2.1: Water

Biotech Focus

Strategies for Cleaner Water

Industrial activities produces a large amount of wastewater containing pollutants such as pesticides or industrial dyes. Using biotechnology, wastewater treatment can be more efficient, sustainable and cost effective. Several strategies have been developed, using microorganisms, enzymes, or plants specifically engineered to break down pollutants and contaminants, ultimately making the water safer for consumption.

For more information about the different approaches to treat water, read Biological wastewater treatment technology: Advancement and drawbacks

Introduction

Do you ever wonder why scientists spend time looking for water on other planets? It is because water is essential to life; even minute traces of it on another planet can indicate that life could or did exist on that planet. Water is one of the more abundant molecules in living cells. Approximately 60–70 percent of the human body is made up of water. It is critical our viability by serving many essential functions (Figure \(\PageIndex{1}\)). In humans, water forms saliva and keeps mucosal membranes moist. It allows the body's cells to grow, reproduce and survive. Water flushes out wastes through urination. It provides lubrication to joints. It is needed by the brain to manufacture hormones and neurotransmitters and acts as a shock absorber for the brain, spinal cord, and fetus. Without water, life simply would not exist.

Figure \(\PageIndex{1}\): The functions of water. Water is the major component of most body parts and serves a number of essential functions like keeping membrane moist, flushing wastes from the body, lubricating joints, and regulates body temperature.​​​​ (The water in you: What water does for your body by USGS.gov, public domain)

Water has several important properties, including:

1. it is polar
2. it has the ability to form hydrogen bonds
3. it is a solvent
4. it has high cohesion
5. it has high adhesion
6. it moderates temperature

Learning Objectives

Water is one of the most abundant molecules on Earth. By the end of this section, you will be able to:

• Describe the properties of water that are critical to maintaining life
• Explain why water is an excellent solvent
• Provide examples of water’s cohesive and adhesive properties
• Discuss the role of acids, bases, and buffers in homeostasis

The Polarity of Water & Hydrogen Bonding

The bonds between the oxygen (O) and hydrogen (H) atoms in the water molecule are known as covalent bonds. Electrons are shared between two atoms in a covalent bond. Oxygen has 6 electrons in its outermost shell and hydrogen has only 1 electron. As a result, oxygen shares each of its outer electrons with a hydrogen atom. While there is no overall charge to a water molecule, the shared electrons are found more closely associated with the center of the oxygen atom than they are with the center of a hydrogen atom. This results in "unequal sharing" of electrons within the covalent bond and produces a slight positive charge on each hydrogen atom and a slight negative charge on the oxygen atom. Because of these charges, the slightly positive hydrogen atoms repel each other and form the unique shape (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Left image: The electron configuration in a molecule of water produces polar covalent bonds. The two hydrogen electrons are "pulled" closer to the center of the oxygen atom, making the hydrogen atoms slightly positive in charge and the oxygen atom slightly negative. This "unequal sharing" of electrons is known as a polar covalent bond. (Electron sharing by Kareen Martin, CC BY-NC 4.0)
Right image: Hydrogen bonds form and reform between the partial positive end of one water molecule (i.e., the hydrogen atom) and the partial negative charge of an adjacent water molecule (i.e. the oxygen atom.) The partial charge in a polar bond is denoted with the Greek letter "delta". A molecule of water can interact with 4 hydrogen bonds with 4 other molecules of water. (Hydrogen bonds by Kareen Martin, CC BY-NC 4.0)

As a result of the positive and negative charges in the different parts of the water molecule, each water molecule attracts other water molecules (Figure \(\PageIndex{2}\)). These attractions create hydrogen bonds between water molecules. Hydrogen bonds are weak interactions that form between a hydrogen with a partial positive charge of one molecule and another atom with a partial negative charge. The partial charge in a polar bond is denoted with the Greek letter "delta". Hydrogen bonds can break and reform easily giving water's fluidity.

Concept in Action

Animation: How polarity makes water behave strangely

Water is an Excellent Solvent

The slight positive and negative charges of water molecules enables them to easily dissolve ionic compounds and polar molecules through their formation of hydrogen bonds with surrounding water molecules. A compound that dissolves in water is referred to as hydrophilic (“water-loving”). Because nonpolar molecules do not have partial positive or partial negative charges, they are not attracted to water molecules, explaining why oil separates when added to water. Or vinegar, a polar substance, does not mix with olive oil (a non-polar substance) as seen in Figure \(\PageIndex{3}\). These nonpolar compounds are called hydrophobic (“water-fearing”).

Figure \(\PageIndex{3}\): Balsamic vinegar (black), a polar compound and olive oil (yellow), a non polar one, do not mix. (Plate with vinegar by Tim Ereneta, CC BY-NC 2.0)

Because water is capable of dissolving another substance, it is considered to be a solvent. The compounds dissolved in a solvent are called solutes. When polar and ionic compounds are dissolved in water, hydrogen bonds will form between the charged compounds and adjacent water molecules, resulting in the compound becoming surrounded by a layer of water molecules. This layer is called a hydration shell and it serves to keep solutes separated or dispersed in the water. For example, when table salt (NaCl) is dissolved in water (Figure \(\PageIndex{4}\)), the sodium and chloride ions separate, or dissociate, in the water, and hydration shells form around each ion. Each positively-charged sodium ion is surrounded by negatively-charged oxygen atoms in water molecules. Each negatively-charged chloride ion is surrounded by positively-charged hydrogen atoms.

Figure \(\PageIndex{4}\): When table salt (NaCl) is mixed in water, each ion becomes surrounded by a hydration shell of water molecules. (Hydration shell by Kareen Martin, CC BY-NC 4.0)

The polarity of the water molecule makes it an effective solvent and is important in its many roles in living systems. However, one common mistake or misconception about water is that it dissolves everything because it is the “universal solvent." Water has the ability to dissolve many substances but the term “universal solvent" is misleading. Water is able to dissolve other polar molecules and ions but cannot dissolve non-polar compounds.

Water Moderates Temperature

Temperature is defined as a measure of the motion, or the kinetic energy of molecules. There is a direct relationship between kinetic energy and temperature. As kinetic energy increases, temperature increases. However, water is able to moderate changes in temperature within organisms and in the environment. This is because water has a high specific heat capacity. Specific heat is defined as the amount of energy it takes to raise 1 gram of water by 1 degree Celsius. Water's high specific heat capacity means that it takes a lot of energy to raise the temperature of water by a single degree, and it is the result of hydrogen bonding. Increasing kinetic energy disrupts the hydrogen bonds found between water molecules. Because these bonds can break and re-form rapidly, water can absorb large increases in energy before its temperature will change. In fact, water has the highest specific heat capacity of any liquid.

As energy input continues, the balance between hydrogen-bond formation and destruction swings toward the destruction side. More bonds are broken than are formed. This process results in the release of individual water molecules in gas form in a process called evaporation. As with specific heat capacity, it takes a significant amount of energy to convert liquid water to gaseous water (i.e. steam). This is because water has a high heat of vaporization. Evaporation of sweat, which is 90 percent water, allows for cooling of an organism, because breaking hydrogen bonds requires an input of energy and takes heat away from the body.

As kinetic energy decreases and temperatures drop, less energy is present to break the hydrogen bonds between water molecules. At a specific temperature, these hydrogen bonds will remain intact and begin to form a rigid, lattice-like structure as the water solidifies into ice (Figure \(\PageIndex{5}\)). When water freezes, the distance between the water molecules increases, making ice less dense then liquid water. This means that ice floats (Figure \(\PageIndex{5}\)). In lakes, ponds, and oceans, ice will form on the surface of the water, 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.

Figure \(\PageIndex{5}\): Hydrogen bonding makes ice less dense than liquid water. The lattice structure of ice, seen on the right side, makes it less dense than the freely flowing molecules of liquid water, (left side) enabling it to float on water. (Icebergs by Ben-Zin, public domain, Water and Ice Structure by Kareen Martin, CC BY-NC 4.0)

Water Is "Sticky" (Cohesion and Adhesion)

Water has both high cohesion and high adhesion. In cohesion, water molecules at a liquid-air interface are attracted to each other (because of hydrogen bonding), keeping the molecules close together and well organized (Figure \(\PageIndex{6}\)). The hydrogen bonds found between water molecules at the surface are stronger than those below them, giving rise to surface tension, the capacity of a substance to withstand rupture when placed under tension or stress. Because of cohesion and surface tension, it is possible to “float” a steel needle on top of a glass of water if the needle is placed gently and does not break the surface tension. This is possible even though the needle is denser than the water. Small insects such as the water strider can walk on water because their weight is not enough to break the surface tension. (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Left image - surface tension in water is due to cohesion (i.e. attraction) between water molecules at the surface of the water as adjacent water molecules form strong hydrogen bonds between one another. (Surface tension by USGS.gov, public domain)
Right image - a "water strider" is able to walk on top of water due to a combination of high surface tension of water and long, hydrophobic legs to help them stay above water. (Water strider by Unsplash, CCO)

Water also exhibits adhesion, which is the attraction of two different molecules to one another. This can be observed when water “climbs” up a straw placed in a glass of water. Because of adhesion, the water molecules are attracted to the plastic of the straw and form bonds with it. It is important to distinguish that adhesion and cohesion are related to one another but are not the same thing. Adhesion takes place because of the interaction between water molecules and another molecule; cohesion takes place because of the interaction between water molecules. Cohesive and adhesive forces are important for sustaining life. For example, because of these forces, water can flow up from the roots to the tops of plants to feed the plant (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{7}\): Left: Cohesion between water molecules allows a drop to form and cohesion results in the drop clinging to the tip of a leaf. Right: With the help of adhesion and cohesion, water can work its way all the way up to the branches and leaves at the top of trees. (Cohesion and Adhesion by Kareen Martin, CC BY-NC 4.0)

Buffers, pH, Acids, and Bases

The pH of a solution is a measure of its acidity or basicity and is determined by the concentration of hydrogen ions in that solution. The overall concentration of hydrogen ions is inversely related to its pH. The more H+ present, the lower the pH the more acidic the solution is; conversely, the fewer hydrogen ions, the higher the pH and the more basic (i.e. alkaline) the solution is. Acidity and basicity is measured using the pH scale (Figure \(\PageIndex{8}\)). The pH scale ranges from 0 to 14. A change of one unit on the pH scale represents a change in the concentration of hydrogen ions by a factor of 10; a change in two units represents a change in the hydrogen ion concentration by a factor of 100. Thus, small changes in pH represent large changes in the concentrations of hydrogen ions. Pure water is neutral. It is neither acidic nor basic, and has a pH of 7.0. Anything below 7.0 (from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Most cells operate within a very narrow window of the pH scale, typically ranging only from 7.2 to 7.6.

Figure \(\PageIndex{8}\): The pH scale ranges from 0 to 14, and most solutions fall within this range. Anything below 7.0 is acidic, and anything above 7.0 is alkaline, or basic. The pH values of common substances like milk, saliva, coffee and blood are shown. (pH Scale by OpenStax College, CC BY 3.0)

An acid is a substance that donates hydrogen ions (H+) and lowers pH. (Figure \(\PageIndex{9}\)). The stronger the acid, the more readily it donates H+. For example, hydrochloric acid and lemon juice are very acidic and readily give up H+ when added to water. Conversely, a base is a substance that accepts H+ ions and raises pH. A base can also be defined as a substance that donates hydroxide ions (OH–). The OH– ions from the base combine with H+ from the acid to produce water, which raises a substance’s pH. Sodium hydroxide (NaOH) and many household cleaners are very alkaline and give up OH– rapidly when placed in water, thereby raising the pH.

Figure \(\PageIndex{9}\): Left image - in an acidic solution (e.g. hydrochloric acid/HCl), ionic dissociation gives rise to an excess of H⁺. The pH of an acidic solution ranges between 1 and 6. Center image - in a neutral solution, water molecules dissociate into an equal number of hydrogen ions (H⁺) and hydroxide ions (OH^-) and recombine to reform water. The resulting pH is 7.0. Right image - in a basic solution (e.g. sodium hydroxide/NaOH), ionic dissociation gives rise to an excess of OH^- that can combine with H⁺ to produce water molecules. As a result, the number of H⁺ decreases and the pH of the solution becomes basic. The pH of a basic solution ranges comprised between 8 and 14. (pH Solutions by Kareen Martin, CC BY-NC 4.0)

Organisms can maintain the pH of their cells and fluids within a relatively narrow range using buffers. A buffer is a solution that resists pH changes when small amounts of acid or base are added. In other words, buffers readily absorb excess H+ or OH–, maintaining the pH of the organism at a desired level. Carbon dioxide (CO₂) is part of a prominent buffer system used by many organisms called the carbonic acid-bicarbonate buffer system. This buffer system involves carbonic acid (H₂CO₃) and bicarbonate (HCO₃^–) anion (Figure \(\PageIndex{10}\)). If too a solution has too much H+, bicarbonate will combine with the H+ to create carbonic acid and limit the decrease in pH of the solution. Likewise, if too much OH– is introduced into a solution, carbonic acid will rapidly dissociate into bicarbonate and H+ ions. The H+ ions can combine with the OH– ions, limiting the increase in pH.

Figure \(\PageIndex{10}\): Carbonic acid acts as a buffer. Water and carbon dioxide (CO₂) combine to form carbonic acid (H₂CO₃). Carbonic acid dissociates to form H+ and bicarbonate ions (HCO₃^–). When an acid is added to a fluid, HCO₃^– react with the excess H⁺ to form carbonic acid and prevent a drop in pH. When a base is added, carbonic acid donates H+ to neutralize the excess OH- and form more carbonic acid (modified from Buffer by CNX OpenStax; CC BY 4.0).

Many organisms rely upon this buffering system to maintain the pH of their blood plasma (and tissue fluids) between 7.2 and 7.4. If the blood becomes too acidic, the organism will increase its respiration rate to remove more carbon dioxide and decrease the concentration of carbonic acid. This will decrease the amount of H+ in the blood produced by carbonic acid dissociation. If the blood becomes too basic, the organism can slow its breathing rate, increasing carbonic acid levels and lowering pH through the production of more H+.

Key Concepts

Water has many properties that are critical to maintaining life. These properties of water are intimately connected to the biochemical and physical processes performed by living organisms, and life would be very different if these properties were altered, if it could exist at all.

Some important concepts to remember:

• Water is a polar molecule, allowing for the formation of hydrogen bonds with other molecules, including other water molecules
• Water is an excellent solvent. Its hydrogen bonds allow ions and other polar molecules to dissolve in water
• Water stabilizes temperature. The hydrogen bonds between water molecules cause the water to have a high heat capacity, meaning it takes considerable added heat to raise its temperature.
• The evaporation of water takes considerable amounts of energy; this allows organisms to cool their internal temperatures
• Water is less dense when in solid form; this means that ice floats
• The attraction of water molecules to one another at the surface of a volume of water is called cohesion; cohesion produces surface tension
• The attraction between a water molecule and another molecule is called adhesion
• The pH of a solution is a measure of hydrogen ion concentration
• Acids and bases can change pH values, but buffers tend to moderate the changes they cause

Glossary

Acid - a substance that increases the concentration of hydrogen ions (H⁺) in a solution (i.e. a H+ donor)

Adhesion - the attraction between molecules of different substances; caused by intermolecular forces such as hydrogen bonding, van der Waals forces, or dipole interactions

Base - a substance that increases the concentration of hydroxide ions (OH⁻) or accepts protons (H⁺) in a solution (i.e. an OH- donor)

Buffer - a solution that resists changes in pH when small amounts of acid or base are added; helps to maintain a stable pH in biological and chemical systems

Cohesion - the attraction between molecules of the same substance due to intermolecular forces like hydrogen bonding, van der Waals forces, or dipole interactions

Evaporation - the process by which a liquid turns into a gas at a temperature below its boiling point; it occurs at the surface of the liquid, where molecules with sufficient kinetic energy escape into the air as vapor

Hydrophilic - a substance that is attracted to and dissolves in water

Hydrophobic - a substance that repels water and does not dissolve in it

Kinetic energy - the energy of motion

pH scale - a measurement system used to express how acidic or basic a solution is; ranges from 0 to 14

Solute - a substance that dissolves in a solvent

Solvent - a solution that dissolves a solute

Surface tension - a property of liquids that causes the surface to act like a membrane; a result of strong cohesive attraction between water molecules at the surface of the liquid

Temperature - a measure of the average kinetic energy of the particles in a substance

Attributions

text adapted from Open Stax Biology 2e, licensed CC BY-4.0