Chemistry 9th Chap 7 : Topic 4 - Maryam Publications

Chemistry Chapter 7 : Topic 4

Free Element Oxidation

1 / 22

In a chemical reaction study, it is observed that elements such as sodium metal, chlorine gas, and oxygen gas exist independently without combining with other elements. What is the correct statement regarding their oxidation number in this state?

A) It is always positive due to atomic attraction forces

B) It varies according to temperature and pressure

C) It depends on the type of bonding present in the element

D) It is always zero regardless of physical form or structure

2 / 22

Which of the following correctly represents the oxidation number of oxygen in O₂ gas under standard conditions?

A) - 1

B) 0

C) - 2

D) + 2

3 / 22

A student states that chlorine in Cl₂ has an oxidation number of -1 because chlorine is highly electronegative. What is the correct explanation?

A) The statement is correct only at high temperature.

B) The statement is incorrect because Cl₂ is elemental chlorine.

C) The statement is incorrect because chlorine does not exist in nature.

D) The statement is correct because chlorine always shows -1 oxidation state.

4 / 22

Which of the following always has oxidation number equal to zero?

A) O in H₂O

B) Na⁺ in NaCl

C) Fe in Fe(s)

D) S in H₂SO₄

5 / 22

In which of the following species is the oxidation number of sulfur zero?

A) H₂S

B) SO₂

C) S₈

D) H₂SO₄

6 / 22

Which statement best explains why noble gases have oxidation number zero in their natural form?

A) They are highly reactive gases.

B) They form ions easily.

C) They exist as monoatomic free elements.

D) They are already in combined form.

7 / 22

What is the oxidation number of phosphorus in white phosphorus (P₄)?

A) 0

B) - 3

C) + 5

D) + 3

8 / 22

A chemist analyzes graphite and diamond. What is the oxidation number of carbon in both forms?

A) - 5

B) - 3

C) 0

D) + 4

9 / 22

Why is the oxidation number of hydrogen in H₂ gas equal to zero?

A) Because hydrogen is combined with oxygen.

B) Because hydrogen always forms + 1 ions.

C) Because it is in its elemental diatomic form.

D) Because it forms hydrides.

10 / 22

Which of the following contains an element with oxidation number zero?

A) O₃

B) H₂O

C) CO₂

D) NaCl

11 / 22

In which case is the oxidation number of an element NOT zero?

A) S₈

B) Cl₂

C) Fe(s)

D) Na⁺ in NaCl

12 / 22

A student claims that oxidation number depends on number of atoms in the molecule. Which example disproves this?

A) NaCl has zero oxidation number.

B) H₂O has zero oxidation number.

C) O₂ and O₃ both have zero oxidation number.

D) CO₂ has variable oxidation number.

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Which condition correctly defines “free state” of an element?

A) Element has lost electrons.

B) Element exists in compound form.

C) Element is always dissolved in water.

D) Element exists in uncombined elemental form.

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What is the oxidation number of chlorine in elemental chlorine gas used in laboratories?

A) - 1

B) + 1

C) 0

D) + 7

15 / 22

Which statement is TRUE regarding oxidation number in free elements?

A) It depends on valency .

B) It is always negative.

C) It is always zero.

D) It is always positive.

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Why do metals like sodium (Na) have oxidation number zero in solid form?

A) They are in ionic state.

B) They form negative ions.

C) They exist as elemental metals.

D) They are chemically combined with oxygen.

17 / 22

Which of the following pairs both have oxidation number zero in their elemental state?

A) CO₂ and SO₂

B) O₂ and Fe(s)

C) HCl and NH₃

D) NaCl and H₂O

18 / 22

A student mistakenly assigns oxidation number - 2 to oxygen in O₂. What is the correct correction?

A) Oxygen in elemental form has oxidation number zero.

B) Oxygen does not exist in free form.

C) Oxygen is always - 2.

D) Oxygen is always positive.

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Which of the following is NOT an example of element in free state?

A) Cl₂

B) S₈

C) Fe(s)

D) NaCl

20 / 22

The oxidation number of all atoms in elemental hydrogen (H₂), oxygen (O₂), and nitrogen (N₂) is:

A) variable

B) + 1

C) 0

D) - 1

21 / 22

Which concept best explains why oxidation number is zero in free elements?

A) Elements are always ionic.

B) Atoms are in neutral, uncombined state.

C) Electron transfer has already occurred.

D) Atoms are in ionic lattice.

22 / 22

A chemist observes an element in nature in its pure form. What can be concluded about its oxidation number?

A) It is zero.

B) It depends on temperature.

C) It is always + 1.

D) It is always - 1.

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