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Clouds are colloidal dispersion of water particles in air. These water particles carry some charge over them. On spraying oppositely charged colloidal dust or sand particles over a cloud from an aeroplane, the colloidal water particles present in the cloud will get neutralized and as a result they will come closer and will grow in size to form bigger water drops and ultimately will coagulate or precipitate causing artificial rain.

(i) The process of removing an adsorbed substance from a surface on which it is absorbed.

(ii) The formation of micelles takes place only above a particular concentration called CMC.

(iii) The catalytic reaction that depends upon the pore structure of the catalyst and size of the reactant and product molecules.

Associated colloids are the colloids which act as electrolyte at low concentration and show colloidal behaviour at high concentration.

Example: Soap solution.

(i) CMC is Critical Micelle Concentration and is defined as minimum concentration above which micelle formation takes place. Below the CMC, the substance forming micelle behave as electrolyte. 

For example, CMC for soaps is 10^-3 - 10^-4 mol L^-1 

(ii) Kraft's temperature: It is a temperature (T_k) above which micelle formation takes place.

(iii) McBain micelle : The micelle generally contain at least 100 surfactant molecules. At CMC, the micelles are almost spherical, but at concentration slightly greater than CMC, micelles form flattened spheres. At still higher concentrations, they form extended parallel sheets (the thickness of sheet is about twice the diameter of one molecule). They are called Laminar micelle or McBain micelle because existence of plate like micelles of soaps was first studied by McBain (1913).

(i) The nature of the substance whether colloid or crystalloid depends upon size of the solute particles. When the size of solute particles lies between 1 to 1000 nm, it behaves as a colloid. 

(ii) The colloidal water particles of the clouds get neutralized and coagulated to bigger water drops by spraying salt over clouds and as a result artificial rain is caused.

(i) The particles of the dispersed phase have no affinity for the dispersion medium /solvent repelling (hating) colloidal sols. 

Example, metal and their sulphides.

(ii) The reactant and the catalyst are in the same phase.

CH₃COOCH₃(I) + H₂O(I) + HCl(I) → CH₃COOH(aq) + CH₃OH(aq)

(iii) Oil is dispersed in water/oil is dispersed phase and water is dispersion medium. 

Example- milk

There are some substances which at low concentration behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates.

(i) Scattering of light by the colloidal particles takes place and the path of Iight becomes illuminated. This is called Tyndall effect.

(ii) The positively charged colloidal particles of hydrated ferric oxide sol get coagulated by the oppositely charged ions provided by electrolyte NaCl.

(iii) On passing direct current, the colloidal particles move towards the oppositely charged electrode where they lose their charge and get coagulated.

(i) A colloidal solution of sulphur can be obtained by passing hydrogen sulphide gas into a solution of sulphur dioxide in water.

SO₂ + 2H₂S → 3S + 2H₂O

(ii) A colloidal solution of gold in water can be prepared by reducing auric chloride with stannous chloride.

2AuCl₃ + 3SnCl₂ →  2Au + 3SnCl₄

 (i) This is due to scattering of light by the colloidal particles (called Tyndall effect).

(ii) This is due to charge on the colloidal particles so that they migrate towards the oppositely charged electrode.

(iii) Ferric hydroxide is a positively charged sol and is coagulated by SO₄^2- ions produced by K₂SO₄ . 

Sulphur sol forms multimolecular colloids.

(NH₄)₃ PO₄/PO₄^3-

AlCl₃ is more effective in causing coagulation of negatively charged sol because according to Hardy and Schulze rule greater the valency of the tlocculating ion, greater is its ability io bring coagulation.

Hydrated ferric oxide sol

AlCl₃/AI³⁺

 K₄[Fe(CN)₆].

Greater the valence of the flocculating ion, greater is its ability to bring coagulation as per Hardy-Schulze rule. Thus, K₄[Fe(CN)₆] is most effective in coagulating AgI/Ag⁺ . 

K₄[Fe(CN)₆]/[Fe(CN)₆]^4-

(i) Soil particles in river water is a colloid and is precipitated by electrolytes of sea water and river stream splits in two streams and so on. 

(ii) In medicines colloidal form have larger surface area and are effectively absorbed. 

The coagulation power increases with increase in charge on the ions used for coagulation. Thus, BaCl₂ is more effective in causing coagulation.

Ba⁺⁺ > K⁺

Animal hide is colloidal in nature and has positively charged particles. When it is soaked in tanin which has negatively charged colloidal particles, it results in mutual coagulation taking place.

Positively charged sol of hydrated ferric oxide is formed and on adding excess of NaCl, negatively charged chloride ions coagulate the positively charged sol of hydrated ferric oxide.

A positively charged colloidal solution of hydrated ferric oxide is formed. On adding sodiurn chloride, negatively charged chloride ions neutralise the positive charge of the colloidal solution. Coagulation of the soi takes place.

Milk. It is an emulsion whereas others are solid foam

Blood is colloidal solution of substances like albumin in water. It has negatively charged particles. The fresh solution of alum or ferric chloride is added on flowing blood which coagulates blood by Al³⁺ or Fe³⁺ ions and stop flow of blood.

(c) blood is coagulated and blood vessel is sealed

Blood is a colloidal sol. When we nib the injured part with moist alum then coagulation of blood takes place. Hence main reason is coagulation, which stops the bleeding. Therefore bleeding stop by rubbing moist alum.

(b) It is true, Fe³⁺ ions coagulate blood which is a negatively charged sol

Promoters are substance that enhance the activity of catalyst while poisons decrease the activity of catalyst. 

N₂ + 3H₂ →2NH₃ (g) 

Fe = catalyst, Mo= promoter 

1. Kraft temperature (T_k ) – The temperature above which micelle formation occurs.

Critical micelle concentration (CMC) – The concentration above which micelle formation occurs.

2. Soap is sodium or potassium salt of higher fatty acids. At a particular concentration the soap molecules will rearrange in such a way that its polar end is towards water and non-polar end towards oil.

These molecules aggregate to form ionic micelles around the oil/dirt particles. Since the polar groups can interact with water, the micelles are pulled in water and are removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats.

V₂O₅ accelerates the rate of reaction by the formation of an activated complex with reactants.

On the basis of pore size of the catalyst and size of reactant and products.

On the basis of pore size of the catalyst and size of reactant and products. 

Dispersed phase- liquid ; Dispersion medium solid.

 Heat of adsorption is Negative

The accumulation of molecules of a species at the surface rather in the bulk of a solid or liquid is termed adsorption.

Colloids which have both dispersed and dispersion medium in solid phase .

1. Electrical disintegration method – Bredig’s Arc Method.

2. An electric arc is struck between electrodes of the metal immersed in dispersion medium. The heat produced vapourises the metal which then condenses to form colloids.

3. Difference between sol and gel with suitable examples :

• Sol – Colloids in which dispersed phase is solid and dispersion medium is liquid, e.g. Paints, Cell fluids.
• Gel – Colloids in which dispersed phase is liquid and dispersion medium is solid, e.g. Cheese, Butter. 

1. Tyndall cone is observed due to Tyndall effect. Scattering of light by colloidal particles is called Tyndall effect.

2. The coagulation of ferric hydroxide sol is observed. This is in accordance with Hardy-Schulze rule.

1. Scattering of light by the colloidal particles takes place and the path of light become illuminated. This is called Tyndall effect.

2. The positively charged colloidal particles of Fe(OH)₃ sol get coagulated by the oppositely charged ion provided by the electrolyte.

3. On passing direct current, colloidal particles move towards the oppositely charged electrode where they lose their charge and get coagulated.

The movement of colloidal particles under an applied electric potential is called Electrophoresis.

1. Double decomposition 

2. Oxidation 

3. Hydrolysis

1. A colloid can be precipitated by an ion having opposite charge to that of the colloid. Greater the valency of coagulating ion added greater its power to precipitate.

eg: Al³⁺ > Mg²⁺ > SO₄^2- > Na⁺ > PO³⁺ > Cl^-

2. K⁺ < Ca²⁺< Fe³⁺

3. These are arranged based on Hardy-Shulze rule.

1. Yes

2. properties of colloids :

• Colligative properties 
• Mechanical properties 
• Optical properties 
• Electrical properties 

 Physisorption: 

(a)The forces operating are weak vander Waal’s forces 

(b)The heat of adsorption is low 20-40 KJ Mol^-1 

(c) Does not require any activation energy 

(d) Forms multimoleculer layer 

Chemisorption: 

(a) Forces acting are similar to those of chemical bonds 

(b) The heat of adsorption is high 80-240 KJ Mol^-1 

(c) Requires activation energy 

(d) Forms unimolecular layer

1. It is a colloidal form in which liquid is dispersed in liquid.

2. Milk is an oil in water type emulsion and butter is a water in oil type emulsion.

1. Emulsions are colloidal solution in which a liquid is dispersed in another liquid.

2. There are two types of emulsions

• Oil in water (0/W)
• Water in Oil (W/O) 

3. Oil in Water : Oil dispersed in water

 e.g., mayonnaise. icecrearn.

Water in Oil: Water dispersed in Oil.

e.g., stiff grease, butter, cold cream.

The colloidal solution in which the dispersed phase and dispersion medium are liquids is called an emulsion.
There are two types of emulsions:
(a) Oil in water type:
Here, oil is the dispersed phase while water is the dispersion medium. For example milk, vanishing cream, etc.
(b) Water in oil type:
Here, water is the dispersed phase while oil is the dispersion medium. For example cold cream, butter, etc.