Solutions: Properties, Types, and Solubility (Chapter 9 Study Notes)

Chapter 9: Solutions

Introduction to Solutions

Solutions are homogeneous mixtures composed of two or more substances. They are fundamental in chemistry and biology, as most chemical and biochemical reactions occur in solution, especially in water.

• Solvent: The component present in the larger amount; dissolves the solute.

• Solute: The component present in the smaller amount; is dissolved by the solvent.

• Solution: The uniform mixture of solute(s) and solvent.

Example: In a saline solution, water is the solvent and NaCl is the solute.

Properties of Solutes and Solvents

• Solutes can be solid, liquid, or gas.

• Solutes are dispersed evenly throughout the solution.

• Solutes cannot be separated by filtration, but can be separated by evaporation or distillation.

• Solutes are not visible as particles but may impart color to the solution.

• Solvents are the major component and must have sufficient intermolecular interactions with the solute to dissolve it.

• Water is the most common solvent due to its polarity and ability to form hydrogen bonds.

Water as a Solvent

Water is a polar molecule with polar O–H bonds, making it an excellent solvent for many substances. Solute molecules interact with water via ion-dipole, hydrogen bonding, or dipole-dipole interactions.

• The human body is 45–75% water by mass; infants have about 74% water.

• Reactions in water are called aqueous reactions.

• The label (aq) indicates a substance is dissolved in water.

Types of Solutes and Solvents

Solutes and solvents may be solids, liquids, or gases. The following table summarizes common examples:

Type	Example	Primary Solute	Solvent
Gas in gas	Air	O2(g)	N2(g)
Gas in liquid	Soda water	CO2(g)	H2O(l)
Liquid in liquid	Vinegar	HC2H3O2(l)	H2O(l)
Solid in liquid	Seawater	NaCl(s)	H2O(l)
Solid in solid	Brass	Zn(s)	Cu(s)

Solubility and "Like Dissolves Like" Principle

Solutions form when the solute and solvent have similar polarities. This is summarized as "like dissolves like." Polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.

Solute	Solvent	Will Solution Form?
Polar	Polar	Yes
Nonpolar	Nonpolar	Yes
Polar	Nonpolar	No
Nonpolar	Polar	No

• Common polar solvents: water (H2O), acetone (CH3COCH3), ethanol (CH3CH2OH)

• Common nonpolar solvents: hexane (C6H14), toluene (C6H5CH3), carbon tetrachloride (CCl4)

Example: Octane (C8H18) dissolves in toluene (nonpolar), but not in water (polar).

Ionic Solutes in Water: Ion-Dipole Interactions

Ionic compounds (e.g., NaCl) dissolve in water via ion-dipole interactions. Water molecules surround and stabilize the separated ions, a process called hydration:

• $\mathrm{NaCl}(s) \xrightarrow{H_2O} \mathrm{Na}^+(aq) + \mathrm{Cl}^-(aq)$

• Polyatomic ions (e.g., NO3-, SO42-) remain intact when dissolved.

Dissociation of Ionic Compounds

When ionic compounds dissolve, they dissociate into their constituent ions. The following table summarizes the types and relative amounts of ions produced:

Ionic Compound	Types of Ions Produced	Relative Amounts (Cations)	Relative Amounts (Anions)
MgBr2	Mg2+, Br-	1 mole Mg2+	2 moles Br-
AlCl3	Al3+, Cl-	1 mole Al3+	3 moles Cl-
Na2SO4	Na+, SO42-	2 moles Na+	1 mole SO42-
K3PO4	K+, PO43-	3 moles K+	1 mole PO43-

Electrolytes and Non-Electrolytes

Solutes in solution are classified as electrolytes or non-electrolytes:

• Electrolytes: Dissociate into ions in water; solutions conduct electricity due to the presence of hydrated ions.

• Non-electrolytes: Do not dissociate into ions; solutions do not conduct electricity.

Examples of electrolytes: NaCl, KBr, HCl, H2SO4

Examples of non-electrolytes: Sucrose (C12H22O11), ethanol (CH3CH2OH), urea (H2NCONH2)

Strong vs. Weak Electrolytes

• Strong electrolytes: Dissociate completely (~100%) into ions (e.g., NaCl, HCl).

• Weak electrolytes: Dissociate only slightly; exist as a mixture of ions and molecules (e.g., acetic acid, NH3).

Equilibrium in weak electrolytes is represented by the double arrow ($\rightleftharpoons$).

Summary Table: Classification of Solutes in Aqueous Solutions

Type of Solute	In Solution	Type(s) of Particles	Conducts Electricity?	Examples
Strong electrolyte	Dissociates completely	Ions only	Yes	NaCl, KBr, HCl
Weak electrolyte	Ionizes partially	Molecules and ions	Weakly	HF, H2O, NH3, CH3COOH
Non-electrolyte	No ionization	Molecules only	No	C12H22O11, CH3OH

Solubility of Solutes in a Solvent

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

• Solubility of NaCl: 36 g per 100 g water at 25°C (very soluble)

• Solubility of CaCO3: ~15 mg per 100 g water at 25°C (not very soluble)

Unsaturated vs. Saturated Solutions

• Unsaturated solution: Contains less than the maximum amount of solute; more solute can dissolve.

• Saturated solution: Contains the maximum amount of solute; undissolved solute is present, and an equilibrium exists between dissolving and crystallizing solute.

Equilibrium process: solute + solvent $\rightleftharpoons$ saturated solution

Summary of Key Concepts

• Solutions are homogeneous mixtures of solute(s) and solvent.

• Water is the most common solvent due to its polarity and hydrogen bonding ability.

• "Like dissolves like": Polar solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.

• Ionic compounds dissolve in water via ion-dipole interactions, producing hydrated ions.

• Electrolytes conduct electricity in solution; non-electrolytes do not.

• Solubility depends on the nature of the solute and solvent, as well as temperature.

Example Equations:

• $\mathrm{NaCl}(s) \xrightarrow{H_2O} \mathrm{Na}^+(aq) + \mathrm{Cl}^-(aq)$

• $\mathrm{CaBr}_2(s) \xrightarrow{H_2O} \mathrm{Ca}^{2+}(aq) + 2\mathrm{Br}^-(aq)$