1

### JEE Main 2016 (Online) 10th April Morning Slot

Gold numbers of some colloids are : Gelatin : 0.005 - 0.01, Gum Arabic : 0.15 - 0.25; Oleate : 0.04 - 1.0; Starch : 15 - 25. Which among these is a better protective colloid ?
A
Gelatin
B
Gum Arabic
C
Oleate
D
Starch
2

### JEE Main 2017 (Offline)

A metal crystallises in a face centred cubic structure. If the edge length of its unit cell is ‘a’, the closest approach between two atoms in metallic crystal will be:
A
$2\sqrt 2 a$
B
$\sqrt 2 a$
C
${a \over {\sqrt 2 }}$
D
2a

## Explanation

For a fcc unit cell

$r = {{\sqrt 2 a} \over 4}$

$\therefore$ Closest distance (2r) = ${{\sqrt 2 a} \over 4} = {a \over {\sqrt 2 }}$
3

### JEE Main 2017 (Offline)

The Tyndall effect is observed only when following conditions are satisfied:
(a) The diameter of the dispersed particles is much smaller than the wavelength of the light used.
(b) The diameter of the dispersed particle is not much smaller than the wavelength of the light used.
(c) The refractive indices of the dispersed phase and dispersion medium are almost similar in magnitude.
(d) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.
A
(b) and (d)
B
(a) and (c)
C
(b) and (c)
D
(a) and (d)

## Explanation

Tyndall effect is observed only when

(i) The diameter of the dispersed particle is not much smaller than the wavelength of the light used.

(ii) The refractive indices of the dispersed phase and dispersion medium differ greatly in magnitude.

So, (b) and (d) are correct.
4

### JEE Main 2017 (Online) 8th April Morning Slot

Among the following, correct statement is :
A
Brownian movement is more pronounced for smaller particles than for bigger–particles
B
Sols of metal sulphides are lyophilic.
C
Hardy Schulze law states that bigger the size of the ions, the greater is its coagulating power.
D
One would expect charcoal to adsorb chlorine more than hydrogen sulphide.

## Explanation

(a)   Brownian motion caused by the bombardment of the particles by molecules of the median. In smaller particle the bombardment is not effective enough so that it can change it's direction and perform zig zag motion. But for bigger particle zig zag motion happens as bombardment is effective.

(b)    Inorganic sols are lyophobic. Metal sulphates are inorganic sols so it is lyophotic.

(c)    Hardly Schulze does not depends on the size of the ion it depends on the size of the ion it depends on the charge on the ion.

(d)   Molar mass of Cl2 is 71 and molar mass of H2S is 34 and in both the molecule Nanderwaal force of attraction present. As Cl2 has more mass than H2S So it's vanderwall force of attraction will be more and tendency to melt will also be more. So, Cl2 will get easily absorbed.

NEET