Which of the following is a pure substance? MCQ with Answer and Explanation

Which of the following is a pure substance?
A. Brass
B. Milk
C. Sea water
D. Oxygen gas
Answer: Option D
Solution (By JKExamLibrary)
Oxygen (O₂) is an element, a pure substance. Milk is a colloid; brass is an alloy (mixture); sea water is a solution (mixture). Pure substances have fixed composition.

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Question #1
The diffusion of gases is explained by:
A. Boyle's law
B. Graham's law
C. Charles' law
D. Avogadro's law

Correct Answer: Option B


Explanation:
Graham's law of diffusion/effusion states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Lighter gases diffuse faster. This law helps in separating isotopes (e.g., ²³⁵U enrichment) and understanding gas behavior. Boyle's and Charles' deal with P-V-T relationships. Avogadro's deals with V and number of moles.

This question belongs to: Science chemistry
Question #2
Which of the following molecules has a net dipole moment of zero due to its geometry?
A. ClF3
B. NF3
C. H2S
D. BF3

Correct Answer: Option D


Explanation:
BF3 has a trigonal planar geometry with bond angles of 120 degrees. The three B-F bond dipoles are equal in magnitude and symmetrically arranged, so their vector sum cancels out completely, resulting in a net dipole moment of zero. H2S is bent, NF3 is pyramidal, and ClF3 is T-shaped.

This question belongs to: Science chemistry
Question #3
The setting time of cement is controlled by adding:
A. Limestone
B. Sand
C. Clay
D. Gypsum

Correct Answer: Option D


Explanation:
Gypsum (CaSO₄·2H₂O) is added to clinker to slow down the rapid setting of C₃A, providing workable time. Without gypsum, flash set occurs.

This question belongs to: Science chemistry