1. A reaction is called redox mainly because it involves what linked change?
ⓐ. Oxidation and reduction occurring together
ⓑ. Dissolving and crystallisation occurring in the same vessel
ⓒ. Heating and cooling occurring at equal rates
ⓓ. Colour change and gas evolution occurring together
Correct Answer: Oxidation and reduction occurring together
Explanation: A redox reaction contains oxidation and reduction as connected processes. One substance undergoes oxidation while another undergoes reduction in the same overall chemical change. The name redox itself is formed from reduction and oxidation. Colour change, heat, or gas formation may accompany a reaction, but they do not define redox. A reaction is identified as redox by the chemical changes in the reacting species, not simply by visible observations.
2. Magnesium burns in oxygen to form magnesium oxide: \( \mathrm{2Mg + O_2 \rightarrow 2MgO} \). In the classical oxygen-addition sense, what happens to magnesium?
ⓐ. Magnesium is reduced because it gives a bright flame
ⓑ. Magnesium is oxidised because it combines with oxygen
ⓒ. Magnesium is neutralised because oxygen is a gas
ⓓ. Magnesium is decomposed because a new compound forms
Correct Answer: Magnesium is oxidised because it combines with oxygen
Explanation: In the classical definition, oxidation means addition of oxygen to a substance. Magnesium combines with oxygen and forms \( \mathrm{MgO} \), so magnesium is described as oxidised. The bright flame is an observation, not the basis of the redox term. The reaction is not neutralisation because no acid-base salt-and-water pattern is involved. Here, the chemical clue is the conversion of \( \mathrm{Mg} \) into an oxide.
3. In the reaction \( \mathrm{CuO + H_2 \rightarrow Cu + H_2O} \), the change \( \mathrm{CuO \rightarrow Cu} \) is best described classically as:
ⓐ. oxidation, because copper gains oxygen
ⓑ. oxidation, because hydrogen is present
ⓒ. reduction, because copper becomes black
ⓓ. reduction, because oxygen is removed
Correct Answer: reduction, because oxygen is removed
Explanation: Classical reduction can mean removal of oxygen from a substance. In this reaction, \( \mathrm{CuO} \) loses oxygen and is converted into copper metal, \( \mathrm{Cu} \). Hydrogen takes the oxygen and forms \( \mathrm{H_2O} \), but the asked change is specifically \( \mathrm{CuO \rightarrow Cu} \). Colour is not a reliable definition of reduction. The phrase “oxygen is removed” is the deciding classical clue in this example.
4. Removal of hydrogen from a substance is classified as which process in the older hydrogen-based language?
ⓐ. Oxidation
ⓑ. Reduction
ⓒ. Hydration
ⓓ. Precipitation
Correct Answer: Oxidation
Explanation: In the classical hydrogen-based description, oxidation may mean removal of hydrogen. This definition was useful for many early reactions where oxygen transfer was not the only visible change. Reduction, in the same older language, can mean addition of hydrogen. Hydration means addition or association of water, not removal of hydrogen. The word “oxidation” here is being used through the hydrogen-loss idea, not only through oxygen addition.
5. Nitrogen combines with hydrogen in \( \mathrm{N_2 + 3H_2 \rightarrow 2NH_3} \). Under the classical hydrogen-addition view, nitrogen is:
ⓐ. oxidised because it forms a compound
ⓑ. reduced because it gains hydrogen
ⓒ. unchanged because no oxygen appears
ⓓ. decomposed because the number of molecules changes
Correct Answer: reduced because it gains hydrogen
Explanation: Classical reduction includes addition of hydrogen to a substance. In \( \mathrm{N_2 + 3H_2 \rightarrow 2NH_3} \), nitrogen becomes part of \( \mathrm{NH_3} \), where it is combined with hydrogen. The absence of oxygen does not prevent the hydrogen-based classical definition from being used. A change in the number of molecules is not the criterion for reduction. This example shows why both oxygen-transfer and hydrogen-transfer descriptions were used in early redox language.
6. The paired changes \( \mathrm{CuO \rightarrow Cu} \) and \( \mathrm{H_2 \rightarrow H_2O} \) occur in the same reaction. What is the most suitable redox interpretation?
ⓐ. Both substances are reduced
ⓑ. Both substances are oxidised
ⓒ. \( \mathrm{CuO} \) is reduced while \( \mathrm{H_2} \) is oxidised
ⓓ. \( \mathrm{CuO} \) is oxidised while \( \mathrm{H_2} \) is reduced
Correct Answer: \( \mathrm{CuO} \) is reduced while \( \mathrm{H_2} \) is oxidised
Explanation: \( \mathrm{CuO} \) loses oxygen and becomes \( \mathrm{Cu} \), so it undergoes reduction in the classical oxygen-removal sense. \( \mathrm{H_2} \) gains oxygen and becomes \( \mathrm{H_2O} \), so it undergoes oxidation in the classical oxygen-addition sense. The two changes occur together in one reaction, making it a redox process. It is not enough to look at only one reactant; the paired chemical changes must be followed. This reaction is a useful bridge between old oxygen-transfer language and the broader redox idea.
7. Assertion: Oxidation and reduction occur together in an ordinary redox reaction.
Reason: The change that one substance undergoes is linked with the opposite change in another substance.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: A redox reaction is not just an isolated oxidation or an isolated reduction. The substance that is oxidised is accompanied by another substance that is reduced. In classical examples, one substance may gain oxygen while another loses oxygen. The Reason explains the linked nature of the two changes. Treating oxidation and reduction as separate unrelated events misses the central meaning of redox.
8. Match the basic redox terms in Column I with their meanings in Column II.
| Column I | Column II |
|---|
| P. Oxidised species | 1. Species that undergoes oxidation |
| Q. Reduced species | 2. Species that undergoes reduction |
| R. Oxidising agent | 3. Substance that brings about oxidation of another substance |
| S. Reducing agent | 4. Substance that brings about reduction of another substance |
ⓐ. P-2, Q-1, R-4, S-3
ⓑ. P-1, Q-2, R-3, S-4
ⓒ. P-3, Q-4, R-1, S-2
ⓓ. P-4, Q-3, R-2, S-1
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: The oxidised species is the one that itself undergoes oxidation. The reduced species is the one that itself undergoes reduction. An oxidising agent is named by what it causes in another substance: it brings about oxidation. A reducing agent brings about reduction of another substance. The agent name and the process undergone by the agent are not the same idea, so the wording must be read carefully.
9. A description says, “Oxidation always means addition of oxygen.” The most careful conclusion is:
ⓐ. the statement is complete for all redox reactions
ⓑ. the statement is useless even for burning reactions
ⓒ. an incomplete early definition of oxidation
ⓓ. the statement applies only to reactions in water
Correct Answer: an incomplete early definition of oxidation
Explanation: Addition of oxygen is a valid classical meaning of oxidation. It explains many familiar reactions such as combustion and oxide formation. However, redox reactions are not limited to oxygen addition. Some redox reactions involve hydrogen transfer or other changes that need a broader explanation. The classical definition is useful as a starting point, but it should not be treated as the whole meaning of redox.
10. For a balanced redox equation, the basic conservation requirement is that:
ⓐ. only the number of molecules must be equal on both sides
ⓑ. only the mass of solid substances must be conserved
ⓒ. the same physical states must appear on both sides
ⓓ. atoms and total charge must be conserved
Correct Answer: atoms and total charge must be conserved
Explanation: Chemical equations must obey conservation of mass, so the number of atoms of each element must match on both sides. In ionic redox equations, total charge must also be the same on both sides. Equal numbers of molecules are not required; coefficients are adjusted to conserve atoms and charge. Physical states may help describe the reaction, but they do not replace balancing. Charge conservation becomes especially important when ions such as \( \mathrm{Fe^{2+}} \), \( \mathrm{Cu^{2+}} \), or \( \mathrm{Cl^-} \) appear.
11. Rusting of iron is commonly introduced as an oxidation process because iron:
ⓐ. changes into a liquid
ⓑ. loses its metallic colour only
ⓒ. separates from oxygen to form pure metal
ⓓ. combines with oxygen in moist air
Correct Answer: combines with oxygen in moist air
Explanation: Rusting is a familiar example where iron is oxidised. In the classical sense, oxidation involves addition of oxygen, and rust contains iron combined with oxygen in a hydrated form. Moisture helps the process occur, but the redox idea is connected with the chemical change of iron. A colour change alone is not the definition of oxidation. The observation becomes meaningful only when linked with the formation of an oxygen-containing iron compound.
12. The entry that needs repair in the classical redox table is:
| Row | Change described | Classical label |
|---|
| P | Addition of oxygen | Oxidation |
| Q | Removal of hydrogen | Oxidation |
| R | Removal of oxygen | Reduction |
| S | Addition of oxygen | Reduction |
ⓐ. Row S
ⓑ. Row P
ⓒ. Row Q
ⓓ. Row R
Correct Answer: Row S
Explanation: Addition of oxygen is classically called oxidation, so Row P is sound. Removal of hydrogen is also a classical description of oxidation, so Row Q is sound. Removal of oxygen is classically called reduction, so Row R is sound. Row S reverses the oxygen-transfer idea because addition of oxygen should not be labelled reduction. The safest way to read this table is to pair oxygen gain with oxidation and oxygen loss with reduction.
13. Which statements follow the classical redox orientation?
I. Addition of oxygen to a substance can be called oxidation.
II. Removal of oxygen from a substance can be called reduction.
III. A reaction without oxygen or hydrogen can never be redox.
ⓐ. I only
ⓑ. II and III only
ⓒ. I, II and III
ⓓ. I and II only
Correct Answer: I and II only
Explanation: Statement I agrees with the classical oxygen-addition definition of oxidation. Statement II agrees with the classical oxygen-removal definition of reduction. Statement III is too narrow because classical definitions are limited and later redox ideas cover reactions without direct oxygen or hydrogen transfer. The first two statements describe useful starting definitions, while the third treats those starting definitions as final. A narrow definition can explain some reactions well without explaining all redox reactions.
14. In \( \mathrm{PbO + C \rightarrow Pb + CO} \), carbon changes into \( \mathrm{CO} \). Under the oxygen-transfer view, carbon is:
ⓐ. oxidised because it gains oxygen
ⓑ. reduced because it gains oxygen
ⓒ. reduced because \( \mathrm{PbO} \) contains oxygen
ⓓ. unchanged because carbon remains carbon
Correct Answer: oxidised because it gains oxygen
Explanation: Carbon starts as \( \mathrm{C} \) and becomes part of \( \mathrm{CO} \), so oxygen has been added to carbon. By the classical oxygen-transfer definition, addition of oxygen is oxidation. At the same time, \( \mathrm{PbO} \) loses oxygen and is reduced to \( \mathrm{Pb} \). The element name remaining carbon does not mean the chemical change is absent. The process is judged by the change in chemical combination, not by whether the element symbol still appears.
15. In older oxygen-hydrogen language, removal of hydrogen from a compound is classified as ______.
ⓐ. reduction
ⓑ. oxidation
ⓒ. hydrolysis
ⓓ. neutralisation
Correct Answer: oxidation
Explanation: The blank asks for the classical process name linked with loss of hydrogen. In that language, removal of hydrogen is treated as oxidation. The paired idea is that addition of hydrogen is treated as reduction. Hydrolysis involves reaction with water, and neutralisation involves acid-base reaction, so they are not suitable here. This definition is especially useful when no direct oxygen addition is being discussed.
16. Read the reaction note below.
Black \( \mathrm{CuO} \) is heated in a stream of dry \( \mathrm{H_2} \). A reddish-brown solid \( \mathrm{Cu} \) forms, and water droplets are observed in the cooler part of the tube.
The best interpretation of the note is:
ⓐ. \( \mathrm{CuO} \) is oxidised and \( \mathrm{H_2} \) is reduced
ⓑ. both \( \mathrm{CuO} \) and \( \mathrm{H_2} \) are reduced
ⓒ. \( \mathrm{CuO} \) is reduced and \( \mathrm{H_2} \) is oxidised
ⓓ. no redox change occurs because copper is a metal
Correct Answer: \( \mathrm{CuO} \) is reduced and \( \mathrm{H_2} \) is oxidised
Explanation: \( \mathrm{CuO} \) becomes \( \mathrm{Cu} \), so oxygen is removed from copper oxide. That is reduction in the classical oxygen-removal sense. \( \mathrm{H_2} \) becomes \( \mathrm{H_2O} \), so oxygen is added to hydrogen. That is oxidation in the classical oxygen-addition sense. The observations are useful because they support the chemical changes: copper formation and water formation occur together.
17. A notebook pairs some changes with classical labels. The unsupported pairing is:
| Row | Change | Label |
|---|
| P | \( \mathrm{Mg \rightarrow MgO} \) | Oxidation |
| Q | \( \mathrm{CuO \rightarrow Cu} \) | Reduction |
| R | Loss of hydrogen from a substance | Oxidation |
| S | Addition of hydrogen to a substance | Oxidation |
ⓐ. Row P
ⓑ. Row S
ⓒ. Row Q
ⓓ. Row R
Correct Answer: Row S
Explanation: \( \mathrm{Mg \rightarrow MgO} \) shows addition of oxygen to magnesium, so Row P is supported. \( \mathrm{CuO \rightarrow Cu} \) shows removal of oxygen from copper oxide, so Row Q is supported. Loss of hydrogen is a classical form of oxidation, so Row R is also supported. Addition of hydrogen is classically associated with reduction, not oxidation. The pair “loss of hydrogen” and “gain of hydrogen” must not be given the same label.
18. Use the arrangement described below.
A clean zinc strip is dipped into blue \( \mathrm{CuSO_4} \) solution. After some time, a reddish-brown deposit appears on the strip, and the blue colour fades.
Why does the older oxygen-hydrogen definition feel insufficient for this reaction?
ⓐ. The reaction has no visible observation
ⓑ. The reaction occurs only because water evaporates
ⓒ. The change must be neutralisation because \( \mathrm{CuSO_4} \) is a salt
ⓓ. The change is redox, not an old oxygen-hydrogen case
Correct Answer: The change is redox, not an old oxygen-hydrogen case
Explanation: In this displacement reaction, zinc changes into zinc ions while copper ions change into copper metal. The visible copper deposit and fading blue colour show that a chemical change has occurred. However, the simple oxygen-addition or hydrogen-removal definitions do not directly describe this process. Such examples motivate the need for a broader redox idea. The older definitions are useful, but metal displacement reactions show why they are not enough.
19. A reaction record says that substance \( \mathrm{P} \) gains oxygen while substance \( \mathrm{Q} \) loses oxygen in the same reaction. The record should be read as:
ⓐ. \( \mathrm{P} \) is reduced and \( \mathrm{Q} \) is oxidised
ⓑ. both \( \mathrm{P} \) and \( \mathrm{Q} \) are oxidised
ⓒ. \( \mathrm{P} \) is oxidised and \( \mathrm{Q} \) is reduced
ⓓ. both \( \mathrm{P} \) and \( \mathrm{Q} \) are reduced
Correct Answer: \( \mathrm{P} \) is oxidised and \( \mathrm{Q} \) is reduced
Explanation: In the classical oxygen-transfer definition, gaining oxygen means oxidation. Therefore \( \mathrm{P} \), which gains oxygen, is oxidised. Losing oxygen means reduction, so \( \mathrm{Q} \) is reduced. The same reaction contains both changes, so it fits the linked redox pattern. The direction of oxygen transfer decides the label, not the order in which the substances are named.
20. The first limitation of defining oxidation only as oxygen addition is that this definition:
ⓐ. cannot explain combustion of carbon
ⓑ. makes oxide formation impossible to classify
ⓒ. does not cover many displacement reactions
ⓓ. removes the need to study reduction
Correct Answer: does not cover many displacement reactions
Explanation: Oxygen addition explains many familiar oxidations, including combustion and oxide formation. It also gives a simple entry point into redox language. However, many reactions later treated as redox do not show direct oxygen addition. Metal displacement reactions are a common early example where the oxygen-only definition becomes too narrow. This limitation does not make the classical definition wrong; it shows why a broader definition is needed.
