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1

### JEE Main 2021 (Online) 27th August Morning Shift

Deuterium resembles hydrogen in properties but :
A
reacts slower than hydrogen
B
reacts vigorously than hydrogen
C
reacts just as hydrogen
D
emits $$\beta$$+ particles

## Explanation

The bond dissociation energy of D2 is greater than H2 and therefore D2 reacts slower than H2.
2

### JEE Main 2021 (Online) 26th August Evening Shift

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : Heavy water is used for the study of reaction mechanism.

Reason (R) : The rate of reaction for the cleavage of O - H bond is slower than that of O - D bond.

Choose the most appropriate answer from the options given below :
A
Both (A) and (R) are true but (R) is not the true explanation of (A).
B
Both (A) and (R) are true and (R) is the true explanation of (A).
C
(A) is false but (R) is true.
D
(A) is true but (R) is false.

## Explanation

D2O in used for the study of reaction mechanism. Rate of reaction for the cleavage of O-H bond > O-D bond.
3

### JEE Main 2021 (Online) 26th August Morning Shift

Which one of the following methods is most suitable for preparing deionized water?
A
Synthetic resin method
B
Clark's method
C
Calgon's method
D
Permutit method

## Explanation

Pure demineralised (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the H+ form) and an anion exchange (in the OH$$-$$ form) resins.
4

### JEE Main 2021 (Online) 26th August Morning Shift

The conversion of hydroxyapatite occurs due to presence of F$$-$$ ions in water. The correct formula of hydroxyapatite is :
A
[3Ca3(PO4)2 . Ca(OH)2]
B
[3Ca(OH)2 . CaF2]
C
[Ca3(PO4)2 . CaF2]
D
[3Ca3(PO4)2 . CaF2]

## Explanation

The F$$\Theta$$ ions make the enamel on teeth much harder by converting hydroxyapatite, [3Ca3(PO4)2] . Ca(OH)2], the enamel on the surface of the teeth into much harder fluroappatite. [3Ca3(PO4)2 . CaF2]

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