Chapter 9: Solutions – Structure, Properties, and Biological Relevance

Solutions

Definition and Components

Solutions are homogeneous mixtures composed of two or more substances. They form when there is sufficient attraction between solute and solvent molecules. The solvent is present in a much larger amount, while the solute is present in a smaller amount.

• Solute: The substance present in lesser amount.

• Solvent: The substance present in greater amount.

• Solutes may be liquids, gases, or solids and are evenly distributed throughout the solution.

• Solutes mix with solvents so that both have the same physical state.

• Solutes cannot be separated by filtration but can be separated by evaporation.

• Solutes are not visible but can impart color to the solution.

Types of Solutes and Solvents

Solutes and solvents can be solids, liquids, or gases. The combination of these states leads to various types of solutions.

Type	Example	Primary Solute	Solvent
Gas in gas	Air	Oxygen (gas)	Nitrogen (gas)
Gas in liquid	Soda water	Carbon dioxide (gas)	Water (liquid)
Liquid in liquid	Vinegar	Acetic acid (liquid)	Water (liquid)
Solid in liquid	Seawater	Sodium chloride (solid)	Water (liquid)
Solid in solid	Brass	Zinc (solid)	Copper (solid)

Water as a Solvent

Water is one of the most common solvents in nature. It is a polar molecule due to polar O—H bonds, and its molecules form hydrogen bonds, which are important in many biological compounds.

• Hydrogen bonds occur between the partial negative charge on oxygen and partial positive charge on hydrogen.

Formation of Solutions: Like Dissolves Like

Solutions form when solute–solvent interactions are strong enough to overcome solute–solute and solvent–solvent interactions. The principle "like dissolves like" means that solutes and solvents with similar polarities will form solutions.

Solutions Will Form	Solutions Will Not Form
Polar solute + Polar solvent	Polar solute + Nonpolar solvent
Nonpolar solute + Nonpolar solvent	Nonpolar solute + Polar solvent

Solutions with Ionic and Polar Solutes

Ionic compounds such as NaCl dissolve in water through hydration, where water molecules surround each ion and pull it into solution. Polar molecular compounds, like methanol (CH3OH), are soluble in water due to their ability to form hydrogen bonds with water.

• Ionic solutes require polar solvents for dissolution.

• Polar solutes require polar solvents for a solution to form.

Electrolytes and Nonelectrolytes

Electrolytes in the Body

Electrolytes such as sodium, potassium, chloride, and bicarbonate are crucial for maintaining proper cell and organ function. Their concentrations can be measured in blood tests.

Strong, Weak, and Nonelectrolytes

• Strong electrolytes dissociate 100% in water, producing ions and forming solutions that conduct electricity strongly.

• Weak electrolytes dissociate only slightly, forming solutions with few ions and mostly undissociated molecules.

• Nonelectrolytes dissolve as molecules and do not produce ions or conduct electricity.

Type of Solute	In Solution	Particles in Solution	Conducts Electricity	Examples
Strong electrolyte	Dissociation completely	Ions	Yes	NaCl, KBr, NaNO3
Weak electrolyte	Dissociation partially	Mostly molecules, few ions	Weakly	HF, H2O, NH3
Nonelectrolyte	No ionization	Molecules only	No	CH3OH, C12H22O11

Equivalents and Milliequivalents

An equivalent (Eq) is the amount of an electrolyte or ion that provides 1 mole of electrical charge. The charge of positive ions is always balanced by the charge of negative ions in solution. Concentrations are often expressed in milliequivalents per liter (mEq/L).

Ion	Ionic Charge	Number of Equivalents in 1 Mole
Na+	1+	1 Eq
Ca2+	2+	2 Eq
Fe3+	3+	3 Eq
Cl-	1-	1 Eq
SO42-	2-	2 Eq

Calculating Equivalents

To convert between equivalents and moles, use the following relationships:

• $0.50\ L \times \frac{8.8\ mEq\ Ca^{2+}}{1\ L} \times \frac{1\ Eq\ Ca^{2+}}{1000\ mEq\ Ca^{2+}} = 0.0044\ Eq\ Ca^{2+}$

• $0.0044\ Eq\ Ca^{2+} \times \frac{1\ mole\ Ca^{2+}}{2\ Eq\ Ca^{2+}} = 0.0022\ mole\ Ca^{2+}$

Electrolytes in Body Fluids and IV Solutions

Electrolyte concentrations in blood plasma and intravenous solutions are carefully regulated to maintain health.

Electrolyte	Concentration (mEq/L)
Na+	138
K+	5
Mg2+	3
Ca2+	4
Cl-	110
HCO3-	30
HPO42-	4
Proteins	6

Solubility

Definition and Factors Affecting Solubility

Solubility is the maximum amount of solute that dissolves in a specific amount of solvent. It is temperature sensitive and is usually expressed as grams of solute per 100 grams of solvent.

Unsaturated and Saturated Solutions

• Unsaturated solutions contain less than the maximum amount of solute and can dissolve more solute.

• Saturated solutions contain the maximum amount of solute and have undissolved solute at the bottom. Dissolving and recrystallization occur in equilibrium.

Effect of Temperature and Pressure on Solubility

Solubility of most solids increases with temperature, while solubility of gases decreases as temperature increases. According to Henry’s law, the solubility of a gas in a liquid is directly related to the pressure of that gas above the liquid.

Solubility Rules for Ionic Compounds

Not all ionic compounds are soluble in water. Solubility rules help predict which compounds will dissolve.

Soluble If It Contains	Insoluble If It Contains
Li+, Na+, K+	None
NO3-, C2H3O2-	None
Cl-, Br-, I-	Ag+, Pb2+, Hg22+
SO42-	Ba2+, Pb2+, Ca2+, Sr2+, CO32-, PO43-, OH-

Concentration of a Solution

Units of Concentration

Concentration is the amount of solute in a given amount of solution. Common units include mass percent (m/m), volume percent (v/v), mass/volume percent (m/v), and molarity.

Mass Percent

Mass percent (% m/m) is the concentration by mass of solute in a solution, calculated as grams of solute per 100 grams of solution.

• $\text{mass percent (m/m)} = \frac{\text{g of solute}}{\text{g of solute} + \text{g of solvent}} \times 100\%$

• $\text{mass percent} = \frac{\text{g of solute}}{100\text{ g of solution}}$

Volume Percent

Volume percent (% v/v) is the percent volume of solute (liquid) to volume of solution.

• $\text{volume percent (v/v)} = \frac{\text{mL of solute}}{100\text{ mL of solution}}$

• $\text{volume percent (v/v)} = \frac{\text{mL of solute}}{\text{mL of solution}} \times 100\%$

Mass/Volume Percent

Mass/volume percent (% m/v) is the percent mass (g) of solute to volume (mL) of solution.

• $\text{mass/volume percent (m/v)} = \frac{\text{g of solute}}{100\text{ mL of solution}}$

• $\text{mass/volume percent (m/v)} = \frac{\text{g of solute}}{\text{mL of solution}} \times 100\%$

Properties of Solutions

Solutions, Colloids, and Suspensions

Mixtures can be classified as solutions, colloids, or suspensions based on particle size and behavior.

• Solutions: Transparent, do not separate, contain small particles that pass through semipermeable membranes.

• Colloids: Medium-size particles, cannot be filtered, can be separated by semipermeable membranes.

• Suspensions: Heterogeneous mixtures with large particles that settle out and can be filtered.

Osmosis and Osmotic Pressure

Osmosis is the flow of water from a lower to a higher solute concentration across a semipermeable membrane. Osmotic pressure is the pressure required to prevent the flow of additional water into the more concentrated solution and increases with the number of dissolved particles.

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic solution: Exerts the same osmotic pressure as body fluids; RBCs retain normal volume.

• Hypotonic solution: Lower solute concentration than RBCs; water flows into cells, causing swelling and hemolysis.

• Hypertonic solution: Higher solute concentration than RBCs; water leaves cells, causing shrinkage (crenation).

Dialysis

In dialysis, solvent and small solute particles pass through an artificial membrane, while large particles are retained. Hemodialysis is used to remove waste particles such as urea from blood.

Additional info: These notes cover all major aspects of Chapter 9: Solutions, including solution formation, types, properties, concentration units, solubility, electrolytes, and biological relevance. All included images directly reinforce the adjacent explanations and are strictly relevant to the content.