Understanding Different Types of Substances

The question asks us to identify the correct statement among the given options regarding the nature of substances like oxygen, air, gases, and elements. Let's examine each statement carefully.

We need to evaluate the following statements:

• Oxygen is always a molecule.
• Air is always a mixture.
• Any gas is always a compound.
• Element is always an atom.

Analysing Statement 1: Oxygen is always a molecule.

Oxygen most commonly exists as a diatomic molecule ($\text{O}_2$), which is what we breathe. However, oxygen can exist in other forms. For example:

• Atomic oxygen ($\text{O}$) exists at very high altitudes.
• Oxygen can be part of compounds, like water ($\text{H}_2\text{O}$) or carbon dioxide ($\text{CO}_2$). In these compounds, oxygen atoms are chemically bonded to other atoms, not necessarily forming a separate $\text{O}_2$ molecule.
• Ozone ($\text{O}_3$) is another molecular form of oxygen.

Since oxygen can exist as individual atoms, in compounds, or as $\text{O}_3$ molecules, it is not always in the form of an $\text{O}_2$ molecule. Therefore, the statement "Oxygen is always a molecule" is incorrect.

Analysing Statement 2: Air is always a mixture.

Air is the gaseous envelope surrounding the Earth. It is primarily composed of nitrogen gas ($\text{N}_2$), oxygen gas ($\text{O}_2$), argon gas ($\text{Ar}$), carbon dioxide gas ($\text{CO}_2$), small amounts of other gases, and variable amounts of water vapor.

A mixture is a substance comprising two or more components not chemically bonded together. The components retain their individual properties and can often be separated by physical means.

In air, the different gases like nitrogen, oxygen, and argon are physically mixed. They do not react chemically with each other under normal atmospheric conditions. The proportion of these gases can vary slightly in different locations or conditions (e.g., humidity affecting water vapor content), which is characteristic of a mixture, not a compound.

Therefore, air is fundamentally a physical combination of various gases, meaning air is always a mixture. This statement appears to be correct.

Analysing Statement 3: Any gas is always a compound.

Gases can be elements, compounds, or mixtures:

• Elements as gases: Many elements exist as gases at standard temperature and pressure, such as Helium ($\text{He}$), Neon ($\text{Ne}$), Argon ($\text{Ar}$), Krypton ($\text{Kr}$), Xenon ($\text{Xe}$), Radon ($\text{Rn}$) (noble gases), Hydrogen ($\text{H}_2$), Nitrogen ($\text{N}_2$), Oxygen ($\text{O}_2$), Fluorine ($\text{F}_2$), Chlorine ($\text{Cl}_2$) (diatomic molecules).
• Compounds as gases: Many compounds exist as gases, such as Carbon Dioxide ($\text{CO}_2$), Methane ($\text{CH}_4$), Ammonia ($\text{NH}_3$), Water vapor ($\text{H}_2\text{O}$).
• Mixtures as gases: Air is a common example of a gaseous mixture.

Since gases can be elements or mixtures, it is incorrect to state that any gas is always a compound.

Analysing Statement 4: Element is always an atom.

An element is a pure substance consisting only of atoms that all have the same numbers of protons in their nuclei. An atom is the smallest unit of an element that retains the chemical properties of the element.

However, elements can exist in different forms:

• Some elements exist as individual atoms (e.g., noble gases like $\text{He}$, $\text{Ne}$).
• Many elements exist as molecules composed of multiple atoms of that element (e.g., $\text{O}_2$, $\text{N}_2$, $\text{S}_8$, $\text{P}_4$). In these cases, the element exists as molecules, not just isolated atoms.
• Metallic elements exist as a large collection of atoms held together in a metallic lattice structure.

So, while an element is defined by its atoms, the substance itself (the element) is not always present as single, isolated atoms. Therefore, the statement "Element is always an atom" is incorrect.

Conclusion

Based on the analysis of each statement:

• "Oxygen is always a molecule" is incorrect.
• "Air is always a mixture" is correct.
• "Any gas is always a compound" is incorrect.
• "Element is always an atom" is incorrect.

The only correct statement is that air is always a mixture.