Mixtures & Physical Changes

• Mixtures are naturally occurring and must often be separated to collect useful or precious chemicals.

Mixtures and Physical Changes

• A mixture consists of two or more substances physically combined.
• Mixtures contain combinations of gases, liquids, and solids.
• Mixtures are separated through physical means.
• Separation of a mixture involves a physical change.
• A physical change alters physical properties without changing chemical composition.
• Physical changes do not involve the breaking or forming of chemical bonds.
• Examples of physical changes:
  • Changing states of matter
  • Dissolving or precipitating from a solution
  • Physically separating mixture components

Physical Properties in Separation

• Separation techniques consider physical properties:
  • Particle size
  • Density
  • Melting/boiling points
  • Solubility
  • Magnetism
  • Electrostatic attraction
• Pure substances in a mixture retain distinct physical properties that allow for separation.

Sieving

• Sieving separates particles by size.
• Smaller particles pass through the sieve's holes while larger particles are retained.
• Example: Sieving separates gold coins from sand, where the sieve holes are sized to retain the coins but allow sand to pass through.
• Heterogeneous mixture: variable composition and properties.
• Homogeneous mixture: uniform composition and properties.

Density-Based Separation Techniques

• Density is mass per unit volume, measured in kg/m³ or g/cm³.
• Density = Mass/Volume
• Mass/weight differs from density.
• Example: 1 kg of textbooks and 1 kg of feathers have the same mass, but feathers have lower density due to their larger volume.
• Several separation techniques rely on density differences:
  • Sedimentation
  • Decantation
  • Centrifugation
  • Separation funnels

Sedimentation

• Sedimentation allows insoluble particles in mixtures to settle out from liquids or gases, often with the mixture stationary.
• Denser solids that cannot dissolve fall to the bottom.

Decantation

• Decantation involves pouring a liquid from above settled insoluble particles.
• The liquid is poured off, leaving the denser solid behind.

Miscibility and Immiscibility

• Miscibility: The ability of a liquid solute to dissolve in a liquid solvent.
• Miscible liquids mix in any proportion to form a homogeneous mixture.
• Immiscibility: When two substances cannot dissolve in each other and form two distinct layers, creating a heterogeneous mixture.
• Example: Ethanol and water are miscible, while oil and water are immiscible.

Centrifugation

• Centrifugation rapidly spins a mixture to separate substances of different densities.
• Solids and liquids, or immiscible liquids, separate based on density, with denser substances moving to the bottom.
• Example: Separating milk into layers to collect cream.

Separation Funnels

• Separation funnels separate two immiscible liquids of different densities.
• The mixture is poured into the funnel, shaken, and allowed to settle.
• The denser liquid is drained from the bottom, leaving the less dense liquid in the funnel.
• Example: Separating water from lamp oil, as they are immiscible.