Solutions Part 7
In dissolving the salt, water molecules surround ions that make up the salt, holding them in place through the electrical attraction of opposite charges. Due to the differences in electronegativity that give the water molecule its high polarity, the hydrogen atoms are more positively charged, and thus these attract the negatively charged chlorine ion. Similarly, the negatively charged oxygen end of the water molecule attracts the positively charged sodium ion. As a result, the salt is “surrounded” or dissolved.
PLASMA AND AQUEOUS-SOLUTION REACTIONS IN THE BODY.
Whereas saltwater is an aqueous solution that can eventually kill someone who drinks it, plasma is an essential component of the life process—and in fact, it contains a small quantity of sodium chloride. Not to be confused with the phase of matter also called plasma, this plasma is the liquid portion of blood. Blood itself is about 55% plasma, with red and white blood cells and platelets suspended in it.
Plasma is in turn approximately 90% water, with a variety of other substances suspended or dissolved in it. Plasma also contains ions, which prevent red blood cells from taking up excess water in osmosis. Prominent among these ions are those of salt, or sodium chloride. Plasma also transports nutrients such as amino acids, glucose, and fatty acids, as well as waste products such as urea and uric acid, which it passes on to the kidneys for excretion.
Much of the activity that sustains life in the body of a living organism can be characterized as a chemical reaction in an aqueous solution. When we breathe in oxygen, it is taken to the lungs and fed into the bloodstream, where it associates with the iron-containing hemoglobin in red blood cells and is transported to the organs. In the stomach, various aqueous-solution reactions process food, turning part of it into fuel that the blood carries to the cells, where the oxygen engages in complex reactions with nutrients.
Aqueous-solution reactions can lead to the formation of a solid, as when a solution of potassium chromate (K 2 CrO 4 ) is added to an aqueous solution of barium nitrate (Ba[NO 3 ] 2 to form solid barium chromate (BaCrO 4 ) and a solution of potassium nitrate (KNO 3 ). This reaction is described in the essay on Chemical Reactions.
We have primarily discussed liquid solutions, and in particular aqueous solutions. The air we breathe is a solution, not a compound: in other words, there is no such thing as an “air molecule.” Instead, it is made up of diatomic elements (those in which two atoms join to form a molecule of a single element); monatomic elements (those elements that exist as single atoms); one element in a triatomic molecule; and two compounds.
