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Chemistry - Matter (Kinetic Molecular Theory)

Grade 8ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Matter is composed of very small particles called atoms or molecules (e.g., H2OH_2O, O2O_2).

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Intermolecular Space (IMSIMS): The empty space existing between the molecules of a substance. It is minimum in solids and maximum in gases.

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Intermolecular Force of Attraction (IMFIMF): The force with which molecules attract each other. There is an inverse relationship: IMF∝1IMSIMF \propto \frac{1}{IMS}.

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Kinetic Energy (K.E.K.E.): Molecules are in a state of continuous random motion. The K.E.K.E. of molecules increases with an increase in temperature (TT).

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Change of State: Physical states change when heat energy is added or removed, affecting the K.E.K.E. and IMFIMF. For example, Melting involves Solid→HeatLiquidSolid \xrightarrow{Heat} Liquid.

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Sublimation: The process where a solid directly converts into gas without passing through the liquid state (e.g., NH4ClNH_4Cl, I2I_2, or Naphthalene C10H8C_{10}H_8).

📐Formulae

K.E.∝TK.E. \propto T

IMF∝1Intermolecular SpaceIMF \propto \frac{1}{Intermolecular\ Space}

Density (ρ)=Mass (m)Volume (V)\text{Density } (\rho) = \frac{\text{Mass } (m)}{\text{Volume } (V)}

Solid⇌FreezingMeltingLiquid⇌CondensationBoilingGas\text{Solid} \xrightleftharpoons[Freezing]{Melting} \text{Liquid} \xrightleftharpoons[Condensation]{Boiling} \text{Gas}

💡Examples

Problem 1:

Why does the smell of hot sizzling food reach you several meters away, but to get the smell from cold food you have to go close?

Solution:

The rate of diffusion depends on the Kinetic Energy of the particles. Since K.E.∝TK.E. \propto T, particles of hot food have higher temperature and move faster.

Explanation:

At higher temperatures, the molecules of the food vapors possess higher K.E.K.E., allowing them to mix with air and diffuse across large distances rapidly compared to cold food particles.

Problem 2:

Identify the process when Ammonium Chloride (NH4ClNH_4Cl) is heated in a China dish covered with an inverted funnel.

Solution:

The process is Sublimation: NH4Cl(s)→ΔNH4Cl(g)NH_4Cl(s) \xrightarrow{\Delta} NH_4Cl(g).

Explanation:

According to Kinetic Molecular Theory, providing heat to NH4ClNH_4Cl increases the K.E.K.E. of molecules so significantly that they overcome the IMFIMF of the solid state and escape directly into the vapor state.

Problem 3:

Compare the Intermolecular Space (IMSIMS) and Intermolecular Force (IMFIMF) between a piece of Iron (FeFe) and Oxygen gas (O2O_2).

Solution:

IMSIron<IMSOxygenIMS_{Iron} < IMS_{Oxygen} and IMFIron>IMFOxygenIMF_{Iron} > IMF_{Oxygen}.

Explanation:

In solids like FeFe, particles are closely packed due to very strong IMFIMF, resulting in negligible IMSIMS. In gases like O2O_2, particles are far apart due to negligible IMFIMF, resulting in very large IMSIMS.