part one : h20
margie sanchez, contributing science writer
It covers almost 75% of the earth’s surface. Approximately 97% of the earth’s supply of this compound is found in our oceans. It accounts for about 66 to 70% of the entire mass of the human body, and 93% of a baby’s body during the first month of gestation. It comprises much of the mass in vegetables and fruits like celery, cucumbers, and watermelon. By now, most of you have guessed what “it” is. The watermelon was a dead giveaway, in case you missed it. Yes, I am talking about water. Everything living thing on this planet depends on the presence of water for survival. Our atmosphere is integrated into a very delicate, balanced relationship with the earth’s oceans as well as fresh bodies of water. Our bodies require water to maintain homeostasis, the equilibrium that drives all biological systems into synergistic relationships among the various physiological pathways so that we can achieve optimal health. Therefore, it is not an exaggeration to claim that water is the single most abundant substance on our planet as well as our bodies. One might venture to designate Earth and the human body as “water worlds”. So, why is water so important? And what makes water so unique among all other compounds?
To truly understand the significance of water, let us first examine the atomic structure of water because the physics and chemistry of water can better explain its incredible properties. Every molecule of water is made of two atoms of hydrogen, the most abundant element in the universe, and one atom of oxygen which is the most abundant element on the earth’s surface. It’s a polar molecule. The oxygen side is slightly negative while the hydrogen side is slightly positive. Because the charge on the oxygen is 2- and each hydrogen has a 1+ charge, the net charge of water is neutral. It’s the polarity of that positive charge on the hydrogen and the negative charge on the oxygen that confers an attraction between water molecules (and other polar molecules). The hydrogen of a water molecule is attracted to the oxygen of another water molecule, while the oxygen of a water molecule is attracted to a hydrogen of another water molecule.
Moreover, the polarity of the water molecule results in a bent, not linear, shape. Intramolecular forces of the negative charge on the oxygen and the positive charges on each hydrogen account for an angle of 104.5 ⁰ on the H-O-H bond. Compared to the bond angle of the water molecule, a linear or straight molecule has a bond angle of 180⁰ . That H-O-H bond angle is less than the expected angle of a tetrahedral molecule like water, but the diminished bond angle is probably due to a repulsive interaction between the two lone pairs of electrons of the oxygen atom. The shape of a water molecule is quite significant because it is very specific to the behavior of water as it reacts and interacts with other molecules, including other water molecules.
The three physical states of water- solid, liquid and gas- are familiar to everyone. We drink water in its liquid form. When we heat water it is converted to the gas state we recognize as steam. Most of us use plenty of water in its solid state as ice, especially on hot days. Did you know that water in its solid state as ice is unique compared to most substances? You see, most substances expand when heated, and then contract when cooled. Thus, as most substances cool to form a solid state, they increase in density. That is, the substance becomes compacted, taking up less space but still retaining the same mass or weight. However, even though water contracts at first as it cools from 100⁰C to 3.98⁰C, once it reaches below 3.98⁰C liquid water expands again as it continues to freeze to its solid state of ice. This phenomenon results in a decrease in density that makes ice less dense than liquid water. Same mass, but spread out over a larger area. This property of water is extremely significant to the earth’s ecosystems. Ice and other substances with a density less than water will float.
If water contracted when it froze, the increase in density would send the ice to the bottoms of lakes, ponds, rivers, and oceans. The results would be catastrophic, as all marine life would perish. Instead, ice floats at the top of bodies of water, insulating fish and other water breathers, while still giving them plenty of liquid water from which to extract oxygen. Marine mammals find holes in the ice to come up for air or break through thin patches of ice to breathe.
Water is known as the universal solvent. A solvent is a fluid in which other substances called the solutes can be dissolved to form a solution. Water’s unique properties allow it to react as a base or an acid, depending on the environment or situation. For instance, when water reacts with HCL (hydrochloric acid), water acts like a base that accepts a proton (as hydrogen) to make a hydronium ion, HO+. But when water reacts with a base like the amide ion, –NH2 (ammonium ion), water acts like an acid that donates a proton to make ammonia, NH3 , and a hydroxide ion, HO-.
Water will react with alkali metals like sodium, the heavier alkaline earth metals like calcium, and the halogens. With most other elements, water does not react at room temperature. (That can be very advantageous when we need containers to hold water.) Solid compounds that contain water are known as hydrates. Hydrates can be heated to dry out the water molecules. The result is a substance called an anhydrous compound. Some of these anhydrous compounds will readily absorb water from the air around them, so they can be very useful as drying agents. In other words, no matter where water is located, it usually has something to contribute. Industrial applications alone make water very useful. But water also provides practical uses for us in routine applications.
As the universal solvent, many things can be dissolved in water. Think of all the things you dissolve when you cook, make drinks like lemonade, or wash your dishes and clothes. Oh yes, heat makes water even more interesting. Heat can increase the rate of a reaction with water, like when you steep tea in hot water, boil pasta, or wash your dishes in a dishwasher. We do simple chemistry every time we make lemonade, tea, coffee, soups, etc. Anytime we dissolve something in water we make something new. Water is the solvent which dissolves another substance, makes cleaning easier, and provides the basis of good hygiene.
Reactions like the ones we see in our homes also occur in our bodies. Water can help break up larger molecules by breaking chemical bonds. This process is called hydrolysis and is very important in making good use of the food we eat. Water provides the perfect biological solvent for numerous processes in our bodies. Remember, a solvent is a fluid in which something else, called a solute, can be dissolved. Water works very well as a solvent for biological purposes because of its ability to dissolve a wide variety of solutes. As previously mentioned, water can act like an acid or a base, depending on the substances added to it. The polarity of water makes it the perfect biological solvent because most of the molecules in our cells are also polar. That means that polar molecules will form both hydrogen and ionic bonds with water molecules.
If a solute has an affinity for water, thereby dissolving easily in water, the solute (molecule) is called hydrophilic, which means “water-loving”. In fact, most of the small organic (carbon based) molecules in our cells are hydrophilic. I’m referring to molecules like sugars, organic acids, and some amino acids. Some organic molecules have no net charge, that is, they are overall neutral like water. However, they can still be hydrophilic because they have regions that are positively charged and other regions that are negatively charged, just like a water molecule. Water molecules will cluster around these regions, and the resulting electrostatic interactions between the solute and the water molecule prevent the solute molecule from associating with one another.
On the other hand, hydrophobic (“water fearing”) molecules like hydrocarbons, proteins and fatty acids have no polar regions and will not demonstrate electrostatic interactions with water. Such hydrophobic molecules disrupt water’s hydrogen bonds. So, these molecules tend to coalesce in an aqueous medium, associating with each other rather than the water. Think of oil and vinegar, and how the oil in a vinaigrette dressing will always eventually float to on top of the vinegar, which is a weak aqueous (water based) acid.
In general, polar molecules and ions like NaCl (salt) and sugars are hydrophilic, and non-polar molecules like lipids (fats) and proteins found in biological membranes are hydrophobic. But some biological macromolecules (large molecules), especially proteins, contain both hydrophilic and hydrophobic regions. Overall, water accounts for most of the interactions, reactions, and intermediate steps in all biological systems and pathways, from regulating blood pressure to processing residues and waste products in our liver and kidneys. Without water, we couldn’t make tears, sweat, mucous, blood, and urine. If not for water, our skin would resemble paper, our urine would be toxic, and our blood would be nothing more than sludge. By the time our brain receives the signal to indicate thirst, we are already water deficient and should promptly heed this signal to replenish the 70% mass of water necessary for our bodies to achieve optimum health.