01.06 Mixtures of Substances and Diffusion

Introduction to Mixtures

• Definition: A mixture consists of two or more substances combined physically, not chemically. The individual substances retain their own properties and can be separated by physical means.
• States of Matter in Mixtures: Mixtures can involve any combination of solids, liquids, and gases.

Types of Mixtures

1. Solutions
2. Suspensions

1. Solutions

Definition: A solution is a homogeneous mixture where one substance (solute) is completely dissolved in another (solvent), resulting in a single phase.

Components of a Solution:

• Solute: The substance that is dissolved (e.g., salt in saltwater).
• Solvent: The substance that dissolves the solute (commonly a liquid like water).

Example:

• Saltwater: Solid salt (solute) dissolves in liquid water (solvent) to form a salt solution (Figure 1.18).

Characteristics of Solutions:

• Homogeneous: Uniform composition throughout.
• Invisible Particles: Solute particles are dispersed at the molecular level and cannot be seen.
• Single Phase: Exists entirely as one phase (solid, liquid, or gas).

Common Solvents:

• Water: The most common solvent, often referred to as the “universal solvent”.
• Organic Solvents: Used in organic chemistry (e.g., ethanol, acetone).

Diagram Reference:

• Figure 1.18: Illustrates how solute particles are completely dispersed in the solvent, making the solute invisible.

Example of Formation:

• Dissolving Sugar in Tea: Sugar (solute) dissolves in tea (solvent) forming a homogeneous sweetened tea solution.

2. Suspensions

Definition: A suspension is a heterogeneous mixture where small particles of a solid are dispersed throughout a liquid but do not dissolve. The particles remain separate and can eventually settle out.

Characteristics of Suspensions:

• Heterogeneous: Non-uniform composition; different parts can be distinguished.
• Visible Particles: Solid particles or liquid droplets are visible to the naked eye or under a microscope.
• Multiple Phases: Exists as more than one phase (solid and liquid).

Example:

• Sediment in Water: Fine particles of clay suspended in water, visible and can settle over time.

Applications:

• Precipitation Reactions: Often produce suspensions, such as when insoluble salts form after mixing solutions of soluble salts (Chapters 12 and 22).

Example of Formation:

• Mud in Water: When soil is mixed with water, the soil particles form a suspension.

Key Terms

Physical Separation Methods

Mixtures can be separated using physical methods based on the properties of their components:

1. Evaporation: Separates a solute from a solvent by heating (e.g., obtaining salt from saltwater).
2. Filtration: Separates suspended solids from liquids (e.g., filtering sand from water).
3. Distillation: Separates components based on different boiling points (e.g., separating alcohol from water).
4. Decanting: Separates liquid from solid by pouring off the liquid.
5. Chromatography: Separates components based on their movement through a medium.

Example:

• Making Sea Salt: Evaporate seawater to remove water, leaving behind salt crystals.

Diffusion in Gases and Liquids

Definition: Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached.

Factors Affecting Diffusion:

• State of Matter: Diffusion occurs faster in gases, slower in liquids, and slowest in solids.
• Temperature: Higher temperatures increase the rate of diffusion.
• Concentration Gradient: A steeper gradient (greater difference in concentration) increases the rate of diffusion.
• Molecular Size: Smaller particles diffuse faster than larger ones.

Examples:

• Gas Exchange in Lungs: Oxygen diffuses from alveoli into blood; carbon dioxide diffuses out.
• Perfume in Air: The scent spreads through the room as perfume molecules diffuse in the air.
• Tea Coloring: Adding a drop of food coloring to tea demonstrates diffusion as the color spreads without stirring.

Endothermic and Exothermic Processes

Endothermic Reactions:

• Definition: Absorb heat from the surroundings.
• Sign: ΔH is positive.
• Example: Dissolving ammonium nitrate in water (used in cold packs).

Exothermic Reactions:

• Definition: Release heat into the surroundings.
• Sign: ΔH is negative.
• Example: Combustion of fuel (e.g., burning of methane).

Summary

• Mixtures are combinations of substances not chemically bonded and can be separated physically.
• Solutions are homogeneous mixtures with solutes completely dissolved in solvents.
• Suspensions are heterogeneous mixtures with visible dispersed particles that can settle.
• Diffusion is the movement of particles from high to low concentration, influenced by various factors.
• Endothermic and exothermic processes describe whether a reaction absorbs or releases heat.

Study Tips

• Understand Definitions: Make sure you can define and differentiate between mixtures, solutions, and suspensions.
• Use Examples: Relate concepts to everyday examples to better grasp their applications.
• Practice Separation Techniques: Familiarize yourself with different methods to separate mixtures and know when to use each.
• Visualize Processes: Draw diagrams like Figure 1.18 to visualize how solutes and solvents interact.
• Memorize Key Terms: Use flashcards to remember key vocabulary and their definitions.

Practice Questions

1. Identify the Mixture:
   • Is a salad a solution or a suspension? Explain your answer.
2. Diffusion Scenario:
   • Describe what happens when you spray perfume in one corner of a room.
3. Separation Technique:
   • Which physical separation method would you use to separate sand from saltwater? Why?
4. Endothermic vs. Exothermic:
   • Give an example of an endothermic reaction and explain why it is classified as such.

Answers to Practice Questions

1. Salad is a suspension because the different components (vegetables, dressing) remain separate and can be distinguished from one another.
2. When you spray perfume in one corner of a room, diffusion occurs as perfume molecules move from the area of high concentration (spray) to areas of lower concentration, spreading the scent throughout the room.
3. To separate sand from saltwater, you would use filtration to remove the sand particles from the liquid, followed by evaporation to obtain the salt from the water.
4. Dissolving ammonium nitrate in water is an endothermic reaction because it absorbs heat from the surroundings, resulting in a cooling effect.