MCAT Chemistry > Solutions
Solutions are homogeneous mixtures of 2+ substances in a single phase. Solutions are greatly imported in biochemistry because the human body is made up of 2/3 water in terms of total weight. Solutions consist of the solvent or the medium in which atoms and ions (NaCl/salt) or other molecules such as sugar dissolve into. These types of substances introduced into the solvent are known as the solute.
Within the human body most substances are dissolved within water. This means that most chemical reactions take place within the medium or solvent of water. Water helps absorb and retain the heat produced from the reactions and also provides lubrication. In solutions ions undergo dissociation and become known as electrolytes which conduct electricity within a solution. Polar molecules are responsible for forming hydration spheres around ions and polar molecules which keep them within the solution.
|Molecule or Compound||Osmotically Active Compounds|
|NaCl||Na+ + Cl-|
|(NH4)2SO4||NH4+ + NH4+ + SO42-|
Atoms that are linked through chemical bonds are polar and interact with water through hydrogen bonds. This means they are also known as hydrophilic and are molecules such as sugars that "love water" as it translates to.
Dissociated atoms or ions also interact with water and are linked with non-polar covalent bonds. These do not interact with water in contrast and will repel water. These compounds are known as hydrophobic (water fearing) and examples of them are oil.
Other biological molecules such as fatty acids are both hydrophobic and hydrophilic at the same time.
Law of Mass Action
AmBn ←→ mAn+ + nBm-
ksp = [An+]m[Bn-]n
Solubility product constants, like all constants are temperature dependent. The solubility product increases with increasing temperature for a non-gas solution, and inversely decreases for a gas solute. This is because higher pressures favor dissolution of gas solutes; therefore the constant will be larger for gases at higher pressure than lower ones.
Ion Product is used to determine where a solution is with respect to equilibrium
Q = I.P. = [An+]m[Bn-]n
- If I.P < keq then solution is unsaturated and not at equilibrium. For unsaturated solutions, dissolution is thermodynamically favored over precipitation.
- If I.P > keq then solution is super-saturated which is only possible through extreme heating and rapid cooling. The product is thermodynamically unstable and could easily fall out
- If I.P = keq, then equilibrium is achieved. The k of dissolution is equal to the k of precipitation.
Common Ion Effect is a reduction in the molar solubility of a molecule due to being dissolved in a solution that already contains common ionic constituents. It is basically Le Chatelier's Principle, which says that adding more reactants will shift the graph to the left (towards solid salts), as solubility is decreased and precipitation favored over dissolution.