What is Vapour Pressure Lowering?

S7-SA5-0666

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 lowering is when the vapour pressure of a liquid decreases after a non-volatile solute (something that doesn't easily evaporate) is added to it. This happens because the solute particles reduce the number of solvent particles at the liquid's surface that can escape into the gas phase.

Simple Example

Quick Example

Imagine you have a glass of water. If you leave it in the sun, some water will evaporate. Now, if you add a spoonful of sugar (which doesn't evaporate easily) to the water, the water will evaporate slower. This is because the sugar molecules are blocking some of the water molecules from escaping, causing the 'pressure' of the water vapour above the solution to be less than before.

Worked Example

Step-by-Step

Let's say pure water has a vapour pressure of 20 mmHg at a certain temperature. We add 1 mole of glucose (a non-volatile solute) to 9 moles of water. We want to find the new vapour pressure.

Step 1: Understand Raoult's Law, which states that the vapour pressure of a solution is equal to the mole fraction of the solvent multiplied by the vapour pressure of the pure solvent. P_solution = X_solvent * P_pure_solvent.
---Step 2: Calculate the total moles in the solution. Total moles = moles of water + moles of glucose = 9 moles + 1 mole = 10 moles.
---Step 3: Calculate the mole fraction of the solvent (water). Mole fraction of water (X_water) = moles of water / total moles = 9 moles / 10 moles = 0.9.
---Step 4: Apply Raoult's Law. P_solution = X_water * P_pure_water = 0.9 * 20 mmHg = 18 mmHg.
---Step 5: Calculate the lowering in vapour pressure. Vapour Pressure Lowering = P_pure_water - P_solution = 20 mmHg - 18 mmHg = 2 mmHg.

Answer: The new vapour pressure is 18 mmHg, and the vapour pressure lowering is 2 mmHg.

Why It Matters

Understanding vapour pressure lowering is crucial in fields like medicine for creating stable drug formulations and in engineering for designing efficient cooling systems. It's also vital in chemical industries for separating substances and in food processing to preserve items longer, impacting careers in pharma research, chemical engineering, and food technology.

Common Mistakes

MISTAKE: Thinking that adding any solute will lower vapour pressure. | CORRECTION: Vapour pressure lowering primarily occurs with non-volatile solutes. Volatile solutes (like alcohol in water) can increase or decrease the overall vapour pressure depending on their own vapour pressure.

MISTAKE: Confusing vapour pressure lowering with boiling point elevation. | CORRECTION: Vapour pressure lowering is the decrease in vapour pressure. Boiling point elevation is a consequence of this lowering, where the solution needs a higher temperature to boil because its vapour pressure is lower.

MISTAKE: Forgetting to use mole fraction of the solvent in Raoult's Law. | CORRECTION: Raoult's Law directly relates the vapour pressure of the solution to the mole fraction of the solvent, not the solute.

Practice Questions

Try It Yourself

QUESTION: If pure benzene has a vapour pressure of 100 mmHg and you add a non-volatile substance such that the mole fraction of benzene becomes 0.8, what is the new vapour pressure of the solution? | ANSWER: 80 mmHg

QUESTION: A solution contains 2 moles of urea (non-volatile) and 8 moles of water. If the vapour pressure of pure water is 50 mmHg, what is the vapour pressure lowering? | ANSWER: 10 mmHg

QUESTION: The vapour pressure of pure ethanol is 44 mmHg. When 1 mole of a non-volatile solute is dissolved in 9 moles of ethanol, what is the percentage lowering of the vapour pressure? | ANSWER: 10%

MCQ

Quick Quiz

Which of the following will cause a decrease in the vapour pressure of water?

Adding alcohol to water

Heating the water

Adding sugar to water

Stirring the water vigorously

The Correct Answer Is:

Adding a non-volatile solute like sugar reduces the number of solvent particles at the surface, thus lowering the vapour pressure. Heating water increases vapour pressure, and alcohol is volatile.

Real World Connection

In the Real World

This concept is used when making traditional Indian sweets like 'chashni' (sugar syrup). When sugar is dissolved in water, the resulting syrup has a lower vapour pressure than pure water. This means it needs a higher temperature to boil and evaporate, allowing it to become thicker and more concentrated without drying out too quickly, which helps in making jalebis or gulab jamuns.

Key Vocabulary

Key Terms

VAPOUR PRESSURE: The pressure exerted by the vapour in equilibrium with its liquid phase at a given temperature | NON-VOLATILE SOLUTE: A substance that does not easily evaporate or turn into a gas | SOLVENT: The substance that dissolves the solute to form a solution (usually present in larger quantity) | MOLE FRACTION: The ratio of the number of moles of one component to the total number of moles in the solution | RAOULT'S LAW: A law stating that the partial vapour pressure of each component in an ideal mixture of liquids is equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture.

What's Next

What to Learn Next

Next, you should explore 'Boiling Point Elevation'. This concept directly builds on vapour pressure lowering because a lower vapour pressure means you need to heat the solution to a higher temperature to reach the boiling point, which is when the vapour pressure equals atmospheric pressure. Keep up the great work!