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Physics - Matter (Kinetic Theory)

Grade 8ICSE

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

🔑Concepts

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Matter is composed of extremely small particles called atoms or molecules.

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Intermolecular space (SinterS_{inter}): 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 (FinterF_{inter}): The force of attraction between the molecules of matter. It is strongest in solids and weakest in gases.

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

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Solids have a definite shape and volume because FinterF_{inter} is very strong, keeping molecules in fixed positions.

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Liquids have a definite volume but no definite shape because FinterF_{inter} is weak enough to allow molecules to move within the boundary of the liquid.

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Gases have neither definite shape nor definite volume because FinterF_{inter} is negligible and SinterS_{inter} is very large.

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Change of state: When heat is supplied, the K.E.K.E. of molecules increases, which helps them overcome the FinterF_{inter}, leading to phase changes like melting or vaporization.

📐Formulae

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

K.E.∝T (where T is the absolute temperature in Kelvin)K.E. \propto T \text{ (where } T \text{ is the absolute temperature in Kelvin)}

P=FA (Pressure exerted by gas molecules on container walls)P = \frac{F}{A} \text{ (Pressure exerted by gas molecules on container walls)}

💡Examples

Problem 1:

Explain the process of sublimation in iodine crystals using the kinetic theory of matter.

Solution:

In iodine (I2I_2) crystals, the intermolecular forces are weak. When heated, the K.E.K.E. of the molecules increases significantly and quickly overcomes the FinterF_{inter}.

Explanation:

Because the molecules gain enough energy to break free directly into the gaseous state without passing through the liquid state, the solid iodine turns directly into I2I_2 vapor.

Problem 2:

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 is higher at higher temperatures. Molecules of hot food have higher K.E.K.E. than those of cold food.

Explanation:

Since K.E.∝TK.E. \propto T, the molecules of the hot food move with higher velocities (vv), allowing them to mix with air and reach greater distances quickly.

Problem 3:

Calculate the density of a substance if a mass of 500 g500\text{ g} occupies a volume of 250 cm3250\text{ cm}^3.

Solution:

ρ=MV=500 g250 cm3=2 g/cm3\rho = \frac{M}{V} = \frac{500\text{ g}}{250\text{ cm}^3} = 2\text{ g/cm}^3

Explanation:

Density is defined as mass per unit volume. Using the formula ρ=MV\rho = \frac{M}{V}, we find the concentration of matter in the given space.

Matter (Kinetic Theory) - Revision Notes & Key Formulas | ICSE Class 8 Science