Question

The temperature at which the vapour pressure of a liquid becomes equals of the external pressure is its. (A) Melting point (B) sublimation point (C) Critical temperature (D) Boiling point

Short answer

The temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure is its \( \boxed{\text{(D) Boiling point}} \).

Step-by-step solution

Understanding the given statement

The statement can be rephrased as follows: "The temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure." This means that at this particular temperature, the liquid has enough energy to overcome the external pressure and change its phase from liquid to gas.

Analyzing each option and selecting the correct one

Now, let's go through each option and understand their meanings: (A) Melting point: The temperature at which a solid turns into a liquid under standard atmospheric pressure. This option doesn't match our statement since it refers to a solid-to-liquid phase transition. (B) Sublimation point: The temperature at which a substance changes directly from the solid phase to the gas phase without passing through the liquid phase. This option also doesn't match our statement since it doesn't involve a liquid-to-gas phase transition. (C) Critical temperature: The temperature above which a gas cannot be liquefied, regardless of the pressure applied. This option also doesn't match our statement since it doesn't describe the situation when vapor pressure equals the external pressure. (D) Boiling point: The temperature at which a liquid's vapor pressure equals the external (atmospheric) pressure, causing it to vaporize and change from the liquid phase to the gas phase. This option matches the given statement perfectly, as it describes the exact scenario mentioned in the statement.

Selecting the correct answer

Based on our analysis, the correct answer is (D) Boiling point, as it accurately describes the temperature at which the vapor pressure of a liquid becomes equal to the external (atmospheric) pressure, causing the liquid to change its phase from liquid to gas.

Key concepts

Vapour Pressure

Vapour pressure is a key concept when studying the boiling point of liquids. It refers to the pressure exerted by the vapor that is in equilibrium with its liquid phase at a given temperature. Consider a closed container partly filled with a liquid. Some molecules from the surface of the liquid will escape into the gas phase, creating a pressure in the space above the liquid. This pressure is called the vapour pressure.

Vapour pressure increases with temperature because higher temperatures provide molecules with more kinetic energy, allowing more molecules to transition into the vapor phase. It's crucial to understand that different liquids have different vapour pressures at the same temperature. For example:

• Water has a relatively low vapour pressure compared to alcohol at room temperature.
• This means alcohol evaporates more readily than water at the same temperature.

The boiling point of a liquid is actually the temperature at which its vapour pressure matches the external pressure acting on it. Hence, when vapour pressure equals atmospheric pressure, the liquid can turn into gas, that's its boiling point.

Phase Transition

Phase transition refers to the change of a substance from one state of matter to another. Common phase transitions include:

• Solid to liquid (melting)
• Liquid to solid (freezing)
• Liquid to gas (vaporization/boiling), and vice versa (condensation)
• Solid to gas (sublimation), and gas to solid (deposition)

Each of these transitions involves either absorbing or releasing energy. For example, during boiling, energy is absorbed to overcome intermolecular forces, allowing the liquid to transition into the gas phase. **It is imperative to note that during a phase transition, the temperature remains constant**. This is because the energy added or removed is used to change the phase, not the temperature.

Understanding phase transitions is critical when discussing boiling points as it is the energy absorbed by the liquid that allows it to reach the vapour pressure necessary to match external pressures.

External Pressure

External pressure, often synonymous with atmospheric pressure, is the pressure exerted by the weight of air in the atmosphere on any surface. It depends on altitude and weather conditions. At sea level, the standard atmospheric pressure is 1 atm or 101.3 kPa.

External pressure plays a crucial role in determining the boiling point of a liquid. If you increase the external pressure, more energy (or higher temperature) is needed for the liquid to match this pressure with its vapour pressure and thus boil. Conversely, reducing the external pressure lowers the boiling point.

• This is why water boils at a temperature lower than 100°C at high altitudes.
• Reduced atmospheric pressure at high altitudes results in lower boiling points, as it requires less energy for the vapour pressure to equal the lower external pressure.

In summary, the boiling point of a liquid is dependent on the external pressure, emphasizing the principle that vaporization occurs only when the vapour pressure of the liquid equals the external pressure.