Colloids, Emulsion, Gel and Their properties with application Questions in English

(A) To disperse an organic solvent like benzene in water,a surfactant (emulsifying agent) is required. $A$ surfactant molecule has a dual nature: a long hydrophobic hydrocarbon tail that interacts with the organic phase (benzene) and a hydrophilic polar head that interacts with the aqueous phase (water). Option $A$ represents a soap molecule (sodium salt of a long-chain fatty acid),which acts as an effective surfactant for this purpose.

(C) The colloidal mixture of a liquid in a liquid is called an emulsion,not a gel. $A$ gel is a colloidal system where a liquid is dispersed in a solid.

(C) The $Fe(OH)_3$ sol is a positively charged colloid.
In the presence of an electric field,the dispersion medium (water) moves towards the cathode due to electro-osmosis.
Since container $C$ is connected to the cathode (negative electrode),the water will move through the semi-permeable animal membrane $S$ from container $B$ into container $C$.
Therefore,the water level in container $C$ will increase.

(D) The reaction between $AgNO_3$ and $KI$ is: $AgNO_3 + KI \rightarrow AgI(s) + KNO_3$.
$A$ negatively charged colloidal sol of $AgI$ is formed when $KI$ is in excess,as $I^-$ ions are adsorbed on the surface of $AgI$ particles.
In option $A$: $100 \ mL \times 0.1 \ M = 10 \ mmol$ of $AgNO_3$ and $10 \ mmol$ of $KI$. This results in a neutral sol.
In option $B$: $100 \ mL \times 0.1 \ M = 10 \ mmol$ of $AgNO_3$ and $50 \ mL \times 0.2 \ M = 10 \ mmol$ of $KI$. This results in a neutral sol.
In option $C$: $200 \ mL \times 0.1 \ M = 20 \ mmol$ of $AgNO_3$ and $200 \ mL \times 0.1 \ M = 20 \ mmol$ of $KI$. This results in a neutral sol.
In option $D$: $100 \ mL \times 0.1 \ M = 10 \ mmol$ of $AgNO_3$ and $100 \ mL \times 0.15 \ M = 15 \ mmol$ of $KI$. Here,$KI$ is in excess $(15 \ mmol > 10 \ mmol)$,leading to the adsorption of $I^-$ ions and the formation of a negatively charged sol.

(B) The gold numbers of $A, B, C$ and $D$ are $0.04, 0.002, 10$ and $25$ respectively.
The protective power of a colloid is inversely proportional to its gold number.
Mathematically,$\text{Protective Power} \propto \frac{1}{\text{Gold Number}}$.
Comparing the given values:
$B (0.002) < A (0.04) < C (10) < D (25)$.
Therefore,the order of protective power is $B > A > C > D$.

(D) $Gel$ is a colloidal system in which a liquid is dispersed in a solid medium.
Butter is an example of a $Gel$ (liquid dispersed in solid).
Milk is an emulsion (liquid dispersed in liquid).
Cloud is an aerosol (liquid dispersed in gas).
Therefore,both Milk and Cloud are not examples of $Gel$.

(A) The sols of metal sulfides like $Sb_2S_3$ and $CdS$,as well as metal sols like Gold $(Au)$,are known to be negatively charged sols.
These particles adsorb anions from the dispersion medium,resulting in a negative charge on their surface.

(B) There are some substances which at low concentrations behave as normal strong electrolytes,but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates.
These aggregated particles are called micelles.
These are also known as associated colloids.
The formation of micelles takes place only above a particular temperature called Kraft temperature $(T_{k})$ and above a particular concentration called critical micelle concentration $(CMC)$.
Therefore,at high concentrations in water,soap acts as an associated colloid.

(C) According to the $Hardy-Schulze$ law,the coagulating power of an electrolyte depends on the valency of the ion carrying a charge opposite to that of the colloidal particles.
For a positive colloid,the negative ion with the highest valency is the most effective coagulating agent.
Comparing the negative ions:
$Cl^-$ (valency $1$)
$SO_4^{2-}$ (valency $2$)
$[Fe(CN)_6]^{4-}$ (valency $4$)
Since $[Fe(CN)_6]^{4-}$ has the highest valency,$K_4[Fe(CN)_6]$ is the most effective electrolyte.

(D) The $Fe(OH)_3$ sol is prepared by the hydrolysis of ferric chloride.
It is a classic example of a positively charged colloid.
It is classified as a multi-molecular colloid because it consists of aggregates of a large number of atoms or smaller molecules (with diameters less than $1 \ nm$) held together by van der Waals forces.
However,in the context of charge,it is specifically known as a positively charged sol.

(B) The $Tyndall \ effect$ is an optical property of colloids caused by the scattering of light by colloidal particles. It depends on the size of the particles and the refractive index difference between the dispersed phase and the dispersion medium,but it is not dependent on the charge of the colloidal particles.

(A) colloidal system is classified based on the physical state of the dispersed phase and the dispersion medium.
Fog is formed when liquid water droplets are dispersed in air (a gas).
Therefore,it is a system of $liquid$ in a $gas$ (aerosol).

(C) Multimolecular Colloids: When a large number of atoms or small molecules (having diameters of less than $1 \, nm$) of a substance combine together in a dispersion medium to form aggregates having size in the colloidal range,the colloidal solutions thus formed are called multimolecular colloids.
These particles are held together by Van der Waals' forces.
Examples include gold sol,sulphur sol,etc.
$A$ gold sol contains particles of various sizes composed of several atoms of gold.
Therefore,the correct answer is gold sol.

(D) Colloidal solutions are prepared by various methods like chemical methods (hydrolysis,reduction,oxidation,double decomposition) and physical methods (peptization,Bredig's arc method).
Electrophoresis is a phenomenon involving the movement of colloidal particles under the influence of an electric field.
It is used for the purification or characterization of colloids,not for their preparation.
Therefore,the correct answer is $D$.

(C) Electro-osmosis is a phenomenon where the movement of colloidal particles is prevented by a semi-permeable membrane,and only the dispersion medium is allowed to move under the influence of an applied electric field.
$Fe(OH)_3$ sol is a positively charged colloid.
To maintain electrical neutrality,the dispersion medium must carry a negative charge.
Therefore,under an electric field,the negatively charged dispersion medium moves towards the anode.

(D) The $As_2S_3$ sol is a negatively charged sol.
This negative charge is acquired due to the preferential adsorption of common sulphide ions $(S^{2-})$ from the dispersion medium on the surface of the colloidal particles.
This adsorption is also responsible for the stability and peptization of the sol.

(D) According to the Hardy-Schulze rule,the coagulation power of an ion is directly proportional to the magnitude of its charge.
Greater the valence of the flocculating ion added,the greater is its power to cause precipitation.
Comparing the charges: $K^{+}$ $(+1)$,$Ca^{2+}$ $(+2)$,$Al^{3+}$ $(+3)$,$Sn^{4+}$ $(+4)$.
Since $Sn^{4+}$ has the highest charge $(+4)$,it possesses the highest coagulation power.

(B) The process of settling of colloidal particles is called coagulation or precipitation of the sol.
When a positively charged colloidal solution is mixed with a negatively charged colloidal solution,they neutralize each other's charges,leading to mutual coagulation.
According to the Hardy-Schulze rule,the coagulating power of an ion is directly proportional to its valence (charge). For a negatively charged colloidal solution,the coagulating power of cations increases with their charge.
Therefore,the order of coagulating power is $Al^{3+} > Ba^{2+} > Na^{+}$.

(B) colloidal system is classified based on the physical state of the dispersed phase and the dispersion medium.
In pumice stone,a gas is dispersed in a solid medium.
This type of colloidal system is known as a $Solid \ foam$.

(A) The charge on a colloidal particle is determined by the preferential adsorption of ions from the dispersion medium.
$1$. For $[Fe(OH)_3]$,the sol is positively charged due to the adsorption of $Fe^{3+}$ ions. Thus,option $A$ is correct.
$2$. For $[As_2S_3]$,the sol is negatively charged due to the adsorption of $S^{2-}$ ions. However,the standard representation is $[As_2S_3] : S^{2-}$,but option $A$ is a more direct representation of the primary charge-determining ion.
$3$. For $[AgI]$,if $AgNO_3$ is in excess,$Ag^+$ ions are adsorbed,making it positively charged $([AgI] : Ag^+)$.
$4$. For $[AgI]$,if $KI$ is in excess,$I^-$ ions are adsorbed,making it negatively charged $([AgI] : I^-)$.
Therefore,option $A$ is the correctly matched pair.

(B) Lyophobic colloids are solvent-hating colloids that do not readily form stable colloidal solutions when mixed with the dispersion medium.
$Gelatin$,$Starch$,and $Gum$ are examples of lyophilic colloids,as they have a strong affinity for the dispersion medium (water).
$Phosphorous$ $(P)$ sol is an example of a lyophobic colloid because it does not have a natural affinity for the dispersion medium and requires special methods for preparation.

(C) The classification of colloids is based on the physical state of the dispersed phase and the dispersion medium:
$1$. Liquid Aerosol: Liquid dispersed in gas (e.g.,Fog) $\rightarrow A-Q$.
$2$. Solid Sol: Solid dispersed in solid (e.g.,Alloy) $\rightarrow B-S$.
$3$. Emulsion: Liquid dispersed in liquid (e.g.,Milk) $\rightarrow C-P$.
$4$. Jelly (Gel): Liquid dispersed in solid (e.g.,Gel) $\rightarrow D-R$.
Therefore,the correct matching is $A-Q, B-S, C-P, D-R$.

(B) The correct answer is $B$.
$(a)$ Colloidal particles are small enough to pass through ordinary filter paper.
$(b)$ The freezing point of a colloidal solution is the same as that of a true solution at the same molar concentration,because the number of particles in a colloidal solution is much smaller than in a true solution,leading to negligible depression in freezing point. Thus,the statement that it is lower is false.
$(c)$ When $AgNO_3$ is added to $KI$ (excess $KI$),$I^-$ ions are adsorbed on the surface of $AgI$ particles,forming a negatively charged colloid.
$(d)$ Addition of excess electrolyte causes coagulation (precipitation) of colloidal particles due to the neutralization of charge.

(A) Brownian movement is the random motion of particles suspended in a fluid (a liquid or gas) resulting from their collision with the fast-moving atoms or molecules in the fluid.
Since smaller particles have less mass,they are more easily affected by these collisions,making the Brownian movement more pronounced for smaller particles compared to bigger particles.
Therefore,option $A$ is the correct statement.
Metal sulphides are lyophobic (not lyophilic).
Hardy-Schulze law states that the coagulating power of an ion depends on its valency,not its size.
Charcoal adsorbs gases that are more easily liquefiable; $H_2S$ is more easily liquefiable than $Cl_2$ (due to higher boiling point),so $H_2S$ is adsorbed more.

(A) Preparation of egg albumin sol is done in the following two steps:
$(i)$ Prepare $100 \ mL$ of $5 \ \% \ (w/V)$ solution of $NaCl$ in water in a $250 \ mL$ beaker.
$(ii)$ Break one egg in a porcelain dish,pipette out the albumin (the transparent white part),and pour it into the sodium chloride solution. Stir well to ensure the preparation of the sol.

(A) The protective power of a colloid is inversely proportional to its gold number.
$\text{Gold number} \propto \frac{1}{\text{Protective power}}$
This means that the smaller the value of the gold number of a lyophilic sol,the greater is its protective action.
Comparing the given values: Gelatin $(0.005-0.01)$ has the lowest gold number among the options.
Therefore,Gelatin is the best protective colloid.

(D) The Critical Micelle Concentration $(CMC)$ of a surfactant decreases as the length of the hydrophobic hydrocarbon chain increases.
This is because a longer chain increases the hydrophobic effect and the van der Waals forces of attraction between the hydrocarbon tails,which stabilizes the micelle structure.
Comparing the given options:
$A$: $C_9$ chain
$B$: $C_{12}$ chain
$C$: $C_{14}$ chain
$D$: $C_{16}$ chain
Since $CH_3(CH_2)_{15}N^{+}(CH_3)_3Br^{-}$ has the longest hydrocarbon chain $(C_{16})$,it will have the strongest hydrophobic interactions and will form micelles at the lowest molar concentration.

(B) The movement of the dispersion medium under the influence of an applied electric field is known as electro-osmosis.
In contrast,the movement of colloidal particles under an electric field is called electrophoresis or cataphoresis.

(D) According to the Hardy-Schulze rule,the coagulating power of an ion is directly proportional to its valence.
For sol $A$,the flocculation value of $BaCl_2$ $(Ba^{2+})$ is less than $KCl$ $(K^+)$,which implies that the cation $Ba^{2+}$ is more effective at coagulation than $K^+$. Therefore,sol $A$ must be negatively charged.
For sol $B$,the flocculation value of $Na_2SO_4$ $(SO_4^{2-})$ is less than $NaBr$ $(Br^-)$,which implies that the anion $SO_4^{2-}$ is more effective at coagulation than $Br^-$. Therefore,sol $B$ must be positively charged.
Thus,sol $A$ is negatively charged and sol $B$ is positively charged.

(C) Smoke is a colloidal system where solid particles are dispersed in a gas medium.

(B) Gold number is defined as the minimum amount of protective colloid in milligrams that prevents the coagulation of $10 \, mL$ of a standard gold sol when $1 \, mL$ of $10 \% \, NaCl$ solution is added to it.
Given mass of starch $= 0.025 \, g$.
Converting mass to milligrams: $0.025 \, g = 0.025 \times 1000 \, mg = 25 \, mg$.
Since this amount of starch just prevents the coagulation of $10 \, mL$ of gold sol,the gold number of starch is $25$.

(C) Fog is a type of aerosol where the dispersed phase is a liquid and the dispersion medium is a gas.

(B) The process of removing impurities from a colloidal solution is known as purification.
Electrodialysis is a technique used to accelerate the process of dialysis by applying an electric field,which helps in the faster removal of ionic impurities from the colloidal solution.
Therefore,colloidal solutions can be purified by electrodialysis.

(B) Arsenious sulphide $(As_2S_3)$ sol is a negatively charged colloidal sol.
According to the Hardy-Schulze rule,the coagulating power of an electrolyte depends on the valency of the oppositely charged ion.
For a negatively charged sol,the coagulating power increases with the increase in the valency of the cation.
The valencies of the cations are: $Na^+$ $(+1)$,$Ba^{2+}$ $(+2)$,and $Al^{3+}$ $(+3)$.
Since $Al^{3+}$ has the highest valency,$AlCl_3$ is the most effective coagulating agent.

(A) Haemoglobin is a positively charged sol at a $pH$ below its isoelectric point.
Gold sol is a well-known example of a negatively charged sol.

(B) solid sol is a colloidal system in which a solid is dispersed in a solid medium. Gem stones are examples of solid sols where colored pigments are dispersed in a solid crystalline structure.

(B) The classification of colloids based on the physical state of the dispersed phase and dispersion medium is as follows:
$1$. Cheese $(C)$: It is a gel where liquid is dispersed in a solid medium (liquid in solid).
$2$. Milk $(M)$: It is an emulsion where liquid is dispersed in a liquid medium (liquid in liquid).
$3$. Smoke $(S)$: It is an aerosol where solid particles are dispersed in a gas medium (solid in gas).
Therefore,the correct combination is $C$: liquid in solid; $M$: liquid in liquid; $S$: solid in gas.

(D) The statement in option $D$ is false because lyophilic sols are inherently stable due to strong interaction between the dispersed phase and the dispersion medium. They cannot be easily coagulated by adding small amounts of an electrolyte. Coagulation of lyophilic sols requires the addition of a large amount of electrolyte or the addition of a suitable solvent (like alcohol or acetone) to dehydrate the particles. In contrast,lyophobic sols are unstable and are easily coagulated by adding small amounts of an electrolyte. Latex is a colloidal solution of rubber particles which are negatively charged,but since the question asks for the false statement,$D$ is the most scientifically inaccurate statement regarding the ease of coagulation.

(A) An aerosol is a colloidal system in which a solid or liquid is dispersed in a gas.
Examples include fog (liquid in gas) and smoke (solid in gas).

(A) In electrophoresis,colloidal particles move towards the oppositely charged electrode and get neutralized,leading to precipitation. Thus,option $(A)$ is correct.
Brownian motion decreases as viscosity increases.
Colloidal medicines are more effective due to their large surface area.
Addition of alum to water is a standard process for purification (coagulation of impurities).

(B) Peptization is the process of converting a freshly prepared precipitate into a colloidal solution by adding a suitable electrolyte,known as a peptizing agent.

(B) Colloidal particles are aggregates of molecules or ions with diameters ranging from $1 \ nm$ to $1000 \ nm$.
Because colloidal particles are much larger than the solute particles in a true solution,the number of particles in a given mass of colloid is much smaller than in a true solution.
Since osmotic pressure is a colligative property and depends on the number of particles,the osmotic pressure of a colloidal solution is of a lower order than that of a true solution at the same concentration.
Therefore,the statement in option $B$ is $INCORRECT$.

(A) The movement of colloidal particles under an applied electric field is known as electrophoresis.
When the colloidal particles are positively charged,they migrate towards the cathode (the negatively charged electrode).
This specific movement of positively charged particles towards the cathode is known as cataphoresis.

(A) Arsenic sulphide $(As_2S_3)$ sol is a negatively charged sol.
According to the Hardy-Schulze rule,the coagulating power of an electrolyte depends on the valency of the active ion (the ion carrying a charge opposite to that of the sol).
For a negatively charged sol,the coagulating power of cations follows the order: $Al^{3+} > Ba^{2+} > Na^+ = K^+$.
Therefore,$AlCl_3$ is the most effective electrolyte for the coagulation of $As_2S_3$ sol.

(B) Starch sol is a lyophilic colloid.
Lyophilic colloids are stabilized by two factors: charge and solvation (hydration).
To coagulate a lyophilic sol,both factors must be removed.
Adding an electrolyte (like $NaCl$,$BaCl_2$,or $CaSO_4$) only neutralizes the charge but does not remove the solvation layer.
However,adding a dehydrating agent like an alcohol (e.g.,$C_2H_5OH$) removes the solvation layer,leading to coagulation.
Therefore,$C_2H_5OH$ is effective in coagulating starch sol.

(B) When a large number of atoms or smaller molecules of a substance aggregate together in a dispersion medium to form particles having sizes in the colloidal range $(1-1000 \ nm)$,the resulting colloidal systems are called multimolecular colloids.
$Fe(OH)_3$ sol is formed by the aggregation of a large number of $Fe(OH)_3$ molecules,thus it is a multimolecular colloid.

(B) Lyophilic colloids are solvent-attracting colloids. Gelatin is a protein and acts as a lyophilic colloid. It is also used as a protective colloid because it has a very low gold number.

(A) The classification of colloids is as follows:
$1$. $As_2S_3$ Sol is a lyophobic colloid and also a multimolecular colloid $(A \to P, R)$.
$2$. Sulphur Sol is a lyophobic colloid and also a multimolecular colloid $(B \to P, R)$.
$3$. Starch is a macromolecular colloid $(C \to Q)$.
$4$. Soap is an associated colloid $(D \to S)$.
Therefore,the correct matching is $A \to P, R; B \to P, R; C \to Q; D \to S$.

(C) Aqua dag is a colloidal solution of graphite in water.
It is also known as $C$ sol,where $C$ stands for carbon (graphite).

(C) The $As_2S_3$ sol is a negatively charged sol.
According to the Hardy-Schulze rule,the coagulating power of an electrolyte depends on the valency of the ion carrying a charge opposite to that of the colloidal particles.
For a negatively charged sol,the coagulating power increases with the increase in the valency of the cation.
In the given options,the cations are $Zn^{2+}$,$Na^+$,$Al^{3+}$,and $Zn^{2+}$.
The valencies are: $Zn^{2+} = +2$,$Na^+ = +1$,$Al^{3+} = +3$.
Since $Al^{3+}$ has the highest valency $(+3)$,it will have the maximum coagulating power for the negatively charged $As_2S_3$ sol.