In chemistry, a mixture is a material composed of two or more different chemical substances which are not chemically bonded. Unlike compounds, the components of a mixture retain their individual chemical properties, and they can be separated by physical methods. Mixtures do not have a fixed composition, meaning the constituents can be present in any proportion by mass.
Classification of Mixtures
Mixtures are broadly classified into two categories based on the uniformity of their composition and particle distribution.
Homogeneous Mixtures
Homogeneous mixtures possess a uniform composition throughout their mass. The components are completely mixed and indistinguishable from one another, existing in a single phase.
- Key Characteristics: No visible boundaries of separation; uniform properties throughout.
- Examples: Air (mixture of gases like nitrogen, oxygen, argon), brass (solid solution of copper and zinc), saltwater, and vinegar (acetic acid in water).
Heterogeneous Mixtures
Heterogeneous mixtures lack a uniform composition throughout their mass. The components remain physically distinct and exhibit visible boundaries of separation.
- Key Characteristics: Two or more distinct phases; non-uniform properties.
- Examples: Mixture of sand and iron filings, oil and water, muddy water, and concrete.
Comparison Between Homogeneous and Heterogeneous Mixtures
| Parameter | Homogeneous Mixtures | Heterogeneous Mixtures |
| Composition | Uniform throughout | Non-uniform throughout |
| Visibility of Components | Components are not visible to the naked eye or microscope | Components are easily visible or distinguishable |
| Phases | Consists of a single phase | Consists of two or more phases |
| Separation | Relatively difficult to separate physically | Easily separated by mechanical or physical methods |
Types of Mixtures Based on Particle Size
Mixtures are further categorized into solutions, suspensions, and colloids depending on the size of the solute/dispersed particles.
Solutions (True Solutions)
A solution is a homogeneous mixture of two or more substances. It consists of a solute (the substance being dissolved) and a solvent (the medium in which the solute dissolves).
- Particle Size: Less than 1 nanometer (< 1 nm or < 10-9 m).
- Stability: Highly stable; particles do not settle down when left undisturbed.
- Optical Property: Does not scatter a beam of light (No Tyndall Effect).
- Examples: Iodine tincture (iodine in alcohol), soda water (carbon dioxide in water), and alloys.
Colloids (Colloidal Solutions)
Colloids are heterogenous in nature but appear homogeneous to the naked eye. They consist of a dispersed phase (similar to solute) and a dispersion medium (similar to solvent).
- Particle Size: Between 1 nanometer and 1000 nanometers (1 nm to 1000 nm).
- Stability: Quite stable; particles do not settle down easily.
- Optical Property: Scatters light efficiently, making the path of light visible (Exhibits Tyndall Effect).
- Examples: Milk, fog, blood, face cream, and gelatin.
Suspensions
A suspension is a heterogeneous mixture containing solid particles sufficiently large for sedimentation.
- Particle Size: Greater than 1000 nanometers (> 1000 nm or > 10-6 m).
- Stability: Unstable; particles settle down under the influence of gravity when left undisturbed.
- Optical Property: Scatters light when particles are suspended, but the effect ceases once particles settle.
- Examples: Chalk powder in water, smoke from internal combustion engines, and flour mixed in water.
Comprehensive Classification of Colloids
Colloids are classified based on the physical state of the dispersed phase and the dispersion medium.
| Dispersed Phase | Dispersion Medium | Type of Colloid | Common Examples |
| Solid | Solid | Solid Sol | Colored gemstones, milky glass |
| Solid | Liquid | Sol | Paints, cell fluids, milk of magnesia |
| Solid | Gas | Aerosol | Smoke, automobile exhaust, dust |
| Liquid | Solid | Gel | Cheese, butter, jellies |
| Liquid | Liquid | Emulsion | Milk, hair cream, oil-in-water mixes |
| Liquid | Gas | Aerosol | Fog, mist, cloud, insecticide sprays |
| Gas | Solid | Solid Foam | Pumice stone, foam rubber, marshmallow |
| Gas | Liquid | Foam | Froth, whipped cream, soap lather |
Physical Separation Techniques for Mixtures
Separation techniques exploit differences in the physical properties (such as boiling point, melting point, density, and solubility) of the components.
Evaporation
Used to separate a volatile component (solvent) from a non-volatile solid component (solute).
- Application: Obtaining salt from seawater, separating dye from blue/black ink.
Centrifugation
Operates on the principle that denser particles are forced to the bottom and lighter particles stay at the top when spun rapidly.
- Application: Separation of cream from milk, diagnostic testing in blood and urine laboratories, squeezed water extraction in washing machines.
Decantation and Sedimentation
Used to separate a mixture of an insoluble solid and a liquid, or two immiscible liquids, by allowing the heavier component to settle.
- Application: Separating oil and water using a separating funnel; clearing muddy water.
Sublimation
Employed to separate mixtures containing a sublimable component (which transitions directly from solid to gas) from a non-sublimable impurity.
- Application: Separation of ammonium chloride, camphor, naphthalene, or anthracene from common salt.
Chromatography
A technique used to separate solutes that dissolve in the same solvent but travel at different speeds due to varying solubility and adsorption rates.
- Application: Separating colors from a dye, pigments from natural colors, and drugs from blood samples.
Distillation
Used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have a sufficient difference in their boiling points (minimum difference of 25 K).
- Application: Separation of acetone and water.
Fractional Distillation
Utilized when the difference in boiling points of the miscible liquids is less than 25 K. It employs a fractionating column packed with glass beads to provide extra surface area for cooling and condensation.
- Application: Separation of different gases from liquid air, refining of crude petroleum into fractions like petrol, diesel, and kerosene.
Crystallization
A process that separates a pure solid in the form of its crystals from a solution. It is superior to evaporation because it prevents decomposition of solids that char upon dry heating.
- Application: Purification of common salt from seawater, separation of crystals of alum (phitkari) from impure samples.
Key Chemical Laws and Concepts Related to Mixtures
Law of Constant Proportions vs. Mixtures
Unlike compounds, which strictly follow the Law of Constant Proportions (stating that a chemical compound always contains exactly the same proportion of elements by mass), mixtures do not follow this law. The constituents of a mixture can be varied indefinitely.
Tyndall Effect
The phenomenon of scattering of a beam of light by colloidal or suspension particles. It occurs because the particle size is comparable to the wavelength of visible light. True solutions do not show the Tyndall effect.
- Real-world instances: Sunlight passing through the canopy of a dense forest due to mist particles; light beam visible in a dark cinema hall due to dust particles.
Brownian Motion
The continuous, zig-zag, random motion of colloidal particles in a dispersion medium. It is caused by the unequal bombardment of particles by the molecules of the dispersion medium, which prevents colloidal particles from settling down, ensuring stability.
Azeotropes (Constant Boiling Mixtures)
Azeotropes are binary mixtures having the same composition in liquid and vapor phases and boiling at a constant temperature. They cannot be separated into their pure components by fractional distillation.
- Types: Minimum boiling azeotropes (e.g., 95% ethanol + 5% water by volume) showing large positive deviation from Raoult’s Law; Maximum boiling azeotropes (e.g., 68% nitric acid + 32% water by mass) showing large negative deviation from Raoult’s Law.