Chemistry · Class 11

Active learning ideas

Deviation from Ideal Behaviour and Liquefaction of Gases

Ever wondered why the gas in your LPG cylinder is a liquid, or how an AC cools your room? This topic uncovers the real behaviour of gases that makes these technologies possible.

• CBSE Learning Outcomes
• NCERT Class 11 Chemistry: Unit 5 - States of Matter

• 15–40 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Collaborative Problem-Solving
• 30 min · Pairs

Plotting Z vs P

Provide students with data for the compressibility factor (Z) versus pressure (P) for different gases like H₂, He, CH₄, and CO₂. Students plot these on a graph to visually compare their deviation from the ideal gas line (Z=1).

Explain the significance of the 'a' and 'b' constants in the van der Waals equation.

Facilitation Tip

Ask students to correlate the extent of deviation with the known intermolecular forces in each gas.

What to look for

Use an exit ticket asking students to rank three gases (e.g., NH₃, N₂, H₂) in order of their expected deviation from ideal behaviour and to justify their ranking based on intermolecular forces.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 02

• Jigsaw
• 40 min · Small Groups

Van der Waals Constants Jigsaw

Divide the class into 'expert' groups for constant 'a' and constant 'b'. Each group researches its constant's significance and then re-groups to teach their peers what they learned.

Analyze the conditions required for the liquefaction of a gas, including the role of critical temperature.

Facilitation Tip

Provide a simple table for students to fill in, comparing the two constants after the jigsaw activity.

What to look for

A set of numerical problems where students must use the van der Waals equation to calculate P, V, or T for a given real gas, and compare the result with the value obtained from the ideal gas law.

• Understand
• Analyze
• Evaluate
• Relationship Skills
• Self-Management

Activity 03

• Collaborative Problem-Solving
• 15 min · Whole Class

Critical Temperature Analogy

Use an analogy of trying to pack unruly children (gas molecules) into a room. At high energy (high temp), they run around too much to be packed. Only below a certain energy level (critical temp) can they be forced together (liquefied) by pushing the walls in (pressure).

Compare the pressure-volume isotherms of a real gas with those of an ideal gas.

Facilitation Tip

Use this analogy as a quick, memorable hook before diving into the technical details of Andrews' isotherms.

What to look for

Provide students with unlabelled P-V isotherms for a real gas and ask them to identify the critical point, the gaseous phase, the liquid phase, and the liquid-vapour equilibrium region.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

A few notes on teaching this unit

Start by highlighting the failures of the ideal gas law using a Z vs P plot. Introduce the van der Waals equation as a logical correction, focusing on the physical meaning of 'a' and 'b'. Use the CO₂ isotherm graph as a visual tool to explain liquefaction and the critical point, rather than just stating definitions. Connect these concepts back to everyday examples to solidify understanding.

By the end of this topic, your students will be able to explain why real gases don't follow the ideal gas law and describe the specific conditions needed to turn any gas into a liquid.

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Watch Out for These Misconceptions

• Any gas can be liquefied just by applying enough pressure.

  A gas can only be liquefied below a specific temperature called its critical temperature (Tc). Above this temperature, it exists as a gas and cannot be turned into a liquid, no matter how high the pressure.

• The van der Waals constants 'a' and 'b' are just mathematical 'fudge factors' without any real meaning.

  These constants have clear physical significance. 'a' is a measure of the magnitude of intermolecular attractive forces, and 'b' is related to the effective volume occupied by the gas molecules themselves.

• Ideal gas behaviour is a rare, special case.

  All real gases approach ideal behaviour at sufficiently high temperatures and low pressures. Under these conditions, the kinetic energy of molecules overcomes attractive forces and the molecular volume becomes negligible compared to the container volume.

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Methods used in this brief

• Collaborative Problem-Solving
• Jigsaw