What is Vapour Pressure?

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Grade Level:

Class 12

AI/ML, Physics, Biotechnology, FinTech, EVs, Space Technology, Climate Science, Blockchain, Medicine, Engineering, Law, Economics

Definition

What is it?

Vapour pressure is the pressure exerted by the vapour of a liquid in a closed container when the liquid and its vapour are in equilibrium. It tells us how easily a liquid can turn into a gas at a certain temperature.

Simple Example

Quick Example

Imagine you leave a bowl of water uncovered in your room. After some time, the water level goes down because water molecules escape into the air as vapour. Now, if you cover that bowl tightly, some water molecules still escape, but they get trapped and start hitting the lid and the water surface, creating pressure. That trapped pressure is the vapour pressure.

Worked Example

Step-by-Step

Let's say we have two liquids, A and B, at 25 degrees Celsius.

Step 1: Liquid A has a high tendency to evaporate. At 25 degrees Celsius, its vapour molecules exert a pressure of 760 mmHg in a closed container.

---Step 2: Liquid B has a lower tendency to evaporate. At the same 25 degrees Celsius, its vapour molecules exert a pressure of 300 mmHg in a closed container.

---Step 3: We can compare their vapour pressures. Liquid A has a vapour pressure of 760 mmHg.

---Step 4: Liquid B has a vapour pressure of 300 mmHg.

---Step 5: Since Liquid A has a higher vapour pressure (760 mmHg > 300 mmHg), it means Liquid A evaporates more easily than Liquid B at 25 degrees Celsius.

Answer: Liquid A has a higher vapour pressure, indicating it is more volatile.

Why It Matters

Understanding vapour pressure helps engineers design better engines and cooling systems for EVs, and even helps scientists predict weather patterns. In medicine, it's key for storing medicines properly. It's also important for chemical engineers who design industrial processes.

Common Mistakes

MISTAKE: Thinking vapour pressure is the same as atmospheric pressure. | CORRECTION: Vapour pressure is specific to a liquid in a closed system, while atmospheric pressure is the pressure of the air around us.

MISTAKE: Believing all liquids have the same vapour pressure at the same temperature. | CORRECTION: Different liquids have different intermolecular forces, leading to different vapour pressures even at the same temperature. For example, petrol has a higher vapour pressure than water.

MISTAKE: Thinking vapour pressure increases when you increase the amount of liquid in a closed container. | CORRECTION: Vapour pressure depends only on the type of liquid and its temperature, not the amount of liquid, as long as there's enough liquid to reach equilibrium.

Practice Questions

Try It Yourself

QUESTION: If a liquid has a high vapour pressure at room temperature, what does that tell you about how easily it evaporates? | ANSWER: It evaporates very easily.

QUESTION: Why does water boil at a lower temperature on top of a high mountain like Mount Everest compared to a city like Mumbai? | ANSWER: On Mount Everest, atmospheric pressure is lower. Water boils when its vapour pressure equals the external atmospheric pressure. Since the external pressure is lower, water's vapour pressure needs to reach a lower value, which happens at a lower temperature.

QUESTION: Liquid X has a vapour pressure of 500 mmHg at 50 degrees Celsius. Liquid Y has a vapour pressure of 800 mmHg at the same temperature. Which liquid is more volatile (evaporates more easily)? Explain why. | ANSWER: Liquid Y is more volatile. A higher vapour pressure means more molecules are escaping into the gaseous phase, indicating it evaporates more easily.

MCQ

Quick Quiz

What happens to the vapour pressure of a liquid when its temperature increases?

It decreases

It increases

It remains the same

It first decreases then increases

The Correct Answer Is:

As temperature increases, liquid molecules gain more kinetic energy, allowing more of them to escape into the vapour phase, thus increasing the vapour pressure. Options A, C, and D are incorrect because higher temperature always leads to higher vapour pressure.

Real World Connection

In the Real World

When you buy a cold drink or a bottle of water, sometimes you see tiny droplets on the outside. This happens because the water vapour in the warmer air around the bottle cools down and turns back into liquid. This process is related to vapour pressure. Also, petrol pumps have special systems to capture petrol vapour to prevent it from escaping into the air, which is important for air quality and safety, as petrol has a high vapour pressure.

Key Vocabulary

Key Terms

VAPOUR: The gaseous state of a substance that is a liquid or solid at room temperature | EQUILIBRIUM: A state where opposing forces or influences are balanced, so no net change occurs | PRESSURE: Force applied perpendicular to the surface of an object per unit area | VOLATILE: Easily evaporated at normal temperatures.

What's Next

What to Learn Next

Next, you should learn about 'Boiling Point' and 'Raoult's Law'. Boiling point is directly related to vapour pressure, and Raoult's Law will help you understand how adding things to a liquid changes its vapour pressure.