NEET Practice Questions (MCQs) with Answers & Solutions

Electromagnetic waves are self-sustaining oscillations of electric and magnetic fields. They do not require a material medium and propagate even in vacuum, making them transverse waves. The electric and magnetic fields oscillate sinusoidally in space and time, are perpendicular to each other, and to the direction of propagation. (Context: 'They are self-sustaining oscillations of electric and magnetic fields in free space, or vacuum. They differ from all the other waves we have studied so far, in respect that no material medium is involved... The oscillating electric and magnetic fields... are perpendicular to each other, and to the direction of propagation of the electromagnetic wave.')

An oscillating charge produces an oscillating electric field, leading to an electromagnetic wave. The frequency of the electromagnetic wave naturally equals the frequency of oscillation of the charge. (Context: 'The frequency of the electromagnetic wave naturally equals the frequency of oscillation of the charge.')

According to the NCERT text, 'A combination reaction may be denoted in the manner: A + B \rightarrow C. Either A and B or both A and B must be in the elemental form for such a reaction to be a redox reaction.' In option 3, Carbon (C) and Oxygen ($O_2$) are in their elemental forms, and their oxidation states change (0 to +4 for C, 0 to -2 for O). This makes it a redox combination reaction. Options 1 and 4 are decomposition reactions where elements are formed. Option 2 is a decomposition reaction where no element is formed, and oxidation states do not change, hence it is not a redox reaction.

The NCERT text states, 'In a displacement reaction, an ion (or an atom) in a compound is replaced by an ion'. In this reaction, elemental sodium (Na) displaces hydrogen from water, forming hydrogen gas and sodium hydroxide. Sodium (0 oxidation state) is oxidized to Na(+1), and Hydrogen (+1 in water) is reduced to H(0) in $H_2$. Therefore, it is a displacement redox reaction.

The NCERT text specifies, 'Precisely, a decomposition reaction leads to the breakdown of a compound into two or more components at least one of which must be in the elemental state.' It also clarifies, 'It may also be noted here that all decomposition reactions are not redox reactions. For example, decomposition of calcium carbonate is not a redox reaction.' This directly supports option 2 and refutes option 1. Option 3 is incorrect because if it's a redox reaction, oxidation numbers must change. Option 4 is too restrictive; while some are, not all decomposition reactions are the direct reverse of combination reactions involving only elements.

The NCERT text explicitly lists '$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$' as an example under '1. Combination reactions'. Although it looks like a combustion reaction, combustion reactions are a type of combination reaction, especially when involving elemental dioxygen as mentioned in the text: 'All combustion reactions, which make use of elemental dioxygen, as well as other reactions involving elements other than dioxygen, are redox reactions.'

The NCERT text states, 'It may carefully be noted that there is no change in the oxidation number of hydrogen in methane under combination reactions and that of potassium in potassium chlorate in reaction (7.28). This may also be noted here that all decomposition reactions are not redox reactions. For example, decomposition of calcium carbonate is not a redox reaction.' In $CaCO_3 \rightarrow CaO + CO_2$, the oxidation states are: Ca (+2, +2, +2), C (+4, +4), O (-2, -2, -2). Since no oxidation states change, it is not a redox reaction.

The NCERT text states, 'A combination reaction may be denoted in the manner: A + B \rightarrow C. Either A and B or both A and B must be in the elemental form for such a reaction to be a redox reaction.' This implies changes in oxidation states are necessary. The formation of an oxide is a common example but not a necessary condition. For instance, $3Mg(s) + N_2(g) \rightarrow Mg_3N_2(s)$ is a redox combination reaction but does not produce an oxide.

The NCERT text explicitly states, 'Either A and B or both A and B must be in the elemental form for such a reaction to be a redox reaction.' This covers the conditions under which a combination reaction typically becomes a redox reaction due to changes in oxidation states from the elemental state (0).

The NCERT text defines decomposition reactions as 'opposite of combination reactions. Precisely, a decomposition reaction leads to the breakdown of a compound into two or more components at least one of which must be in the elemental state.' If the elemental state is achieved from a compound, a change in oxidation state occurs, making it a redox reaction.