Boiling Point Explained: Vapor Pressure and Atmospheric Pressure

The question asks for the specific temperature at which a liquid, when in an open container, reaches a state where its internal vapour pressure is exactly the same as the pressure of the surrounding atmosphere. Let's explore this concept:

Understanding Vapor Pressure

All liquids have a tendency to evaporate, meaning some molecules escape from the liquid surface and become gas (vapour). In a closed container, these vapour molecules eventually build up enough pressure to counteract further evaporation, reaching a state called equilibrium. This pressure exerted by the vapour in equilibrium with its liquid phase at a given temperature is known as the vapour pressure.

Atmospheric Pressure

The atmospheric pressure is the force exerted by the weight of the air in the atmosphere above a certain point. This pressure varies with altitude and weather conditions.

Defining the Boiling Point

The boiling point is defined as the temperature at which the vapour pressure of a liquid equals the surrounding atmospheric pressure.

• When the liquid is heated, its vapour pressure increases.
• At the boiling point, bubbles of vapour can form not just at the surface but also within the bulk of the liquid, rising and escaping into the atmosphere. This is visible boiling.
• In an open vessel, the liquid is exposed to the external atmospheric pressure. Boiling occurs when the internal push (vapour pressure) overcomes the external push (atmospheric pressure).
• The standard boiling point is usually measured at a standard atmospheric pressure of 1 atmosphere (atm) or 101.325 kilopascals (kPa).

For example, water boils at 100°C (212°F) at standard atmospheric pressure. If the atmospheric pressure is lower (like at higher altitudes), the boiling point is lower. If the atmospheric pressure is higher (like in a pressure cooker), the boiling point is higher.

Analyzing the Options

• Melting point: This is the temperature at which a substance changes from solid to liquid. It's unrelated to vapour pressure equalling atmospheric pressure.
• Boiling point: This perfectly matches the definition provided in the question – the temperature where vapour pressure equals atmospheric pressure in an open vessel.
• Liquid point: This is not a standard scientific term used to describe this phenomenon.
• None of the above: Since boiling point is the correct term, this option is incorrect.

Therefore, the temperature at which the vapour pressure of a liquid in an open vessel becomes equal to the atmospheric pressure is called the boiling point.