81. Write the products formed when aluminium metal is heated with manganese dioxide:
(A)
(B)
(C)
(D)
Answer:
(D)
Explanation:
Aluminium is more highly reactive than manganese. During this thermite displacement reaction, aluminium vigorously strips the oxygen from manganese dioxide, leaving pure manganese () and forming aluminium oxide ().
82. Pick out the chemically most reactive element from the given triads: Li, Na, K and F, Cl, Br
(A) Li and F
(B) Li and Br
(C) K and F
(D) K and Br
Answer:
(C) K and F
Explanation:
For alkali metals, reactivity increases down the group, making Potassium (K) the most reactive. For halogens, reactivity decreases down the group, making Fluorine (F) the most reactive non-metal.
83. Which of the following compounds can not exhibit chain isomerism?
(A) Propane
(B) Butane
(C) Pentane
(D) Hexane
Answer:
(A) Propane
Explanation:
Chain isomerism requires the carbon backbone to be branched. Propane contains only three carbon atoms, making it geometrically impossible to form any structurally branched chain isomers.
84. Name the functional group present in :
(A) Alcohol
(B) Carboxylic acid
(C) Ketone
(D) Aldehyde
Answer:
(C) Ketone
Explanation:
The molecule is propanone (acetone), which features a carbonyl group () structurally bonded between two other carbon atoms, defining it strictly as a ketone.
85. The properties of eka-aluminium predicted by Mendeleev are the same as the properties of later discovered element:
(A) Scandium
(B) Germanium
(C) Gallium
(D) Aluminium
Answer:
(C) Gallium
Explanation:
Dmitri Mendeleev predicted the existence of an undiscovered element below aluminium on his periodic table, which he named “eka-aluminium”. When Gallium was later discovered, its properties matched his predictions flawlessly.
