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Chemistry · Class 9 · Chapter 5: Physical States of Matter

8 min

Key points

Matter exists in three physical states: solid, liquid and gas — these states depend on intermolecular forces and kinetic energy
Kinetic Molecular Theory explains behavior of gases: gas particles are in constant random motion and have negligible intermolecular forces
Boyle's Law: At constant temperature, volume of a gas is inversely proportional to pressure (PV = constant)
Charles's Law: At constant pressure, volume of a gas is directly proportional to absolute temperature (V/T = constant)
Gay-Lussac's Law: At constant volume, pressure of a gas is directly proportional to absolute temperature (P/T = constant)
Ideal Gas Equation combines all gas laws: PV = nRT where R = 8.314 J mol⁻¹ K⁻¹
Plasma is the fourth state of matter found at very high temperatures — exists in stars and fluorescent lights
Evaporation occurs at surface of liquid at all temperatures while boiling occurs throughout liquid at specific temperature
Sublimation is direct conversion of solid to gas without passing through liquid state — examples: dry ice, camphor

Definitions

Kinetic Molecular Theory: A theory that explains the behavior of gases in terms of particles in motion. It states that gas particles are in constant random motion and the average kinetic energy is proportional to absolute temperature.
Boyle's Law: At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.
Charles's Law: At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature.
Sublimation: The process by which a solid changes directly into gas without passing through the liquid state.
Plasma: The fourth state of matter consisting of highly ionized gas with equal numbers of positive ions and electrons.

Worked example

Define the three states of matter and explain the arrangement of particles in each state. (6 marks)

Solid: Matter in which particles are closely packed in regular arrangement with strong intermolecular forces. Particles vibrate at fixed positions. Liquid: Matter in which particles are less closely packed with moderate intermolecular forces. Particles can move freely but remain close together. Gas: Matter in which particles are far apart with negligible intermolecular forces. Particles move randomly in all directions with high kinetic energy.

Common mistakes

Students often forget to convert Celsius to Kelvin when using gas law equations
Confusing evaporation with boiling — evaporation occurs at surface while boiling occurs throughout
Writing wrong units for gas constant R — remember 8.314 J mol⁻¹ K⁻¹
Mixing up direct and inverse relationships in gas laws
Forgetting to mention 'at constant temperature/pressure' conditions in gas law definitions

Quick recap

Matter exists in three states: solid (closely packed particles with strong forces), liquid (moderately packed with medium forces), and gas (widely spaced with weak forces). State changes occur at specific temperatures - melting point and boiling point. Kinetic molecular theory explains particle behavior in each state based on intermolecular forces and particle motion.

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