Chemical Substances, Mixtures, and Physical Separation Techniques

Pure Substances and Mixtures

Substances are formed by the same particles (either atoms or molecules). They cannot be decomposed into other substances by simple procedures. These include elements and compounds.

Mixtures are formed by two or more substances. Mixtures, whether homogeneous or heterogeneous, can always be separated by physical means such as filtration, decantation, distillation, or chromatography.

Separation Methods

Separation methods are generally categorized based on the type of mixture:

• Mechanical Methods: Most useful for heterogeneous mixtures.
• Thermal Methods: Appropriate for homogeneous mixtures.

Mechanical Separation Methods

Sedimentation

Widely used in mining to separate ore from gangue by gravity.

Settling (Decantation)

Used when the mixture is formed by liquids of different densities.

Centrifugation

Splits components by density differences, accelerating the process using a centrifuge, a container rotating at high speed.

Elution

Separation by density, where a stream carries away less dense materials.

Filtration

Allows the separation of solid particles from a liquid through sieves or filters.

Magnetic Separation

Used to separate magnetic mineral ores from gangue, and especially in garbage recycling to separate ferrous metal debris.

Chromatography

Involves dragging a mixture of substances in solution across a solid support, such as paper.

Thermal Separation Methods

Crystallization

If large crystals are desired, the solution is left in a shallow container, allowing the water to evaporate slowly over hours or days.

Evaporation to Dryness

If the size of the crystals is not important, the solution is evaporated completely. The substance will be obtained in powder form.

Distillation

Involves gently heating a mixture of several liquids until they reach their respective boiling points, allowing them to separate.

Fractional Distillation

Used for complex natural blends of organic products (often of industrial interest). For example, crude oil is heated to 400ºC in a distillation tower to separate its components.

Understanding Solutions and Solubility

Solutions are stable homogeneous mixtures.

Solute

The substance found in smaller proportion. This substance often changes state during the process.

Solvent

The component found in greater proportion.

Solubility

The maximum quantity that can dissolve in 100g of water (the solvent) at a given temperature.

Concentration Terms

• Diluted: The amount of solute is very small in relation to its solubility.
• Concentrated: The amount of solute is near the solubility limit.
• Saturated: The solution holds the maximum amount of solute possible; no more can be dissolved.

Colligative Properties of Solutions

Colligative properties are specific properties that depend solely on the number of solute particles in a solution, not on the identity of the solute.

Lowering of the Freezing Temperature (Cryometry)

The decrease in freezing temperature (cryoscopic depression) is proportional to the amount of particles the solute releases upon dissolving.

Increasing the Boiling Temperature (Ebullioscopy)

A higher temperature is needed for the solvent to pass into the gas phase.

Osmosis

A phenomenon where cells in the body are nourished and eliminate waste through their membranes, driven by the movement of solvent across a semipermeable membrane.