In the following set of material–state pairs, which pair is the odd one out based on the actual physical state of the substance?

Introduction / Context:
This question asks you to find the odd pair out from a list of substance–state combinations. Each option links a chemical element or substance with a physical state or classification such as gas or metal. Your task is to identify which pair is factually incorrect or inconsistent with the others based on standard science knowledge about these substances at normal conditions. This is a typical odd one out question combining basic chemistry and logical grouping.

Given Data / Assumptions:

• Options: Oxygen : Gas, Mercury : Gas, Gold : Metal, Uranium : Metal, Copper : Metal.
• We consider the usual physical state of each element at room temperature and standard pressure.
• We assume basic school level chemistry knowledge about gases and metals.

Concept / Approach:
Oxygen is a well known gas at room temperature. Gold, uranium, and copper are all metals that are solid under normal conditions. Therefore, the correct pairs describe the usual state or classification of the substance. The suspicious pair is Mercury : Gas, because mercury is known as a liquid metal at room temperature, not a gas. Thus, Mercury : Gas is the odd one out since it misclassifies the substance, whereas all other pairs classify correctly.

Step-by-Step Solution:

Step 1: Check Oxygen : Gas. Oxygen is a common atmospheric gas, so this pair is correct. Step 2: Examine Mercury : Gas. Mercury is commonly known as a silvery liquid metal at room temperature. Step 3: Examine Gold : Metal. Gold is a precious metal and solid at room conditions. Step 4: Examine Uranium : Metal. Uranium is a heavy metallic element, used in nuclear reactors, and is indeed a metal. Step 5: Examine Copper : Metal. Copper is also a metal, used in electrical wiring and coins. Step 6: Conclude that only Mercury : Gas mislabels the typical physical state of the substance.

Verification / Alternative check:
To double check, recall that all metals listed except mercury are solids under ordinary conditions. Mercury is unique among metals because it is liquid at room temperature. Oxygen is not a metal at all but a diatomic gas. Therefore, pairing oxygen with gas is correct, while pairing mercury with gas is incorrect. Gold, uranium, and copper remain as consistent “metal” classifications, leaving Mercury : Gas clearly inconsistent.

Why Other Options Are Wrong:
The other options are factually correct: Oxygen is a gas; Gold, Uranium, and Copper are metals. Since the task is to find the incorrect or mismatched pair, these cannot be the answers. Only Mercury : Gas fails on a simple scientific fact, because mercury is a liquid metal rather than a gas in ordinary conditions.

Common Pitfalls:
Sometimes candidates are misled by the fact that mercury can evaporate and produce toxic vapours, but that does not make its usual state a gas. Another pitfall is discomfort with uranium, as people associate it more with radioactivity than with its metallic nature. Reviewing the periodic table basics helps avoid such confusions.

Final Answer:
The odd pair out is Mercury : Gas, because mercury is actually a liquid metal, not a gas, whereas the other pairs correctly state the substance and its classification.