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Vapor pressure is a concept that plays a crucial role in determining the boiling point of a liquid. It is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When a liquid is heated, its molecules gain energy and move more vigorously. Some of these molecules then escape into the air as vapor. This escaping vapor creates pressure on the surface of the liquid, known as vapor pressure.

• A higher temperature implies a higher vapor pressure
• Vapor pressure is unique to each liquid
• When vapor pressure equals external pressure, boiling begins

The key to boiling is when the vapor pressure of the liquid matches the external pressure exerted by the surroundings. This balance is what allows bubbles of vapor to form and rise through the liquid, leading to boiling.

External pressure is simply the pressure exerted over the surface of a liquid by the surrounding environment, which typically includes atmospheric pressure. It is an important factor in determining a liquid's boiling point since it influences when the vapor pressure inside the liquid matches the external pressure. When external pressure is reduced, such as at higher altitudes, the boiling point of a liquid decreases because it requires less vapor pressure to match the lower surrounding pressure. Conversely, if the external pressure is increased, like in a pressure cooker, the boiling point rises because the liquid needs to achieve a higher vapor pressure to match the external conditions. Key points to remember about external pressure:

• It determines the boiling conditions of a liquid
• Lower external pressure equals a lower boiling point
• Higher external pressure results in a higher boiling point

The normal boiling point of a liquid is specifically defined as its boiling point under standard atmospheric pressure of exactly 1 atmosphere (atm) or 101.3 kilopascals (kPa). This standardization helps scientists and engineers compare the boiling points of different liquids under consistent conditions. At the normal boiling point, the vapor pressure of the liquid is exactly equal to 1 atm. This is why water, for example, boils at 100°C (212°F) under these specific pressure conditions. It's important to note that while the normal boiling point provides a baseline for comparison, the actual boiling point can vary when atmospheric conditions change. To sum up, the normal boiling point:

• Is measured at 1 atm or 101.3 kPa
• Allows for reliable comparisons of boiling points
• Can differ from "real-world" boiling points due to varying pressures