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Matter and its Composition - Kinetic Theory of Matter

Grade 7ICSE

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

🔑Concepts

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Matter is anything that has mass, occupies space, and can be perceived by our senses. It is composed of tiny particles called atoms or molecules (e.g., H2OH_2O molecules in water).

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According to the Kinetic Theory of Matter, particles of matter are in continuous, random motion and possess kinetic energy (K.E.K.E.).

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Intermolecular Space: The space existing between the molecules of a substance. It is minimum in solids, more in liquids, and maximum in gases.

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Intermolecular Force of Attraction: The force with which molecules of a substance attract each other. It is strongest in solids and weakest in gases. This force is also known as Cohesion for similar molecules and Adhesion for different molecules.

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Kinetic Energy and Temperature: The kinetic energy of particles increases with an increase in temperature (TT). As TT increases, molecules move faster.

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Solids have a definite shape and volume because the intermolecular forces are very strong, keeping the particles in fixed positions.

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Liquids have a definite volume but no definite shape. The particles are less tightly packed, allowing them to glide over each other.

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Gases have neither a definite shape nor a definite volume. The intermolecular forces are negligible, allowing particles to move freely in all directions (VgasV_{gas} expands to fill the container).

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Change of State: Matter can change from one state to another (e.g., Solid→HeatLiquidSolid \xrightarrow{Heat} Liquid) by changing temperature or pressure, which affects the kinetic energy of the molecules.

📐Formulae

Density(D)=Mass(M)Volume(V)Density (D) = \frac{Mass (M)}{Volume (V)}

K.E.∝T (Kinetic Energy is directly proportional to Temperature)K.E. \propto T \text{ (Kinetic Energy is directly proportional to Temperature)}

Total Energy=P.E.+K.E.\text{Total Energy} = P.E. + K.E.

💡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:

At higher temperatures, the kinetic energy (K.E.K.E.) of the food particles is higher.

Explanation:

Since K.E.∝TK.E. \propto T, the particles of hot food move faster and have a higher rate of diffusion in the air. In cold food, the kinetic energy is low, resulting in a slower rate of diffusion.

Problem 2:

When we dissolve a spoon of sugar in a glass of water, the level of water does not rise significantly. Why?

Solution:

The sugar particles occupy the intermolecular spaces of the water molecules.

Explanation:

According to the Kinetic Theory, there are spaces between water molecules (H2OH_2O). When sugar is added, the sugar molecules (C12H22O11C_{12}H_{22}O_{11}) break down and fit into these empty spaces, hence the total volume (VV) does not increase noticeably.

Problem 3:

Compare the intermolecular force (FiF_i) and intermolecular space (SiS_i) between a piece of Iron and Oxygen gas.

Solution:

For Iron (Solid): FiF_i is very high, SiS_i is very low. For Oxygen (Gas): FiF_i is negligible, SiS_i is very high.

Explanation:

Solids like FeFe have particles tightly bound by strong attractive forces, leaving little space. Gases like O2O_2 have particles far apart due to weak attraction.