Physical and Chemical Changes, solution

Total Questions: 33

31. Match List-I (oxidation number) with List-II (the element) and select the correct answer using the codes given below the lists: [I.A.S. (Pre) 2002]

List-IList-II
A. 21.Oxidation number of Mn in MnO₂
B. 32.Oxidation number of S in H₂SO₄
C. 43.Oxidation number of Ca in CaO
D. 64.Oxidation number of Al in NaAlH₄
CodeABCD
(a)3412
(b)4312
(c)3241
(d)4321
Correct Answer: (a)
Solution:The oxidation number of any compound is always zero. This means that sum of the oxidation number of all the elements present in the compound is zero. By this rule, we can now find the oxidation number in given options here:

  1. Suppose, the oxidation number of Mn in MnO₂ is . MnO₂ contains two oxygen ions, each with an oxidation number of (-2). +2 x (−2)=0          X - 4

  2. Suppose, the oxidation number of S in H2SO4 is . H₂SO₄ contains two hydrogen ions, each with an oxidation number of (+1) and oxygen ions with oxidation number  2(+1)+X+4(−2)=0         ⇒ 2+ X = 0                       

  3. The oxidation number of Ca in CaO when oxygen has oxidation number (-2).X−2=0                           ⇒X=2

  4. Suppose, the oxidation number of Al in NaAlH₄  is . NaAlH₄ contains one Na which has oxidation number (+1) and four H which has oxidation number (-1).     (+1)+X+4(−1)=0              

    ⇒    1 + X 4 = 0                         

32. Which of the following shows down the reaction rate ? [69th B.P.S.C. (Pre) 2023]

Correct Answer: (d) A heterogeneous catalyst
Solution:Catalytic promoter or accelerators are those substances used to increase the activity of catalysts positively. Reactions that are facilitated by catalysts can be divided into two major classes: homogeneous catalysis and heterogeneous catalysis. A homogeneous catalyst is present in the same phase as the reactants. A heterogeneous catalyst is present in a different phase from the reactants. All of these facilitate reactions. Hence, option (d) is correct.

33. Example of corrosion is: [Chhattisgarh P.C.S. (Pre) 2016]

Correct Answer: (d) Above three
Solution:Gradual decay of metals by air or chemical substances is called corrosion. Rusting on iron i.e. brown coating on iron, green coating on copper and black coating on silver, all these are examples of corrosion.