Adsorption in Solutions – Examples, Characteristics, Practice Problems and FAQ

Consider that after making potato curry, you decide to taste it first before serving it to your family. Unfortunately, you discover that your curry contains a lot of salt.

What would you do now when everyone is eager to taste it at the dinner table and you don't have time to prepare a new curry?

To find a solution, you decide to approach your mother. Your mother adds flour to the dough and after 5 to 10 minutes, it tastes excellent when you sample it. Right now, the salt in your curry is merely what you find to be appropriate.

What is this hack? What phenomenon is responsible for lowering the amount of salt from your curry?

Adsorption is the name given to this phenomenon. Salt is adsorbed from the curry (a liquid) on the surface of the dough in this sort of adsorption. Let's investigate adsorption in solutions in detail on this concept page.

TABLE OF CONTENTS

Adsorption in Solutions
Examples of Adsorption in Solutions
Factors Affecting Adsorption in Solutions
Positive and Negative Adsorption
Freundlich Adsorption Isotherm for Adsorption in Solutions
Practice Problems
Frequently Asked Questions – FAQ

Adsorption in Solutions

Solids can adsorb solutes from solutions. The process of preferential adsorption of solute or solvent molecules from a solution by an adsorbent is known as adsorption from solution. If the solute is more adsorbed by the adsorbent than the solvent, the solution becomes less concentrated and vice versa.

Examples of Adsorption in Solutions

An aqueous solution of methylene blue is shaken with activated charcoal. Blue coloured solution turns colourless.

A solution of acetic acid in water is shaken with charcoal. A part of the acid is adsorbed by the charcoal and, the concentration of the acid decreases.

Raw brown sugar is passed over the charcoal bed to remove the brown coloured matter and give white sugar crystals. Animal charcoal removes the colours of solutions by adsorbing coloured impurities.

Silica gel sachets are used in many moisture sensitive medicines and electrical appliances due to its hygroscopic properties. Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.

Column chromatography is based on the different adsorbing tendencies of molecules toward silica (stationary phase). Chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical and industrial fields.

Factors Affecting Adsorption in Solutions

The surface area of the adsorbent: The extent of adsorption increases with increase in the surface area of adsorbent.

Temperature: The extent of adsorption decreases with increase in temperature.

Concentration: The extent of adsorption depends on the concentration of the solute in solution. As the concentration of the solute in the solution increases, the extent of adsorption increases.

Nature of the adsorbent and the adsorbate: The extent of adsorption depends on the nature of the adsorbent and the adsorbate.

Positive and Negative Adsorption

Positive Adsorption: Adsorption of solutes from a solution is referred to as positive adsorption. As a result, the concentration of the solution drops. Example: Adsorption of oxalic acid by charcoal.

Negative Adsorption: Adsorption of the solvent molecules from a solution is known as negative adsorption. As a result, the concentration of the solution rises. Example: Adsorption of solvent molecules by black charcoal in a KCl solution.

Freundlich Adsorption Isotherm for Adsorption in Solutions

Frendlich’s equation approximately describes the behavior of adsorption from the solution. But for the solution phase, the concentration of the solution is taken into account instead of pressure.

Frendlich’s equation can be states as:

$\frac{x}{m} = k C^{\frac{1}{n}}$ …………… (i)

Where, k and n are constants

x is the mass of the solute adsorbed

m is the mass of adsorbent

C is the concentration of the solution

Note: At a particular temperature, k and n depend on the nature of the solute and the adsorbent.

Taking log on both the sides of equation (i),

$l o g \frac{x}{m} = l o g k {+ \frac{1}{n} l o g C}^{}$ ……………. (ii)

Comparing (ii) with the equation of a straight line, y=mx +c

Slope (m) = $\frac{1}{n}$

Intercept (c) = log k

Freundlich isotherm is valid if the plot is a straight line. It explains the behavior of adsorption in an approximate manner.

The factor ( $\frac{1}{n}$ ) can have values between 0-1.

Practice Problems

Q1. The value of k and n in the Frendlich’s equation, $\frac{x}{m} = k C^{\frac{1}{n}}$ depend on:

A. Nature of solute
B. Nature of adsorbent
C. Both A and B
D. None of the above

Answer: C
Solution: Frendlich’s equation can be states as $\frac{x}{m} = k C^{\frac{1}{n}}$

Where, k and n are constants

x is the mass of the solute adsorbed

m is the mass of adsorbent

As seen from the equation, the value of k and n depend on both the nature of the solvent and the adsorbent.

So, option C is the correct answer.

Q2. How does the adsorption of solute from a solution phase depend on temperature?

A. Increases with increase temperature
B. Remains constant increase temperature
C. Decreases with increase temperature
D. None of the above

Answer: C

Solution: The extent of adsorption is inversely proportional to temperature, i.e the extent of adsorption decreases with increase in temperature.

So, option C is the correct answer.

Q3. Silica and aluminium gels removes moisture from the surrounding by:

A. Adsorbing moisture
B. Absorbing moisture
C. Both A and B
D. None of the above

Answer: A

Solution: Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity due to their hygroscopic properties.

So, option A is the correct answer.

Q4. In Freundlich’s equation, for the solution phase, __________ of the solution is taken into account instead of pressure.

A. Temperature
B. Concentration
C. Nature
D. Volume

Answer: B

Solution: In Freundlich’s equation, for the solution phase, the concentration of the solution is taken into account instead of pressure.

Frendlich’s equation can be states as:

$\frac{x}{m} = k C^{\frac{1}{n}}$

Where, k and n are constants

x is the mass of the solute adsorbed

m is the mass of adsorbent

C is the concentration of the solution

So, option B is the correct answer.

Frequently Asked Questions – FAQ

Q1. Does the factors affecting adsorption of gases on solid same or different from adsorption from solution phase?
Answer: Both adsorption of gases on solid and adsorption from solution phase depends on same factors. Factors like nature of adsorbent, pressure/concentration, temperature etc. are same for both.

Q2. Which purification technique is based on the principle of adsorption?
Answer: Column chromatography's purification method is based on the concept of adsorption. In the adsorbent, silica gel is used. Basic chemicals are adsorbed onto the silica gel after the eluted acidic molecules.

Q3. Is adsorption from solution phase is physisorption chemisorption?
Answer: In the adsorption process from the solution phase there is no bond formation between the adsorbent and the adsorbate. Therefore, the adsorption from solution phase is a physisorption reaction.

Q4. Why does adsorption decrease with increase in temperature?
Answer: Physisorption is often a reversible process. Weak van der Waals' force is involved in physical adsorption. As the temperature increases, these forces deteriorate. So, adsorption decreases with the increase in temperature.