Chapter 13: Solutions – Introductory Chemistry Study Notes

Solutions

Definition and Types

A solution is a homogeneous mixture of two or more substances. The solvent is the majority component, and the solute is the minority component. Water is the most common solvent, forming aqueous solutions. Solutions can be composed of various combinations: gas-liquid, liquid-liquid, solid-gas, etc.

• Homogeneous mixture: Uniform composition throughout.

• Solvent: The substance present in greater amount.

• Solute: The substance present in lesser amount.

• Aqueous solution: Solution where water is the solvent.

• Example: Ocean water (salt dissolved in water), blood plasma, soda pop (CO2 dissolved in water).

Solubility and Saturation

Solubility Concepts

Solubility is the amount of solute (usually in grams) that dissolves in a given amount of solvent at a specific temperature. A saturated solution contains the maximum amount of solute under given conditions. An unsaturated solution contains less than the maximum, and a supersaturated solution contains more than the normal maximum amount of dissolved solute.

• Saturated solution: No more solute can dissolve.

• Unsaturated solution: More solute can dissolve.

• Supersaturated solution: Holds more solute than normally possible; excess solute may precipitate.

• Example: Soda pop becomes supersaturated when opened, releasing CO2 gas.

How Solids Dissolve in Water

Solvent-Solute Interactions

When a solid is placed in water, the attractive forces between water molecules and the particles of the solid compete with the forces holding the solid together. If the solvent–solute attractions are stronger, the solid dissolves.

• Solvent–solute attraction: Water molecules surround and disperse ions or molecules.

• Example: Sodium chloride (NaCl) dissolves as water dipoles attract Na+ and Cl− ions.

Electrolyte and Nonelectrolyte Solutions

Electrical Conductivity

Electrolyte solutions contain dissolved ions and conduct electricity. Nonelectrolyte solutions contain dissolved molecules and do not conduct electricity.

• Electrolyte: NaCl solution (ions present).

• Nonelectrolyte: Sugar solution (molecules present).

Solubility Varies with Temperature

Temperature Effects

The solubility of solids in water generally increases with increasing temperature. For example, more potassium nitrate dissolves at higher temperatures.

• Solids: Solubility increases with temperature.

• Example: KNO3 solubility rises from 30 g/100 g water at 20°C to 80 g/100 g water at 50°C.

Gas Solubility in Water

Temperature and Pressure Dependence

The solubility of gases in liquids decreases with increasing temperature but increases with increasing pressure. This is described by Henry’s law: the higher the pressure of a gas above a liquid, the more soluble the gas is in the liquid.

• Temperature: Warm soda pop fizzes more because CO2 solubility decreases with temperature.

• Pressure: Soda cans are pressurized to keep CO2 dissolved.

Solution Concentration

Mass Percent

Mass percent is the number of grams of solute per 100 g of solution. It is calculated as:

• Formula: $\text{Mass percent} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

• Other units: Parts per million (ppm), parts per billion (ppb).

• Example: Lake Nyos water contains 8.5% CO2 by mass.

Molarity

Molarity (M) is the number of moles of solute per liter of solution. It is used to quantify concentration in chemical reactions and laboratory preparations.

• Formula: $\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Preparation: Add solute to flask, add water to desired volume.

Using Molarity in Calculations

Ion Concentration

For ionic compounds, the concentration of individual ions can be determined from the overall molarity and the chemical formula. For example, a 1.0 M CaCl2 solution contains 1.0 mol/L Ca2+ and 2.0 mol/L Cl−.

• Complete dissociation: Assumes all formula units dissociate into ions.

• Actual concentration: May be slightly less due to ion pairing.

Solution Dilution

Dilution Equation

To dilute a stock solution to a desired concentration, use the equation:

• Formula: $M_1 V_1 = M_2 V_2$

• Stock solution: Highly concentrated solution used for preparation.

• Example: Preparing 5.00 L of 1.50 M KCl from 12.0 M stock.

Solution Stoichiometry

Stoichiometric Calculations

In reactions involving solutions, use volume and concentration to calculate moles, then use stoichiometric coefficients to convert between reactants and products.

• General solution map: Volume A → Moles A → Moles B → Volume B

• Example: Neutralizing H2SO4 with NaOH.

Colligative Properties

Freezing Point Depression and Boiling Point Elevation

Adding a nonvolatile solute to a liquid lowers its freezing point and raises its boiling point. These effects depend only on the number of solute particles, not their identity, and are called colligative properties.

• Freezing point depression: Solution freezes at lower temperature.

• Boiling point elevation: Solution boils at higher temperature.

• Formula for freezing point depression: $\Delta T_f = m \cdot K_f$

• Formula for boiling point elevation: $\Delta T_b = m \cdot K_b$

Osmosis and Osmotic Pressure

Osmosis

Osmosis is the flow of solvent from a less concentrated solution to a more concentrated solution through a semipermeable membrane. This process is also a colligative property.

• Osmotic pressure: The pressure required to stop osmotic flow.

• Biological relevance: Cell membranes act as semipermeable membranes.

• Example: Seawater draws water out of bodily tissues, promoting dehydration.

Chemistry and Health: Solutions in Medicine

Medical Applications

Solutions administered to patients must have controlled osmotic pressure. Hypoosmotic solutions pump water into cells, hyperosmotic solutions draw water out, and isoosmotic solutions match the osmotic pressure of bodily fluids.

• IV solutions: Usually isosmotic saline (0.9 g NaCl/100 mL solution).

Summary Table: Common Types of Solutions

Solvent	Solute	Example
Liquid	Solid	Salt in water
Liquid	Gas	CO2 in soda pop
Liquid	Liquid	Alcohol in water
Gas	Gas	Air (O2 in N2)

Summary Table: Common Laboratory Solvents

Solvent	Polarity	Common Use
Water	Polar	Aqueous solutions
Acetone	Polar	Organic reactions
Hexane	Nonpolar	Nonpolar solutes

Key Equations

• Mass percent: $\text{Mass percent} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

• Molarity: $\text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Dilution: $M_1 V_1 = M_2 V_2$

• Molality: $m = \frac{\text{moles of solute}}{\text{kg of solvent}}$

• Freezing point depression: $\Delta T_f = m \cdot K_f$

• Boiling point elevation: $\Delta T_b = m \cdot K_b$

Learning Objectives

• Define solution, solute, and solvent.

• Relate solubility of solids in water to temperature.

• Relate solubility of gases in liquids to temperature and pressure.

• Calculate mass percent, molarity, and molality.

• Use dilution equation in calculations.

• Apply stoichiometry to solution reactions.

• Calculate freezing and boiling points for solutions.

• Explain osmosis and its applications.