NCERT Solutions for Class 11 Chemistry Chapter 5: States of Matter
Chapter 5 of Class 11 Chemistry, States of Matter, bridges the gap between abstract atomic theory and the physical world we experience every day. Why does gas fill any container it is placed in? Why does water have a higher boiling point than expected for a molecule of its size? Why does a gas behave differently at very high pressures? All of these questions — and more — are answered in this chapter. At Myclass24, the NCERT solutions for States of Matter are written in a way that helps students connect each law and concept to real, observable phenomena.
The NCERT solutions for chapter covers the three states of matter (solid, liquid, gas) and focuses primarily on the gaseous and liquid states. Students learn about Boyle's Law, Charles' Law, Gay-Lussac's Law, Avogadro's Law, and how they combine into the Ideal Gas Equation. The kinetic molecular theory gives a microscopic explanation for gas behaviour, while concepts like compressibility factor, van der Waals equation, and liquefaction explain why real gases deviate from ideal behaviour. Numerical problems involving gas law calculations, molar mass from gas density, and partial pressure (Dalton's law) are all covered in these solutions. For students across India — from Srinagar to Thiruvananthapuram, from Guwahati to Rajkot — Myclass24 offers free, high-quality content that makes this chapter genuinely understandable rather than a source of stress.
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NCERT Solutions — Class 11 Chemistry Chapter 5: States of Matter
Gas law numericals, real gas concepts, kinetic theory explained | Free PDF on Myclass24
Chapter 5 Explained: Gas Laws, Kinetic Theory & Liquid State
The Three States of Matter
Matter exists in three common states: solid, liquid, and gas. In solids, particles are tightly packed in fixed positions — they vibrate but do not move freely. Liquids have particles that move but stay in contact with each other. Gases have particles that move freely and are far apart. A fourth state, plasma, exists at extremely high temperatures but is not covered in the NCERT Class 11 syllabus. The state of matter a substance is in depends on the balance between intermolecular forces (holding particles together) and thermal energy (trying to pull them apart). One can check out all chapters of NCERT Solutions for Class 11 Chemistry and all subjects of NCERT Solutions for Class 11 from the Myclass24 page.
Gas Laws — The Foundation of This Chapter
The gas laws describe how pressure, volume, temperature, and the amount of gas are related. Each law was discovered experimentally, and together they form a complete picture of ideal gas behaviour.
| Law | Relationship | Mathematical Form | Condition |
|---|---|---|---|
| Boyle's Law | P and V | PV = constant | T and n constant |
| Charles' Law | V and T | V/T = constant | P and n constant |
| Gay-Lussac's Law | P and T | P/T = constant | V and n constant |
| Avogadro's Law | V and n | V/n = constant | T and P constant |
| Ideal Gas Equation | P, V, T, n | PV = nRT | Ideal behaviour |
The gas constant R = 8.314 J mol⁻¹ K⁻¹ (or 0.0821 L·atm mol⁻¹ K⁻¹). At STP (273.15 K and 1 atm), one mole of an ideal gas occupies 22.4 L.
Dalton's Law of Partial Pressures
When two or more non-reacting gases are mixed, the total pressure of the mixture is the sum of the partial pressures of individual gases. Partial pressure of a gas = mole fraction × total pressure. This law is crucial for problems involving gas mixtures and collecting gases over water (where water vapour pressure must be subtracted).
Kinetic Molecular Theory of Gases
The kinetic theory explains gas behaviour at the molecular level with five key postulates:
- Gas molecules are point masses with negligible volume compared to the container.
- No intermolecular forces between molecules (assumption for ideal gas).
- Molecules move in all directions with various speeds, colliding elastically.
- Pressure is due to collision of molecules on container walls.
- Average kinetic energy is proportional to absolute temperature: KE = (3/2)RT
| Speed Type | Formula | Description |
|---|---|---|
| Root Mean Square Speed (u_rms) | √(3RT/M) | Related to KE of molecules |
| Average Speed (u_av) | √(8RT/πM) | Mean speed of all molecules |
| Most Probable Speed (u_mp) | √(2RT/M) | Speed possessed by most molecules |
Real Gases and van der Waals Equation
Real gases deviate from ideal behaviour at high pressures and low temperatures because ideal gas assumptions break down — molecules do have volume, and intermolecular attractions do exist. van der Waals modified the ideal gas equation to account for these:
(P + an²/V²)(V − nb) = nRT
Here, 'a' is the correction for intermolecular attraction (larger for polar molecules), and 'b' is the correction for excluded volume of molecules. The compressibility factor Z = PV/nRT equals 1 for an ideal gas; deviations from 1 indicate non-ideal behaviour.
Liquefaction of Gases and Critical Temperature
Gases can be liquefied by increasing pressure and decreasing temperature. The critical temperature (Tᶜ) is the temperature above which a gas cannot be liquefied no matter how much pressure is applied. For CO₂, Tᶜ = 304.2 K (31.1°C). Gases with stronger intermolecular forces have higher critical temperatures. Below the critical temperature, increasing pressure alone can liquefy the gas.
Liquid State — Surface Tension and Viscosity
Liquids show two important properties. Surface tension is the energy required to increase the surface area of a liquid by one unit — it arises from an inward pull on surface molecules. Viscosity is the resistance to flow — it decreases with increasing temperature because thermal energy overcomes intermolecular forces. Water has unusually high surface tension and viscosity compared to similar-sized molecules, largely due to hydrogen bonding.
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FAQs for NCERT Solutions Class 11 Chemistry Chapter 5 States of Matter
The three states of matter differ in particle arrangement, movement, and intermolecular forces. Solids have fixed shape and volume due to strong forces of attraction. Liquids have fixed volume but no fixed shape, allowing them to flow. Gases have neither fixed shape nor volume because particles move freely with minimal intermolecular attraction. Understanding these differences helps students explain physical properties and phase changes. This topic is fundamental to chemistry and forms the basis for studying gas laws, kinetic theory, and molecular interactions.
Gas laws describe the relationship between pressure, volume, temperature, and the amount of gas. Important laws include Boyle's Law, Charles's Law, Gay-Lussac's Law, and Avogadro's Law. These laws help explain gas behavior under different conditions and form the basis for the ideal gas equation. Understanding gas laws enables students to solve numerical problems and predict changes in gaseous systems. Questions related to gas laws are commonly included in examinations due to their practical applications and mathematical significance.
Kinetic molecular theory explains the behavior of gases by considering the motion of particles. According to this theory, gas particles are in constant random motion and collisions between them are perfectly elastic. The theory assumes negligible particle volume and no intermolecular forces. It successfully explains gas pressure, temperature effects, and diffusion. Understanding kinetic molecular theory helps students interpret gas laws and molecular behavior. This concept provides a scientific basis for understanding the properties of gases and is frequently tested in chemistry examinations.
Intermolecular forces are attractive forces that exist between molecules. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. They influence physical properties such as boiling point, melting point, viscosity, and surface tension. Stronger intermolecular forces generally result in higher boiling and melting points. Understanding these forces helps students explain differences in the behavior of substances. This topic is important because it connects molecular structure with observable physical properties and appears frequently in conceptual examination questions.