COLLOIDS

A comprehensive set of flashcards covering definitions, classifications, properties, purification methods, preparation methods, and applications of colloids based on the provided notes.

What is a colloid and what is the origin of the term?

A colloid is a substance microscopically dispersed within another substance; the term comes from the Greek word 'kolla' meaning glue, reflecting glue-like colloidal particles. Colloidal particles pass through filter paper but not through a semipermeable membrane and can be settled by centrifugation.

What is the typical size range of dispersed-phase particles in a colloid, and how are they usually observed?

Diameters are about 1 nm to 100 nm; they are usually invisible in ordinary optical microscopes but can be observed with an ultramicroscope or an electron microscope.

What are the two phases in a colloidal system?

Dispersed phase (discontinuous) and dispersion medium (continuous).

How do particle sizes compare among solutions, colloids, and suspensions?

Solutions: less than 1 nm; Colloids: 1 to 100 nm; Suspensions: more than 100 nm.

Are colloids homogeneous or heterogeneous, and how does this compare to solutions and suspensions?

Solutions are homogeneous; colloids are homogeneous or heterogeneous; suspensions are heterogeneous.

What do colloids typically look like in terms of appearance and transparency?

Solutions are clear; colloids are cloudy (often translucent); suspensions are often opaque or cloudy.

Do colloidal particles pass through ordinary filter paper or semipermeable membranes?

Colloidal particles pass through ordinary filter paper but do not pass through semipermeable membranes.

What process can be used to settle colloidal particles based on density differences?

Centrifugation.

Can you name some common colloids often cited as examples?

Milk, fog, blood, jam, shoe polish, smoke, among others.

How are colloids classified based on the physical state of dispersed phase and dispersion medium?

Categories include solid in solid (sol), solid in liquid (sol), solid in gas (aerosol), liquid in solid (gel), liquid in liquid (emulsion), and liquid in gas (liquid aerosol).

What is a lyophilic (hydrophilic) colloid and what are its characteristics?

Also called intrinsic colloids; high affinity for the dispersion medium (often water), tend to self-stabilize, reversible, heavily hydrated; examples include starch, gelatin, rubber, and proteins.

What is a lyophobic (hyrophobic) colloid and its typical characteristics?

Extrinsic colloids with little affinity for the dispersion medium; relatively unstable; require electrolytes for stabilization; poorly hydrated; examples include sols of metals like gold and silver.

Differentiate multimolecular colloids from macromolecular colloids with examples.

Multimolecular colloids are aggregates of small atoms or molecules (diameter <10^-7 cm) held by weak forces (e.g., gold sol, sulfur sol). Macromolecular colloids are large dispersed-phase particles, often natural polymers (e.g., proteins, natural polymers).

What are associated colloids (micelles) and how do they behave at different concentrations?

Micelles that behave as electrolytes at low concentrations but as colloids at higher concentrations; examples include soaps and higher alkyl sulphonates; polythene oxide is another example.

What is the difference between sols and gels in terms of appearance?

Sols are colloidal solutions that appear fluid; gels appear solid with varying rigidity (examples: jelly, butter, cheese, curd).

What are positive and negative colloids and give examples of each?

Positive colloids have a positively charged dispersed phase (e.g., Fe(OH)3, Al(OH)2, methylene blue sol); Negative colloids have a negatively charged dispersed phase (e.g., Ag sol, Cu sol).

Name at least two chemical methods to prepare colloids and provide a representative example for each.

Oxidation: e.g., oxidizing H2S with Br2 to form sulfur sol; Reduction: e.g., reducing AuCl3 with SnCl2 to form gold sol; Hydrolysis: e.g., FeCl3 hydrolyzing to Fe(OH)3; Double decomposition: forming arsenous sulfide sol from H2S and As2O3.

What are the three common purification methods for colloids?

Dialysis, electrodialysis, and ultrafiltration.

Describe dialysis and its principle.

Dialysis separates colloidal particles from crystalloids by diffusion through a membrane; colloidal particles cannot pass the cellophane membrane, but ions can; impurities diffuse out with frequent water changes.

What is electrodialysis and its practical application mentioned in the notes?

Dialysis accelerated by applying an electrical potential; used in artificial kidney devices (hemodialyzers) to speed up filtration.

What is ultrafiltration and its role in biology or medicine?

High-pressure filtration through a semipermeable membrane that retains colloidal particles while small solutes and solvent pass through; used in kidney filtration (glomerular filtration).

List some physical properties of colloids.

Heterogeneous two-phase system (dispersed phase and dispersion medium); dispersed particles are not visible to the naked eye but appear homogeneous; generally pass through ordinary filter paper; stability varies (lyophilic sols stable; lyophobic sols require electrolytes); colour depends on particle size.

Explain the Tyndall effect and its significance.

When a beam of light passes through a colloidal solution, the path is illuminated due to light scattering by colloidal particles (Tyndall cone); true solutions do not show this effect; this helps distinguish colloids from true solutions and underpins the use of ultramicroscopy.

What is Brownian movement and its relation to particle size?

Random zigzag motion of colloidal particles due to collisions with dispersion medium molecules; decreases as particle size increases; suspensions do not show this motion.

Define electrophoresis in the context of colloids.

Movement of colloidal particles toward an electrode under an electric field; positively charged particles move toward the cathode, negatively charged toward the anode.

Define electrosmosis.

Movement of the dispersion medium under an electric field when the movement of colloidal particles is hindered by a membrane; the medium moves toward the opposite electrode.

What are some practical advantages of colloids in industry and medicine?

They allow dispersion of normally insoluble materials (e.g., colloidal gold for drug delivery); used in medicines and in the pharmaceutical industry; essential in paints; milk fats stabilize emulsions for digestion and absorption.

How does acidification affect the colloidal particles in milk during fermentation?

Acid production releases ions that neutralize the charges on milk colloidal particles, causing coagulation and curd formation.

What role do soaps play in colloidal chemistry according to the notes?

Soap solutions are colloidal in nature and remove dirt either by adsorption or by emulsifying greasy matter on fabrics.

Name some common foods that are often colloidal in nature.

Milk, butter, ice cream, and many other food items involve colloidal suspensions or emulsions.