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Applied Chemistry

Oxidation and Reduction Practice Questions

20 free Oxidation and Reduction practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 20 Medium

In modern chemistry, oxidation is best defined as:

  1. A Gain of electrons
  2. B Loss of oxygen
  3. C Gain of hydrogen only
  4. D Loss of electrons

Correct answer: Loss of electrons

Oxidation is the loss of electrons by an atom, ion, or molecule. This definition is broader than the older idea that oxidation only means gain of oxygen.

Question 2 of 20 Medium

In modern chemistry, reduction is best defined as:

  1. A Loss of electrons
  2. B Gain of oxygen
  3. C Gain of electrons
  4. D Loss of metal

Correct answer: Gain of electrons

Reduction is the gain of electrons by a substance. When a substance gains electrons, its oxidation state usually decreases.

Question 3 of 20 Medium

Which phrase correctly summarizes oxidation and reduction?

  1. A Oxidation is gain, reduction is loss
  2. B Both oxidation and reduction mean gain of oxygen
  3. C Both oxidation and reduction mean loss of electrons
  4. D Oxidation is loss, reduction is gain

Correct answer: Oxidation is loss, reduction is gain

A common memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain. It refers to the loss or gain of electrons.

Question 4 of 20 Medium

In the reaction Zn + CuSO4 → ZnSO4 + Cu, which substance is oxidized?

  1. A Copper ion
  2. B Sulfate ion
  3. C Zinc metal
  4. D Copper metal

Correct answer: Zinc metal

Zinc loses electrons and changes from Zn to Zn2+ in zinc sulfate. Since loss of electrons is oxidation, zinc is oxidized.

Question 5 of 20 Medium

In the reaction Zn + CuSO4 → ZnSO4 + Cu, which substance is reduced?

  1. A Zinc
  2. B Sulfate ion
  3. C Copper ion
  4. D Zinc sulfate

Correct answer: Copper ion

Copper ions gain electrons and form copper metal. Since gain of electrons is reduction, the copper ion is reduced.

Question 6 of 20 Medium

A reducing agent is a substance that:

  1. A Gains electrons and gets oxidized
  2. B Only gains oxygen
  3. C Does not take part in redox reactions
  4. D Loses electrons and gets oxidized

Correct answer: Loses electrons and gets oxidized

A reducing agent donates electrons to another substance and causes that substance to be reduced. In the process, the reducing agent itself gets oxidized.

Question 7 of 20 Medium

An oxidizing agent is a substance that:

  1. A Donates electrons and gets oxidized
  2. B Always forms a precipitate
  3. C Accepts electrons and gets reduced
  4. D Never changes oxidation state

Correct answer: Accepts electrons and gets reduced

An oxidizing agent accepts electrons from another substance and causes oxidation. While doing so, it gets reduced.

Question 8 of 20 Medium

Which of the following is an example of oxidation in daily life?

  1. A Melting of ice
  2. B Dissolving sugar in water
  3. C Rusting of iron
  4. D Breaking glass

Correct answer: Rusting of iron

Rusting of iron involves reaction with oxygen and moisture to form hydrated iron oxide. This is a common oxidation process.

Question 9 of 20 Medium

Which condition is generally needed for rusting of iron?

  1. A Dry nitrogen only
  2. B Pure hydrogen only
  3. C Absence of air and water
  4. D Oxygen and moisture

Correct answer: Oxygen and moisture

Iron rusts when it is exposed to both oxygen and water or moisture. Removing either oxygen or moisture can slow or prevent rusting.

Question 10 of 20 Medium

Which of the following methods helps prevent rusting?

  1. A Keeping iron wet
  2. B Adding salt water
  3. C Scratching off protective coating
  4. D Painting the iron surface

Correct answer: Painting the iron surface

Painting forms a protective barrier between iron and the surrounding air and moisture. This reduces contact with oxygen and water, preventing rust formation.

Question 11 of 20 Medium

The process of coating iron with zinc to prevent rusting is called:

  1. A Crystallization
  2. B Sublimation
  3. C Galvanization
  4. D Neutralization

Correct answer: Galvanization

Galvanization is the process of coating iron or steel with zinc. Zinc protects iron from corrosion by acting as a barrier and also as a sacrificial metal.

Question 12 of 20 Medium

In the reaction 2Mg + O2 → 2MgO, magnesium is:

  1. A Reduced because it gains extra electrons
  2. B Unchanged because no reaction occurs
  3. C Neutralized by forming a salt
  4. D Oxidized because it reacts with oxygen

Correct answer: Oxidized because it reacts with oxygen

Magnesium combines with oxygen to form magnesium oxide. In electronic terms, magnesium loses electrons and is therefore oxidized.

Question 13 of 20 Medium

In the reaction CuO + H2 → Cu + H2O, copper oxide is:

  1. A Oxidized to copper
  2. B Converted into oxygen gas
  3. C Reduced to copper
  4. D Unchanged

Correct answer: Reduced to copper

Copper oxide loses oxygen and forms copper metal. Loss of oxygen is an older definition of reduction, so copper oxide is reduced.

Question 14 of 20 Medium

In the reaction CuO + H2 → Cu + H2O, hydrogen acts as:

  1. A An oxidizing agent
  2. B A neutral salt
  3. C A catalyst only
  4. D A reducing agent

Correct answer: A reducing agent

Hydrogen removes oxygen from copper oxide and reduces it to copper. Hydrogen itself gets oxidized to water, so it acts as a reducing agent.

Question 15 of 20 Medium

Which statement is true for every redox reaction?

  1. A Only oxidation ever occurs
  2. B Only reduction ever occurs
  3. C Oxidation and reduction both occur
  4. D No electron transfer ever occurs

Correct answer: Oxidation and reduction both occur

In a redox reaction, one substance loses electrons while another gains electrons. Therefore, oxidation and reduction always occur simultaneously.

Question 16 of 20 Medium

What happens to the oxidation number of an element during oxidation?

  1. A It decreases
  2. B It always becomes zero
  3. C It never changes
  4. D It increases

Correct answer: It increases

During oxidation, an element loses electrons and its oxidation number increases. This helps identify oxidation in chemical reactions.

Question 17 of 20 Medium

What happens to the oxidation number of an element during reduction?

  1. A It increases
  2. B It always becomes positive
  3. C It always becomes negative
  4. D It decreases

Correct answer: It decreases

During reduction, an element gains electrons and its oxidation number decreases. This is a useful way to recognize reduction in reactions.

Question 18 of 20 Medium

Which of the following is caused by oxidation of fats and oils in food?

  1. A Freezing
  2. B Distillation
  3. C Rancidity
  4. D Sedimentation

Correct answer: Rancidity

Rancidity occurs when fats and oils are oxidized, producing unpleasant smell and taste. Antioxidants and airtight storage can help slow this process.

Question 19 of 20 Medium

Why are antioxidants added to some packaged foods?

  1. A To make rusting happen faster
  2. B To remove all nutrients from food
  3. C To convert the food into metal
  4. D To reduce fat and oil oxidation

Correct answer: To reduce fat and oil oxidation

Antioxidants slow down oxidation reactions in food, especially oxidation of fats and oils. This helps prevent rancidity and increases shelf life.

Question 20 of 20 Medium

Which of the following changes represents reduction according to the oxygen-based definition?

  1. A Gain of oxygen
  2. B Gain of heat only
  3. C Loss of water only
  4. D Loss of oxygen

Correct answer: Loss of oxygen

According to the older oxygen-based definition, reduction means loss of oxygen from a substance. For example, copper oxide is reduced to copper when oxygen is removed.

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