2.3 Water

Cohesion is the tendency for molecules of the same kind to be attracted to one another. Cohesion allows water molecules to “stick together” via hydrogen bonding. Have you ever filled a glass of water to the top and slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass because of cohesion.

Cohesion gives rise to surface tension, the capacity of a substance to withstand rupture when placed under tension or stress. When you drop a small scrap of paper onto a droplet of water, the paper floats on top of the water droplet, although the object is denser (heavier) than the water. This occurs because of the surface tension created by the water molecules. Cohesion and surface tension keep the water molecules intact and the item floating on top. If you place it gently without breaking the surface tension, it is possible to “float” a steel needle on top of a glass of water.

These cohesive forces are also related to the water’s adhesion property or the attraction between water molecules and other molecules. This is observed when water “climbs” up a straw placed in a glass of water. You will notice that the water appears higher on the sides of the straw than in the middle. This is because the water molecules are attracted to the straw and adhere to it.

Cohesive and adhesive forces are essential for sustaining life. In the human body, cohesion of water molecules ensures that when water moves in and out of capillaries, one water molecule will be followed by many water molecules. This bulk movement of water ensures fluid balance within the body. The cohesion of water also allows water to flow from the roots up to the leaves to feed the plants.

The hydrogen bonds in water allow it to absorb and release heat energy more slowly than many other substances. Temperature is a measure of the motion (kinetic energy) of molecules. As motion increases, energy and temperature increase. Water absorbs a great deal of energy before its temperature rises. Energy must be invested to break the hydrogen bonds that hold water molecules together before those water molecules can move freely. Once the hydrogen bonds have been broken, individual water molecules move faster (gain kinetic energy), resulting in a change in temperature.

The release of individual water molecules at the surface of the liquid (such as a body of water, the leaves of a plant, or the skin of an organism) is called evaporation. Evaporation of sweat, which is 90 percent water, allows for the cooling of an organism because breaking hydrogen bonds requires an input of energy and takes heat away from the body.

Conversely, water also takes a long time to cool. Once you have brewed your tea, you must wait for the tea to reach a drinkable temperature. As water cools, the molecules slow their movement and energy is released as the hydrogen bonds between water molecules re-form. That energy keeps the temperature of the water relatively constant, even though you have stopped heating the water.

The ability for water to stabilize temperature makes Earth habitable. Heat from the atmosphere is absorbed by water in oceans, ensuring that the temperature on Earth’s surface does not exceed a livable temperature. Water can release heat to cooler atmospheric air, again, keeping the temperature on Earth’s surface relatively constant. Life on Earth is possible because of the ability of water molecules to form hydrogen bonds!

For most substances, the solid state of matter is denser than the liquid state of the same substance. As atoms and molecules slow their movement at low temperature, they get closer together. Generally, this increases the density of a substance. Density is a measure of how much mass is contained in a given volume. It indicates how tightly matter is packed together.

Water is one exception to this rule. Ice is less dense (the molecules are farther apart) when frozen compared to liquid water. In liquid water, some molecules are bound together by hydrogen bonds, but some molecules move freely. As liquid water freezes and hydrogen bonds form between water molecules, it creates a rigid, lattice-like structure (e.g., ice) (Figure 2.3.3 a). The fixed geometry of the lattice prevents water molecules from packing close together and, therefore, from becoming denser as it freezes. Instead, water expands upon freezing.

This means that ice floats on the surface of a body of water (Figure 2.3.3 b). Ice will form on the water’s surface in lakes, ponds, and oceans, creating an insulating barrier to protect the animal and plant life beneath from freezing in the water. If this did not happen, plants and animals living in water would freeze in a block of ice and could not move freely, making life in cold temperatures difficult or impossible.

Water is the solvent for most of life’s chemical reactions. A solution is a uniform mixture of a liquid and substances dissolved in that liquid. A solution consists of a solvent (a substance capable of dissolving another substance) and a solute (a substance dissolved in the solvent).

Because water is polar, with slightly positive and negative charges, ionic compounds (such as salt) and polar molecules (such as sugar) can readily dissolve. The charged particles will form hydrogen bonds with a surrounding layer of water molecules. This is a sphere of hydration that keeps the particles separated or dispersed in the water. In the case of table salt (NaCl) mixed in water, the sodium and chloride ions separate, or dissociate, in the water, and spheres of hydration are formed around the ions. A positively charged sodium ion is surrounded by the partially negative charges of oxygen atoms in water molecules. A negatively charged chloride ion is surrounded by the partially positive charges of hydrogen atoms in water molecules. These spheres of hydration are also referred to as hydration shells. The water molecule’s polarity makes it an effective solvent and is essential in its many roles in living systems.

When a substance readily forms hydrogen bonds with water, it can dissolve in water and is called hydrophilic (“water-loving”). Hydrogen bonds are not readily formed with nonpolar substances like oils and fats. These nonpolar compounds are hydrophobic (“water-fearing”) and will not dissolve in water.

Water can dissolve a wide range of substances, which are crucial to facilitate chemical reactions and the transport of nutrients and waste within organisms. For example, blood, which is mostly water, is essential for transporting nutrients, oxygen, and waste products throughout the body.