Surface Chemistry

NEET Chemistry · 98 questions · Page 10 of 10 · Click an option or "Show Solution" to reveal answer

Q91

The conditions given below are in the context of observing Tyndall effect in colloidal solutions : (A) The diameter of the colloidal particles is comparable to the wavelength of light used. (B) The diameter of the colloidal particles is much smaller than the wavelength of light used. (C) The diameter of the colloidal particles is much larger than the wavelength of light used. (D) The refractive indices of the dispersed phase and the dispersion medium are comparable. (E) The dispersed phase has a very different refractive index from the dispersion medium. Choose the most appropriate conditions from the options given below :

A (A) and (E) only

B (C) and (D) only

C (A) and (D) only

D (B) and (E) only

Correct Answer

Option A

Solution

The phenomenon of scattering of light by colloidal particles as a result of which the path of the beam becomes visible is called a tyndall effect.

Smaller the diameter and similar the magnitude of refractive indices, lesser is the scattering and hence the tyndall effect and viced-versa.

The diameter of the dispersed phase particle should not be smaller than the wavelength of light used because they won't be able to scatter the light so, therefore, the diameter of the dispersed particles should be equal or not much smaller than the wavelength of the light used.

The refractive indies (i.e. the ratio of the velocity of light in vacuum to the velocity of light in any medium) of the dispersed phase and the dispersion medium should differ greatly in magnitude than only the particles will be able to scatter the light and tyndall effect will be obersved.

On the other hand, if the refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude, then there will be no-scattering of light and hence, therefore, no Tyndall effect effect is observed.

Hence answer (A) and (E) are correct.

Q92

Which one of the following characteristics is not correct for physical adsorption?

A Adsorption increases with increase in temperature

B Adsorption is spontaneous

C Both enthalpy and entropy of adsorption are negative

D Adsorption on solids is reversible

Correct Answer

Option A

Solution

As adsorption is an exothermic process.

$\therefore$ Rise in temperature will decrease adsorption (according to Le-chatelier principle).

Q93

In Freundlich adsorption isotherm at moderate pressure, the extent of adsorption

\left( {{x \over m}} \right)

is directly proportional to Px. The value of x is :

\infty

B zero

C 1

{{1 \over n}}

Correct Answer

Option D

Solution

According to Freundlich adsorption isotherm, in the median range of pressure

{x \over m} \propto {P^{{1 \over n}}}

$\Rightarrow$

{x \over m}

= kP

^{{1 \over n}}

Hence, x =

{{1 \over n}}

Q94

TLC analysis of a mixture having 3 components (A, B, C) using silica gel as the stationary phase gave following Rf values; for A = 0.72, B = 0.48, C = 0.20. Regarding the above observations, which one of the following statements is not correct for column chromatography of the mixture?

A A is the most non-polar component

B C is the most soluble component in mobile phase

C A will be eluted first

D C is the most strongly absorbed component

Correct Answer

Option B

Solution

The $\mathrm{R}_{f}$ (retardation factor) value is the ratio of the Solute's distance travelled to the Solvent's distance travelled.

Polar molecules will attract more toward adsorbent and cover smaller distance from the baseline.

A has maximum $\mathrm{R}_{f}$ so it will less adsorbed on the adsorbent surface and travel faster and will be eluded first. $\mathrm{C}$ has minimum $\mathrm{R}_{f}$ value.

So it is more strongly adsobred and less soluble in mobile phase.

Q95

Peptization is a :

A process of converting soluble particles to form colloidal solution

B process of converting precipitate into colloidal solution

C process of bringing colloidal molecule into solution

D process of converting a colloidal solution into precipitate

Correct Answer

Option B

Solution

Peptisation is the process of converting a precipitate into a colloidal solution by shaking it with dispersion medium in the presence of small amount of electrolyte.

Q96

A sample of red ink (a colloidal suspension) is prepared by mixing eosin dye, egg white, HCHO and water. The component which ensures stability of the ink sample is :

A HCHO

B Water

C Eosin dye

D Egg white

Correct Answer

Option D

Solution

Egg white is used to stabilise the prepared colloidal sol.of red ink.

Q97

Alum helps in purifying water by :

A forming Si complex with clay particles

B sulphate part which combines with the dirt and removes it

C coagulating the mud particles

D making mud water soluble

Correct Answer

Option C

Solution

Alum furnishes

A{l^{3 + }}

ions which bring about coagulation of negatively charged clay particles, bacteria etc.

Q98

Given below are two statements : Statement I : Emulsions of oil in water are unstable and sometimes they separate into two layers on standing. Statement II : For stabilisation of an emulsion, excess of electrolyte is added. In the light of the above statements, choose the most appropriate answer from the options given below :

A Both Statement I and Statement II are correct.

B Both Statement I and Statement II are incorrect.

C Statement I is correct but Statement II is incorrect.

D Statement I is incorrect but Statement II is correct.

Correct Answer

Option C

Solution

Oil in water emulsions can sometimes separate into two layers on standing.

The most relevant example for the above case is milk, which can separate into two layers on standing for a longer time.

Therefore, statement (I) is correct.

On adding excess of electrolyte, coagulation occurs and emulsion is further destabilised.

Therefore, statement (II) is incorrect.

Ready for a full NEET mock test? Timed · full syllabus · instant results

Take a Mock Test →