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 zero regardless of physical form or structure

B) It is always positive due to atomic attraction forces

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

D) It varies according to temperature and pressure

2 / 22

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

A) - 2

B) 0

C) - 1

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 correct because chlorine always shows -1 oxidation state.

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

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

4 / 22

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

A) Na⁺ in NaCl

B) Fe in Fe(s)

C) S in H₂SO₄

D) O in H₂O

5 / 22

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

A) H₂S

B) H₂SO₄

C) S₈

D) SO₂

6 / 22

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

A) They are already in combined form.

B) They exist as monoatomic free elements.

C) They are highly reactive gases.

D) They form ions easily.

7 / 22

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

A) 0

B) - 3

C) + 3

D) + 5

8 / 22

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

A) 0

B) + 4

C) - 5

D) - 3

9 / 22

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

A) Because hydrogen always forms + 1 ions.

B) Because it is in its elemental diatomic form.

C) Because it forms hydrides.

D) Because hydrogen is combined with oxygen.

10 / 22

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

A) CO₂

B) H₂O

C) O₃

D) NaCl

11 / 22

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

A) Cl₂

B) Na⁺ in NaCl

C) S₈

D) Fe(s)

12 / 22

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

A) O₂ and O₃ both have zero oxidation number.

B) H₂O has zero oxidation number.

C) CO₂ has variable oxidation number.

D) NaCl has zero oxidation number.

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

A) Element has lost electrons.

B) Element exists in uncombined elemental form.

C) Element exists in compound form.

D) Element is always dissolved in water.

14 / 22

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

A) - 1

B) + 1

C) + 7

D) 0

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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 exist as elemental metals.

B) They are in ionic state.

C) They form negative ions.

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) NaCl and H₂O

C) O₂ and Fe(s)

D) HCl and NH₃

18 / 22

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

A) Oxygen is always - 2.

B) Oxygen does not exist in free form.

C) Oxygen in elemental form has oxidation number zero.

D) Oxygen is always positive.

19 / 22

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

A) Fe(s)

B) NaCl

C) Cl₂

D) S₈

20 / 22

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

A) variable

B) + 1

C) - 1

D) 0

21 / 22

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

A) Electron transfer has already occurred.

B) Atoms are in neutral, uncombined state.

C) Atoms are in ionic lattice.

D) Elements are always ionic.

22 / 22

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

A) It depends on temperature.

B) It is always - 1.

C) It is zero.

D) It is always + 1.

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