shared electrons and keeps them closer to itself. This creates a dipole moment, i.e., two electrical poles. This unequal sharing of electrons means that the electrons' negative charges cluster at the oxygen end of the molecule, leaving the hydrogen end of the molecule to be partially positive while the oxygen end is partially negative. Water's polarity leads to it being adhesive to many other substances, and helps make it an excellent solvent.
Polar substances dissolve well in water as "like dissolves like," but nonpolar substances do not dissolve as readily (or at all.) A common example of this solubility is the dissolution of table salt (NaCl) into water to produce Na+ and Cl- ions surrounded by water molecules with their positive and negative poles oriented towards or away from the ions depending on the ion's charge. When a polar solute such as a salt dissolves into positive and negative ions, the water molecules orient themselves to surround the individual ions and point their corresponding poles at the ion based on its charge; water's negative poles point at positive ions and vice versa.
When a nonpolar substance is mixed with water it does not dissolve but remains clumped together and as separate as possible from the water molecules. This is why water and oil remain separate when mixed together. Water is a good solvent of hydrophilic substances such as salts, while poorly dissolving hydrophobic substances such as fats and oils. A substance is unable to dissolve in water when its attraction to the water molecules does not outweigh their attraction to each other; it disrupts the water's intermolecular bonds without replacing them, and is therefore energetically unfavorable. A hydrophilic solute will, in contrast, be quickly surrounded by water molecules and form new tenuous bonds with them.
Water is an amphoteric compound, meaning it can act as a base or an acid in a chemical reaction. This allows it to participate in a large number of reactions, both in the laboratory and in nature. Water is also not easily compressible, meaning it cannot (as a liquid) be easily squeezed into a smaller volume. These various properties of water are very important in the life of the Earth and in the life of individual organisms.
Physical Properties of Water
Water is a liquid at standard temperature and pressure, but is also found in nature in its solid (frozen) and gaseous phases. Water's boiling point (the point at which it shifts from a liquid to a gas, or vice versa) is at 100° Celsius and its freezing point (at which it shifts from a liquid to a solid, or vice versa) is at 0°C. Water has a high heat capacity, which means it can absorb a great deal of energy without its temperature increasing greatly. Water also has a relatively high boiling point compared to the boiling points of similar compounds, meaning that more energy must be put into water to separate the molecules and turn it from a liquid into a gas. These two properties are due to its many intermolecular hydrogen bonds (discussed below in the Special Properties paragraph.)
In its pure state water is tasteless and odorless, however water is a universal solvent and therefore is often found with substances dissolved in it, changing its physical characteristics. Minerals and other impurities give bottled water its flavor, and it is the presence of solutes that gives water its ability to conduct electricity well. If water had no dissolved ions it would be a poor conductor of electricity. However, no water is completely free of ions and even in the absence of a solute it can auto- ionize to form negative hydroxide anions (OH-) and positive hydronium cations (H3O+.) These ions allow water to conduct some electricity even when it is pure or "deionized." Water's electrical conductance as well as water's propensity for dissolving hydrophilic substances both have great importance in chemistry and the biological sciences.
Special Properties of Water
Many of water's special properties are derived from hydrogen bonding. Thanks to their atomic composition and molecular shape, water molecules can be attracted to each other through hydrogen bonding. This weak bond occurs when a hydrogen atom covalently bound to a more electronegative atom is then briefly electrically attracted to a second highly electronegative atom on another separate molecule. The hydrogen acts like the sticky, attractive peanut butter between two pieces of electronegative bread. This brings the two molecules together through the hydrogen's dual attractions. In this way water molecules work like tiny magnets, their partially positive and negative poles interacting together via hydrogen bonds. Water molecules can form up to four hydrogen bonds each, creating a local tetrahedral structure of five hydrogen-bonded water molecules (as seen in the second figure.) Hydrogen bonding gives water its cohesion and surface tension, allowing it to cling to itself. It also greatly affects water's freezing behavior and density.
Liquid water's density is approximately 1 gram per cubic centimeter (1g/cm3.) Unlike most substances, water is denser in the liquid phase than the solid phase; this is due to the water molecules forming a loose hexagonal crystalline structure as they cool and freeze together, locking them into an expanded and less dense form than their freely-moving liquid form. With freezing, the temporary hydrogen bonds between molecules are allowed to stabilize, creating a lattice of water molecules held slightly apart from each other. Because of this difference in density, solid ice can float on liquid water. Thanks to this property, and water's high heat capacity (good insulation), lakes and ponds on Earth freeze slowly from the top down and rarely freeze fully, allowing organisms in the unfrozen water to survive the winter.
Biological Properties of Water
In the body, the major components of cells such as proteins, DNA and organelles are all dissolved in water and the cell is filled with watery cytoplasm. This allows essential molecules to be transported fluidly throughout the cell. The shape of the cell itself is heavily influenced by the water in and around it; too much water and the cell can burst open or "lyse" and too little water and the cell will shrivel. Water's polar nature prevents the hydrophobic phospholipid molecules from floating apart, packing them together in round cell membranes to present as little surface to the water as possible. The nonpolar nature of cell membranes keeps them closed against water leaking between them. Water transport into and out of the cell is strictly regulated, and the components of water (oxygen and hydrogen) are both used in many cellular processes.
Water's ability to conduct electrical impulses as well as it's propensity for dissolving polar substances allow the nerve cells in our brains to communicate with one another via electrical and chemical signals. Water's high heat capacity insulates our bodies from drastic temperature changes. Water's chemical and physical properties contribute to all life on Earth at the planetary, organismal, cellular, and molecular levels.