A Level Chemistry OCR Practice Exam

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What is meant by standard state?

The most stable form of a substance
The physical state of a substance under standard conditions
The condition of a substance at high temperature and pressure
The state of a gas at 1 atm pressure

The correct answer is: The physical state of a substance under standard conditions

The term "standard state" refers specifically to the physical state of a substance under standard conditions. Standard conditions are defined as 1 atmosphere of pressure and a specific temperature, typically 25 degrees Celsius (298 K). In these conditions, the substance is measured in its most stable form, whether that be a solid, liquid, or gas. This definition is crucial because it provides a consistent reference for thermodynamic properties, such as enthalpy and entropy, allowing for meaningful comparison between different substances. By focusing on the physical state under specified standard conditions, we ensure that the data collected is reliable and uniform, which is essential in fields such as thermodynamics, where changes in energy are compared against these standard measurements.