Pressure cookers increase cooking speed by raising the...

Pressure cookers increase cooking speed by raising the boiling temperature of water above its value at atmospheric pressure. (a) What pressure is necessary to raise the boiling point to $120.0^{\circ} \mathrm{C}$ ? (b) What gauge pressure does this correspond to?

Key Concepts

Boiling Point and Phase Equilibrium

The boiling point of a liquid is defined as the temperature at which its vapor pressure equals the external pressure. In other words, when a liquid's vapor pressure reaches the surrounding pressure, bubbles of vapor form within the liquid, leading to boiling. In pressure cookers, by increasing the pressure, the boiling temperature is raised, which accelerates the cooking process since higher temperatures speed up chemical and physical reactions involved in cooking.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It is a key parameter in understanding boiling because it represents the tendency of particles to escape from the liquid phase. As temperature increases, vapor pressure increases, and when it matches the ambient pressure, boiling occurs. Modifying the ambient pressure shifts the boiling point of the liquid.

Clausius-Clapeyron Relation

The Clausius-Clapeyron equation describes the quantitative relationship between vapor pressure and temperature during a phase change. This thermodynamic equation is useful for estimating how much pressure is needed to achieve a higher boiling point or, conversely, how the boiling point changes with pressure. It aids in understanding and predicting the behavior of liquids under different pressures and temperatures.

Absolute vs. Gauge Pressure

Absolute pressure is measured relative to a perfect vacuum, while gauge pressure is measured relative to the atmospheric pressure. In contexts such as pressure cooking, the pressure inside the cooker is often expressed as gauge pressure, which indicates the pressure increase over the local atmospheric pressure. Recognizing the difference between these measurements is important for both theoretical calculations and practical applications in engineering systems.

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