Water as a Universal Solvent

have a large surface area compared to their body weight so they can float without breaking the surface tension of water. Water is amphoteric, meaning it can be an acid or a base. When water is part of a solution, it will change the equilibrium of the solution. When it interacts with a highly acidic solute, it will act as a base and, when it interacts with a highly basic solute, it will act as an acid. Because it has two single hydrogen ions as part of the molecule, it can act as an electron pair donor in biochemical reactions.

Water as a Universal Solvent

Water is considered a universal solvent because it can dissociate or dissolve most organic compounds. This is because it is a highly polar molecule, with a great deal of electronegativity associated with its oxygen component when compared to its hydrogen component. The positive electric charge of the molecule (the hydrogen side) is attracted to the negative parts of the solute being dissolved, while the positive charge of the molecule (the oxygen side) is attracted to the positive parts of the solute. This makes water able to dissociate and break ionic compounds into separate, dissolvable components. There is a chemical term known as the dielectric constant. It refers to the ability of electrons to migrate toward the pole of a positively charged molecule and away from the negatively charged molecules. When this happens, the molecule is called “polarized”. Water has a dipole moment of 6.17 x 10-30 , making it a polarized molecule. The dielectric constant of a molecule is a measurement of how polarized it is. Water has a high dielectric constant because of its high dipole moment. This gives water the ability to surround ions, blocking the charges of the ions so it can be a good solvent for any type of polar or ionic substance. Therefore, it is called a universal solvent as many types of ions can dissolve in it. Because of the polar aspect of water and its ready ability to undergo hydrogen bonding, polar molecules that are uncharged can easily dissolve in it. Water stabilizes these molecules by the effect of its hydrogen-binding ability between any polar molecule and the water molecule. Any molecule that has an N-H bond, alcohols, aldehydes, and ketones all can form hydrogen bonds, making them soluble in water. All of these molecules are biomolecules found in the human body. The hydrogen bonds between an uncharged polar molecule and water will be the strongest when the hydrogen atom in the molecule is in a straight line between two different electronegative atoms. Because of this, the property of hydrogen bonding in the water molecule is very directional. Water has the unique ability to recognize and adapt to the different shapes of organic molecules so that it can keep them in solution. Water also has the capability of forming a hydration shell. This is a shell that forms in a waterbased solution when the solute dissolves in the water. The solute can be surrounded by the

positively-charged hydrogen ions that bind to the solute, allowing it to stay in the watery solution. Water has the ability to be stabilizing to ions like sodium and chloride. It hydrates both ions and decreases the electrostatic interaction between the ions, weakening their tendency to want to go out of solution and form a crystalline lattice. This is also the case with protonated amines, anhydride molecules, carbolic acid, and other biomolecules. Instead of precipitating out as solute-solute solids, they form solute-solvent interactions, keeping the molecules in solution. In effect, water acts as a screen between the polar aspects of the molecule that would have the solute bind with itself instead of water. Water is an extremely versatile solvent. When a crystal of an ionic compound is placed in water, the ions are broken apart by the water solvent. Hydrogen, being negatively charged, will attract any part of the molecule that is positively charged, while oxygen, being positively charged, will attract any part of the molecule that is negatively charged. It effectively forms a shell around each positively and negatively charged component of an ionic substance so that they don’t bind to each other and precipitate out of solution. The substances can move freely in water is a favorable, energy-free occurrence. Water can interact with hydrophobic molecules as well. It causes that part of a hydrophobic molecule to hide within the hydrophilic components of the molecule, leaving the hydrophobic portion hidden and the hydrophilic part exposed. This is an energetically unfavorable interaction but needs to happen to keep the hydrophobic parts of the molecule away from the water. Organic hydrophobic molecules form a micelle, which involves the hydrophilic parts being exposed to water and shielding the hydrophobic parts. Non-polar substances will not dissolve in water and form an unfavorable interaction with water. The hydrocarbon backbone of many organic molecules must form micelles to be dissolved in water and decrease the entropy of water, which is considered an unfavorable process. In humans and animals made from cells surrounded by lipids, water needs to be a part of this system. One typical micelle found in nature is the lipid bilayer of the plasma membrane of the cell. There are polar heads on the phospholipids that make up the lipid bilayer that shield the lipid portions of the bilayer. Fat-soluble molecules can get inside the membrane but ionic compounds cannot. When enzymes and other organic molecules are present in solution, sometimes the water molecules form an ordered structure that is separate from the enzymes and the molecules they work on for the interaction to occur. This increases the disorder of the molecules, increasing the entropy and allowing the process that is to occur between the enzyme and its substrate to become more favorable. This forms an enzyme-substrate complex that starts the enzymatic process. This is considered an example of “desolvation.” Water is not just the solvent the human body uses as the main thing biomolecules are dissolved in. It plays a significant role in many biochemical reactions inside the body. One specific example is the transformation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP), which is how the human body gets its energy. There is a specific condensation that