Preparation of Colloids

A few ways to prepare colloids are given below

•  Chemical method : Colloids can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate to form sols 

• Electrical Disintegration/ Bredig’s arc Method: This is applied to obtain colloidal sols of metals like gold, silver and platinum.  An electric arc is stuck between the electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

• Peptization- It is the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this is called peptizing agent. During peptization the precipitate absorbs the one of the ions of the electrolyte on its surface. This causes +veor –vecharge to develop on the precipitate, which ultimately break up into small particles of the size of a colloid.

Mechanism Of Micelle Formation-

• Soap is the sodium or potassium salt of fatty acid and may be represented as RCOO^- Na (e.g. sodium stearate, (CH₃(CH₂)16COO^-Na⁺]) .  Whendissolved into water , it dissociates into  RCOO^- and  Na⁺ ions

• The RCOO^-ion consists of two parts –  long hydrocarbon chain (also called  non – polar tail)  which is hydrophobic (water repelling ) and a polar group  COO^-   (polar head) which is hydrophilic (water loving )

• At higher concentrations(CMC) RCOO- ions form an aggregate of spherical shape with the hydrocarbon chains pointing towards the centre and the COO- part facing outward on the surface of the sphere. This aggregate is called ionic micelle. It may have as many as 100 ions

On the basis of nature of interaction between dispersed phase and dispersion medium, colloids can be divided into two forms:

(a) Lyophilic Colloids: As the name lyophilic (liquid-loving or solvent attracting) indicates, lyophilic colloids are the colloids exhibiting a strong interaction between the two phases.
The substances like gum, gelatin, starch, when mixed with a suitable liquid as the dispersion medium, directly form the colloidal sol which is also named as lyophilic sol.

(b) Lyophobic Colloids: Lyophobic' means 'liquid hating', which indicates that in these sols there is little or no interaction between the two phases. Substances like metals and their sulphides, when simply mixed with the dispersion medium do not form colloidal sol. They cannot be prepared by simply mixing the two phases. Such substances are called Lyophobic Sols.

Colloids can be classified according to different properties of the dispersed phase and medium.

Firstly, based on the types of particles of the dispersed phase, colloids can be classified as:

• Multimolecular colloids
• Macromolecular colloids
• Associated colloids

Suspension :

Suspension doesn’t mix together entirely, or if they do, the particles may become unmixed and fall to the bottom.

Dispersed particles or solute in suspension are larger than solutes of solution.

The diameter os these particles is of the order 2000A˚ or more.

Example- sand stirred ito water, soot in the air.

Colloids:

The colloids solutions are colloids suspension are intermediate between true solutions and suspensions.

The diameter of the dispersed particles in the solvent of colloid solution ranges from 10A˚ to 2000A˚.

Example: milk, butter.

Colloid:

Depending upon the size of the solute particles the solutions can be categorised as true solution, suspension and colloids. The type of solution in which the size of the solute particle is in the range of 1 to 100 nm, is called a colloid. A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase or colloidal particles) in another substance which is called dispersion medium.

Observation

(i) At low pressure, the graph is almost straight line which indicates that x/m is directly proportional to the pressure.

x/m ∝ pz

x/m = Kp

where K is constant.

(ii) At high pressure, the graph becomes almost constant which means that x/m becomes independent of pressure. This may be expressed as:

x/m = constant

x/m ∝ p⁰

x/m = K p⁰

(iii) Thus, in the intermediate range of pressure, x/m will depend upon the power of pressure which lies between 0 to l i.e., fractional power of pressure (probable range 0.1 to 0.5).

This may be expressed as

x/m ∝ p^1/n

x/m = Kp^1/n

where n can take any whole number value which depends upon the nature of adsorbate and adsorbent. The above relationship is also called Freundlich’s adsorption isotherm.

The adsorption of the gas depends on the nature of the adsorbent. A gas can be adsorbed on different absorbent surfaces in different amounts. For example, <a href="https://byjus.com/chemistry/hydrogen/" rel="noopener" target="_blank">Hydrogen</a> is weakly adsorbed on the alumina surface whereas it is strongly adsorbed on the nickel surface under certain conditions.

Colour of magnesium hydroxide is white.

On July 7, HRD Minister Ramesh Pokhriyal announced a major CBSE syllabus reduction with 30% of the syllabus slashed for the year 2020-21 for classes 9 to 12 because of the reduction in classroom teaching time due to the Covid-19 pandemic and lockdown.

CBSE has rationalized the syllabus with the help of suggestions from NCERT and the same has been notified by a new CBSE notification as well.

Deleted syllabus of CBSE Class 12 Chemistry

D

Ethanol is converted into ethoxy ethane by heating excess of ethanol with conc. sulphuric acid at 140 ^oC.

Nature of the gas

In general, if a gas is more liquefiable it will be more easily absorbed. For example, gases like NH₃, HCl, Cl₂, CO₂, which can be liquefied easily are more readily adsorbed on the solids surface rather than permanent gases like O₂, H₂, etc.

Some of the important applications of adsorption are given below:

1) In gas masks:

Activated charcoal or a mixture of adsorbents is generally used in gas masks to adsorb poisonous and toxic gases from air.

These masks are commonly used by the miners because there are poisonous gases like CO, CH₄ etc. in the atmosphere in the coal mines. Therefore, these masks help to purify the air for breathing.

2) In dyeing of cloth:

Mordants such as alums are used in dyeing of cloth.They adsorb the dye particles which, otherwise, do not stick to the cloth.

3) In Dehumidizers for control of humidity:

Silica and aluminium gels are commonly used to adsorb humidity or moisture from air. 

4) Removal of colouring matter from solutions:

Many substances such sugar, juice and vegetable oils are coloured due to the presence of impurities.They can be decolourised by placing in contact with adsorbents like activated charcoal or fuller’s earth. This method is used in the manufacture of cane sugar. 

5) Heterogeneous catalysis:

Adsorption of reactants on the solid surface of catalysts increases the rate of reaction. The metals such as Fe, Ni, Pt, Pd, etc, are used in the manufacturing process.

Manufacture of ammonia using iron as catalyst (Haber process), manufacture of sulphuric acid by contact process and use of finely divided nickel in the hydrogenation of oils are excellent examples of heterogeneous catalysis.