9.2 Solutions, Concentration & Solubility
Key Takeaways
- A solution is a homogeneous mixture composed of a solute dissolved uniformly within a solvent, with water serving as the primary biological universal solvent.
- Solutions are classified by solute load into unsaturated, saturated (at dynamic equilibrium), and supersaturated (metastable, holding excess solute).
- Temperature increases solid solubility in liquids but decreases gas solubility, whereas pressure increases gas solubility in direct proportion according to Henry's Law (C = k * P).
- Molarity (M = moles of solute / Liters of solution) is the standard chemical unit of concentration, while dilution calculations utilize the conservation formula M1V1 = M2V2.
- Tonicity describes the osmotic effect of a solution on cell volume: hypertonic solutions cause cell shrinkage (crenation), hypotonic solutions cause cell swelling (lysis), and isotonic solutions maintain stable cell volume.
9.2 Solutions, Concentration & Solubility
Solutions play a central role in biological systems, human physiology, and laboratory chemistry. From cellular cytoplasm to human blood plasma, chemical reactions in living organisms occur within aqueous solution environments. For the TEAS 7 exam, students must master solution components, solubility parameters, concentration units, dilution mathematics, and osmotic properties.
Solution Components & Solvation
A solution is a homogeneous mixture consisting of two or more substances blended uniformly at the molecular or ionic level. Solutions consist of two primary parts:
- Solute: The substance that is dissolved and present in the smaller amount (e.g., sodium chloride salt, glucose, or oxygen gas).
- Solvent: The dissolving medium present in the larger amount (e.g., water, ethanol, or benzene).
When water serves as the solvent, the mixture is called an aqueous solution ($aq$). Water is often called the "universal solvent" because its strong polar bent structure and hydrogen-bonding capability allow it to dissolve a wide range of ionic compounds and polar covalent molecules.
The Solvation Process
During solvation (or hydration when water is the solvent), solvent molecules surround individual solute particles. Polar water molecules orient their partially negative oxygen atoms toward positive cations (like $Na^+$) and their partially positive hydrogen atoms toward negative anions (like $Cl^-$). This electrostatic attraction pulls the ions away from the crystal lattice, dispersing them evenly throughout the liquid.
Degrees of Solute Saturation
Solutions are classified based on the quantity of dissolved solute relative to the maximum capacity of the solvent at a given temperature and pressure:
- Unsaturated Solution: Contains less dissolved solute than the solvent's maximum capacity at a specific temperature. If additional solute is added, it will readily dissolve.
- Saturated Solution: Contains the maximum amount of dissolved solute that can remain in stable equilibrium with undissolved solute at a given temperature. At saturation, the rate of dissolution equals the rate of crystallization (dynamic equilibrium):
- Supersaturated Solution: An unstable, metastable condition where a solution contains more dissolved solute than a saturated solution at the same temperature. Supersaturated solutions are prepared by dissolving solute at high temperatures and cooling the mixture slowly without disturbance. Introducing a single "seed crystal" or mechanical shock triggers rapid crystallization of the excess solute.
Factors Affecting Solubility & Henry's Law
Solubility is the quantitative measure of the maximum amount of solute that dissolves in a given quantity of solvent at a specified temperature and pressure.
Temperature Effects
- Solid Solutes in Liquids: For most solid solutes, solubility increases as temperature rises. Higher thermal energy increases solvent molecular motion, helping break lattice interactions.
- Gaseous Solutes in Liquids: For all gases, solubility decreases as temperature rises. As thermal energy increases, dissolved gas molecules gain kinetic energy, escape from the liquid phase, and re-enter the gas phase above the solution (e.g., warm soda loses dissolved $CO_2$ rapidly and goes flat).
Pressure Effects & Henry's Law
Pressure changes have virtually no effect on solid or liquid solubility, but profoundly impact gas solubility in liquids. Henry's Law states that at a constant temperature, the solubility ($C$) of a gas in a liquid is directly proportional to the partial pressure ($P$) of that gas above the liquid surface:
where $k$ is Henry's Law constant. When pressure increases, more gas molecules are forced into contact with the liquid surface per second, driving gas dissolution. In human physiology, Henry's Law explains respiratory gas exchange: high oxygen partial pressure in pulmonary alveoli forces $O_2$ to dissolve into pulmonary capillary blood.
"Like Dissolves Like" Principle
Solubility depends strongly on chemical polarity:
- Polar solvents (like water) dissolve polar covalent solutes (like sugar) and ionic solutes (like $NaCl$).
- Nonpolar solvents (like hexane or oils) dissolve nonpolar solutes (like lipids, grease, and hydrocarbons).
- Polar and nonpolar substances (like oil and water) are immiscible (they separate into distinct layers).
Concentration Units & Dilution Mathematics
Concentration expresses the relative ratio of solute to solvent or total solution.
Molarity ($M$)
Molarity is the most common chemical concentration unit, defined as the number of moles of solute per liter of total solution:
TEAS Calculation Tip: Always convert solution volume from milliliters ($mL$) to liters ($L$) by dividing by 1,000 before calculating molarity ($500\text{ mL} = 0.50\text{ L}$).
Mass Percent $(% w/w)$
Mass percent expresses solute mass as a percentage of total solution mass:
Dilution Equation ($M_1V_1 = M_2V_2$)
During a dilution, solvent is added to a concentrated stock solution. While the total volume increases and molarity decreases, the total moles of solute remain constant. This leads to the fundamental dilution equation:
where $M_1$ and $V_1$ represent initial stock molarity and volume, and $M_2$ and $V_2$ represent final diluted molarity and volume.
Osmotic Pressure & Tonicity
Osmosis is the net movement of water across a semipermeable membrane from a region of lower solute concentration (higher water concentration) toward a region of higher solute concentration (lower solute concentration). Osmotic pressure ($\Pi$) is the minimum external pressure required to prevent this net water flow.
Tonicity and Cellular Responses
Tonicity describes the capability of an extracellular solution to modify cellular volume by altering water movement across the cell membrane.
| Solution Type | Solute Concentration vs. Cell | Net Water Movement | Effect on Red Blood Cells |
|---|---|---|---|
| Hypertonic | Higher solute outside cell | Water leaves cell | Cell shrinks and shrivels (crenation) |
| Hypotonic | Lower solute outside cell | Water enters cell | Cell swells and bursts (hemolysis / lysis) |
| Isotonic | Equal solute inside and outside | No net water movement | Cell maintains normal shape and volume |
In clinical care, intravenous fluids must match blood tonicity. Normal saline ($0.9%\text{ NaCl}$) is isotonic to human red blood cells, ensuring stable fluid balance.
According to Henry's Law, what happens to the solubility of a gas dissolved in a liquid when the partial pressure of that gas above the liquid is doubled at constant temperature?
What is the molarity of a solution prepared by dissolving 0.50 moles of sodium chloride (NaCl) in enough distilled water to yield a total solution volume of 250 mL?
If human red blood cells are placed into a hypotonic solution, what net movement of water occurs across the membrane and what is the biological outcome?