JEE Chemistry Surface Chemistry MCQs Set 11

Answer: (a) aluminium chloride

Explanation:
1. Coagulation is caused by the ion of opposite charge, and the effect grows with its charge (Hardy-Schulze rule).
2. Al\( ^{3+} \) (trivalent cation) is the most effective.

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (b) dialysis

Explanation:
1. Crystalloids pass through a semipermeable membrane but colloidal particles do not.
2. This process is dialysis.

Teacher's Note:
1. Dialysis separates crystalloids from colloids through a semipermeable membrane; ultrafiltration uses graded filters.
2. Peptisation converts a fresh precipitate into a sol by adding a small amount of a suitable electrolyte.

Answer: (b) a layer of dispersion medium on their particles

Explanation:
1. Lyophilic particles are strongly solvated.
2. The layer of dispersion medium around each particle keeps them from coming together.

Teacher's Note:
1. Lyophilic sols are solvent-loving, stable, reversible and stabilised by solvation; lyophobic sols are stabilised only by charge.
2. Examples of lyophobic sols: metal and metal sulphide sols; lyophilic: starch, gelatin, gum.

Answer: (c) solid dispersed in gas

Explanation:
1. Smoke consists of fine solid particles (dispersed phase) in air (dispersion medium).
2. It is an aerosol of a solid in a gas.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (c) Ferric hydroxide

Explanation:
1. Hydrous metal oxides such as Fe(OH)\( _3 \) adsorb cations and form positive sols.
2. Gold, metal sulphides and acidic dyes give negative sols.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (a) millimole per litre

Explanation:
1. Flocculation value is the minimum concentration of electrolyte required to coagulate a sol in a fixed time.
2. It is expressed in millimoles of electrolyte per litre of sol.

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (b) ZnCl2

Explanation:
1. CCl4 is not an electrolyte, so it does not coagulate the sol.
2. Among the electrolytes, Zn\( ^{2+} \) (divalent) is more effective than the monovalent Na\( ^+ \) and K\( ^+ \).

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (d) aquasol

Explanation:
1. A sol in which water is the dispersion medium is an aquasol (hydrosol).
2. An aerosol has gas as the medium and an organosol has an organic liquid.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (b) peptisation

Explanation:
1. The added electrolyte supplies ions that are adsorbed on the precipitate particles.
2. The charged particles repel and break up into a sol; this is peptisation.

Teacher's Note:
1. Dialysis separates crystalloids from colloids through a semipermeable membrane; ultrafiltration uses graded filters.
2. Peptisation converts a fresh precipitate into a sol by adding a small amount of a suitable electrolyte.

Answer: (d) Al\( ^{3+} \)

Explanation:
1. As\( _2 \)S\( _3 \) sol is negatively charged, so cations coagulate it.
2. Al\( ^{3+} \) has the highest charge and the highest coagulating power.

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (b) Wavelength

Explanation:
1. Dialysis, ultrafiltration and Brownian movement are all colloid phenomena.
2. Wavelength is not specific to colloids.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (b) clouds

Explanation:
1. Clouds and fog are tiny liquid droplets in air (a liquid aerosol).
2. Smoke is solid in gas, emulsion is liquid in liquid and jellies are liquid in solid.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (b) Bredig's arc method

Explanation:
1. Bredig's arc method makes sols of metals such as gold, silver, platinum and mercury.
2. An electric arc is struck between metal electrodes under water.

Teacher's Note:
1. Dialysis separates crystalloids from colloids through a semipermeable membrane; ultrafiltration uses graded filters.
2. Peptisation converts a fresh precipitate into a sol by adding a small amount of a suitable electrolyte.

Answer: (d) AlCl\( _3 \)

Explanation:
1. The sol is negatively charged, so the cation does the coagulating.
2. Al\( ^{3+} \) has the highest charge, so AlCl\( _3 \) has the lowest coagulation value (Hardy-Schulze rule).

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (c) has a crystal structure similar to ice

Explanation:
1. Silver iodide crystals resemble ice crystals in structure.
2. They act as nuclei for water vapour to condense into rain.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (c) large aggregates of sulphur molecules

Explanation:
1. A sulphur sol is a multimolecular colloid.
2. Its particles are aggregates of many S\( _8 \) molecules.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (a) lower than that of water

Explanation:
1. Lyophilic sols are viscous and have lower surface tension than the dispersion medium.
2. So it is lower than that of water.

Teacher's Note:
1. Lyophilic sols are solvent-loving, stable, reversible and stabilised by solvation; lyophobic sols are stabilised only by charge.
2. Examples of lyophobic sols: metal and metal sulphide sols; lyophilic: starch, gelatin, gum.

Answer: (c) Solution of urea

Explanation:
1. Urea is a crystalloid that dissolves to form a true solution.
2. Milk, blood and ice cream are colloids.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (d) flocculation value

Explanation:
1. The least concentration of electrolyte that causes coagulation is the flocculation (coagulation) value.

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (d) \( \log\frac{x}{m}=\log K+\frac1n\log C \)

Explanation:
1. Freundlich isotherm: \( \frac{x}{m}=KC^{1/n} \) (or \( KP^{1/n} \)).
2. Taking logarithms: \( \log\frac{x}{m}=\log K+\frac1n\log C \).

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.

Answer: (a) Fe\( ^{3+} \) ion coagulates blood, which is a negatively charged sol

Explanation:
1. Blood is a negatively charged colloid.
2. The trivalent Fe\( ^{3+} \) ion coagulates it and forms a clot.

Teacher's Note:
1. Hardy-Schulze rule: the coagulating power of an ion is greater the higher its charge, and the ion of opposite sign to the sol particles is the effective one.
2. Flocculation value is the minimum electrolyte concentration (millimole per litre) needed to coagulate a sol.

Answer: (c) 10 to 1000 angstrom

Explanation:
1. Colloidal particles range from about 1 nm to 100 nm (10 to 1000 angstrom) in size, intermediate between solutions and suspensions.

Teacher's Note:
1. Colloids have particle sizes between about 1 nm and 1000 nm (10 to 10000 angstrom).
2. Classify a colloid by its dispersed phase and dispersion medium.