Question

Normal boiling point of a liquid is that temperature at which vapour pressure of the liquid is equal to (a) zero (b) $380\mathrm{mm}$ of $\mathrm{Hg}$ (c) $760\mathrm{mm}$ of $\mathrm{Hg}$ (d) $100\mathrm{mm}$ of $\mathrm{Hg}$

Short answer

The correct answer is (c) 760 mm of Hg.

Step-by-step solution

Identify the normal boiling point

The normal boiling point is defined as the temperature at which the vapour pressure of a liquid equals the atmospheric pressure or the pressure at sea level.

Understand the Atmospheric Pressure

The standard atmospheric pressure is equal to 760 mm of Hg. This is defined as the pressure exerted by the weight of the atmosphere at sea level.

Match the given options with Atmospheric Pressure

According the definitions established in step 1 and 2, the normal boiling point of a liquid is that temperature at which vapour pressure of the liquid is equivalent to 760 mm of Hg.

Key concepts

Vapour Pressure

Understanding vapour pressure is crucial in learning about boiling points and phase transitions. Vapour pressure is the pressure created by the vapour released when a liquid or solid evaporates or sublimates. It is important to note that this pressure increases as the temperature increases, because more molecules have sufficient energy to escape from the surface of the liquid. At any given temperature, a liquid will have a specific vapour pressure that is a unique characteristic of that substance.

Imagine a closed container half-filled with water; the water molecules at the surface occasionally escape into the air above and become vapour. Over time, the number of water molecules in the air increases, which raises the vapour pressure. Eventually, the system reaches a state of equilibrium where the same number of molecules evaporate from the liquid as condense back into it, resulting in a constant vapour pressure at a given temperature.

Atmospheric Pressure

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the Earth's atmosphere. It decreases as one moves up in altitude because there is less air mass above. It is responsible for various natural phenomena, from winds to the breathing process of organisms.

Weather Influence

Atmospheric pressure significantly influences weather patterns and can be measured with a barometer. High atmospheric pressure might indicate good weather, while low pressure can be a sign of stormy conditions. Moreover, it’s what gives us the beauty of sunsets at high altitudes, such as on mountains, because of the thinning air showing a broader spectrum of light.

Standard Atmospheric Pressure

Standard atmospheric pressure is defined as the pressure exerted by a 760 mm column of mercury (Hg) at sea level and at a temperature of 15°C (59°F). This exact value is typically represented as 1 atmosphere (1 atm) or 101.325 kilopascals (kPa) and serves as a reference point for many scientific measurements.

Applications and Importance

Standard atmospheric pressure is essential for calibrating instruments like barometers and altimeters. Additionally, it is a crucial variable in various engineering calculations and physical equations, like those necessary for determining the normal boiling point in chemistry. When comparing against standard atmospheric pressure, we can determine how physical properties like boiling point and melting point will change under different environmental conditions.