Identify a substance that is not in its standard state.

• A. Cl₂
• B. Li
• C. H
• D. P₄
• E. Ar

Answer: (C)

Verified step by step guidance

1. Understand the concept of standard state: In chemistry, the standard state of a substance is its most stable form at 1 atmosphere of pressure and a specified temperature, usually 25°C (298 K). For elements, the standard state is typically the form in which the element exists under these conditions.
2. Identify the standard states of the given substances: Chlorine (Cl2) is a diatomic gas at standard conditions, lithium (Li) is a solid metal, phosphorus (P4) is a solid, and argon (Ar) is a noble gas. Hydrogen (H) is typically found as H2 gas in its standard state.
3. Determine which substance is not in its standard state: Compare each substance to its standard state. If a substance is not in its most stable form at 1 atm and 25°C, it is not in its standard state.
4. Consider the form of hydrogen given: The problem lists 'H', which refers to atomic hydrogen. However, hydrogen's standard state is H2 gas, not atomic hydrogen.
5. Conclude which substance is not in its standard state: Based on the comparison, identify that 'H' (atomic hydrogen) is not in its standard state, as hydrogen is typically found as H2 gas under standard conditions.