📘 Vapour Pressure of Liquid Solutions

 Sure! Here's a clear, student-friendly, CBSE Class 12-style explanation of:

📘 Vapour Pressure of Liquid Solutions

(Chapter 1 – Solutions | Class 12 Chemistry – NCERT Based)

💡 What is Vapour Pressure?

Vapour pressure is the pressure exerted by the vapour of a liquid when it is in dynamic equilibrium with its liquid phase at a given temperature.

🔹 It tells us how volatile a liquid is.
🔹 More volatile liquids → higher vapour pressure.

🧪 Vapour Pressure in Liquid Solutions

Liquid solutions can be of two main types:

1️⃣ Solutions of Liquids in Liquids

(e.g., ethanol + water, acetone + benzene)

2️⃣ Solutions of Solids in Liquids

(e.g., sugar in water, salt in water)

🧩 Vapour Pressure of Liquid + Liquid Solutions

In binary liquid solutions (mixture of two liquids), both components may be volatile. The total vapour pressure of the solution depends on:

  • The mole fraction of each liquid

  • The individual vapour pressures of each pure component

  • How the components interact

✅ Raoult’s Law for Ideal Liquid Solutions

In an ideal solution, the partial vapour pressure of each component is directly proportional to its mole fraction in the solution.

📘 Formulas:

pA=xA⋅pA0andpB=xB⋅pB0p_A = x_A \cdot p_A^0 \quad \text{and} \quad p_B = x_B \cdot p_B^0 Total Vapour Pressure:ptotal=pA+pB=xApA0+xBpB0\text{Total Vapour Pressure:} \quad p_{\text{total}} = p_A + p_B = x_A p_A^0 + x_B p_B^0

Where:

  • pA,pBp_A, p_B = partial vapour pressures

  • xA,xBx_A, x_B = mole fractions

  • pA0,pB0p_A^0, p_B^0 = vapour pressures of pure liquids A and B

⚖️ Ideal vs Non-Ideal Solutions

Type Behavior Vapour Pressure
Ideal solution Obeys Raoult’s Law exactly Linear variation with mole fraction
Non-ideal solution Deviates from Raoult’s Law Shows positive/negative deviation

📊 Graphical Representation

(For ideal solutions): A straight-line graph of total vapour pressure vs mole fraction.

📝 Class 12 Board Tip:

  • State Raoult’s Law clearly.

  • Use the formula and mole fraction concepts correctly.

  • Remember: More volatile component = higher vapour pressure.

Would you like this content in slide format, or shall I also explain vapour pressure of solid–liquid solutions next?

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