Solutions, Solubility, and Polarity

Solutions, Solubility, and Polarity

HMH Chapter 4.3 Updated 2026

Mixture Classification

  • Matter:

    • Is it a homogeneous mixture or a pure substance?

    • Pure substance:

      • Can it be simplified chemically? Yes

      • Element: Contains only one type of atom.

      • Compound: Can also be simplified chemically into elements.

    • Mixture:

      • Is it uniform throughout? No: Heterogeneous mixture.

      • Is it uniform throughout? Yes: Homogeneous mixture.

Solutions Basics

  • Homogeneous Mixtures: Uniform throughout.

  • Terms Defined:

    • Solvent: The dissolving agent (e.g., water is referred to as the universal solvent).

    • Solute: The dissolved substance (e.g., salt or any ionic compound, could also be a polar covalent compound).

    • Key Phrase: "Like dissolves like".

    • Miscibility: The ability of two fluids to mix.

Common Terminology in Solutions

  • Soluble:

    • Substances that form a solution in water (e.g., table salt in water, sugar in water).

  • Insoluble:

    • Substances that do not form a solution (e.g., sand in gasoline).

  • Miscible:

    • Fluids that are soluble in each other, applies to either liquid in liquid or gas in gas (e.g., water and ethanol, gases in the atmosphere).

  • Immiscible:

    • Fluids that do not form a homogeneous mixture (e.g., oil and water).

Solutions and Electrolytes

  • Electrolytes:

    • Ions in solution that conduct electricity due to their charges.

    • Distinction between types of electrolytes:

    • Strong Electrolytes:

      • 100% dissociation in water (e.g., HCl, NaOH).

    • Weak Electrolytes:

      • Only partially dissociate in water (e.g., CH3COOH (acetic acid), NH3).

      • Discussed in relation to acids and bases study.

    • Non-electrolytes:

      • Compounds that do not ionize or dissociate (e.g., molecular compounds).

Solubility Concept

  • Solubility:

    • The maximum quantity of solute that will dissolve in a specified quantity of water at a specific temperature.

    • Usually measured using 100g of water.

  • Variables Affecting Solubility Rate:

    • Temperature, pressure, size of crystals, and vigor/duration of stirring.

    • Important note: There exists a maximum solubility limit; once reached, the solution becomes saturated.

Calculating Concentration

  • Concentration:

    • The amount of solute dissolved in a quantity of solvent or solution.

    • Common methods for calculating concentration include:

    • Mass/Volume: Measured in g/L.

    • % Composition: By mass or volume.

    • Molarity (M): Moles of solute per liter of solution - expressed as mol/L. Example: 2.5 M NaOH solution.

    • Molality (m): Moles of solute per kg of solvent - expressed as lowercase "m". Example: 3.7 m HCl solution.

    • Parts per Million (ppm) or Parts per Billion (ppb).

Concentration Calculations - Grams per Liter

  • Formula:
    ext{Grams per Liter (g/L)} = rac{ ext{Grams Solute}}{ ext{Liters of Solution}}

  • Example:

    • Calculate the concentration of 25g of NaCl in 500mL of solution.

    • Use the formula:
      ext{Grams per Liter (g/L)} = rac{25g}{0.5L}

Concentration Calculations - Percent Concentration

  • Formula:
    ext{Percent Concentration} = rac{ ext{Grams Solute}}{ ext{Grams of Solution}} imes 100

  • Example:

    • Calculate the % concentration of 17g of NaCl dissolved in 183g of water.

Concentration Calculations - Parts per Million

  • Formula:
    ext{ppm} = rac{ ext{Grams Solute}}{ ext{Grams of Solution}} imes 1,000,000

  • Example:

    • The EPA set the arsenic standard at 0.010 PPM. Calculate the ppm of 0.12g of Arsenic in 250g of solution to determine safety for drinking.

Concentration Calculations - Molarity

  • Formula:
    ext{Molarity (M)} = rac{ ext{Moles Solute}}{ ext{Liters of Solution}}

  • Example:

    • To digest food, the human body produces 5.6 grams of HCl for every 1.0 L of stomach acid solution. Calculate its molarity.

Concentration Calculations - Molality

  • Formula:
    ext{Molality (m)} = rac{ ext{Moles Solute}}{ ext{kg of Solvent}}

  • Example:

    • Ocean water has a percent concentration of 3.5% NaCl, indicating 3.5g of NaCl in 100g of solution. Calculate its molality, assuming the solution only consists of NaCl and water.

Solubility Review

  • Definition:

    • Maximum quantity of a solute that will dissolve in a defined quantity of water at a specific temperature (usually 100g).

  • Variables Influencing Dissolving Rate:

    • Temperature, pressure, size of crystals, stirring vigor/duration.

    • Saturation point: indicates maximum solubility achieved.

Solubility Curve

  • Solubility Curve: Illustrates how much solute dissolves in 100g of water at a certain temperature.

    • Saturated: Indicates a solution contains maximum solute possible.

    • Unsaturated: Can accommodate more solute.

    • Supersaturated: Contains more solute than solution typically accommodates.

    • Diagrams of both types often accompany descriptions.

Factors Influencing Solubility

  • Key Factors:

    • Not all compounds are soluble in water; solubility is influenced by properties beyond just temperature and pressure, particularly intermolecular forces.

    • The attractive forces between solute and solvent must surpass the solute's own intermolecular forces for dissolution to occur.

  • Precipitate: An insoluble solid forms and separates from a liquid solution, generally following a chemical reaction between two aqueous solutions.

Formation of Precipitate

  • Solubility Rules: These are empirical guidelines drawn from experimental observations, notably:

    • All common ionic compounds of alkali metals (Group 1), nitrates, and NH4^+ are soluble in water (NAG).

    • Sulfates, acetates, and Group 17 (except Fluorine) are typically soluble (SAG), except when combined with:

    • Lead (Pb)

    • Mercury (Hg)

    • Silver (Ag)

    • Calcium (Ca), Strontium (Sr), and Barium (Ba) (CaStroBear).

Solubility Mnemonic Techniques

  • Mnemonic for Solubility Rules:

    • NAGSAG for substances that are generally soluble:

    • Nitrate

    • Ammonium

    • Group 1 elements

    • Sulfates

    • Acetate

    • Group 17 elements (except fluorine)

  • Exceptions - Castro Bear:

    • Calcium (Ca²⁺)

    • Strontium (Sr²⁺)

    • Lead (Pb²⁺)

    • Mercury (Hg²⁺)

    • Barium (Ba²⁺)

Writing Precipitation Reaction Equations

  • Types of Reactions: Focus on double replacement reactions involving solids in aqueous solutions.

  • Equations Types:

    • Molecular: Represents the overall reaction with compounds (e.g.,
      ext{Pb(NO}3 ext{)}2(aq) + 2 ext{KI}(aq)
      ightarrow ext{PbI}2(s) + 2 ext{KNO}3(aq) ).

    • Complete Ionic: Represents all ions involved (e.g.,
      ext{Pb}^{2+}(aq) + 2 ext{NO}3^{-}(aq) + 2 ext{K}^{+}(aq) + 2 ext{I}^{-}(aq) ightarrow ext{PbI}2(s) + 2 ext{K}^{+}(aq) + 2 ext{NO}_3^{-}(aq) ).

    • Net Ionic: Eliminates spectator ions (e.g.,
      ext{Pb}^{2+}(aq) + 2 ext{I}^{-}(aq)
      ightarrow ext{PbI}_2(s) ).

    • Only the species actively participating in the reaction are shown.

    • If all components in the equation are soluble, it is classified as no reaction.