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Matter and its Composition - States of Matter

Grade 7ICSE

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

🔑Concepts

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Matter is defined as anything that has mass, occupies space, and can be perceived by our senses. It is composed of tiny particles called atoms or molecules (H2OH_2O, O2O_2, etc.).

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The empty space between the particles of matter is known as Intermolecular Space. It is minimum in solids and maximum in gases.

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The force of attraction existing between the particles of matter is called Intermolecular Force of Attraction. It is strongest in solids and weakest in gases.

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Kinetic Theory of Matter states that particles of matter are in continuous random motion and possess Kinetic Energy (K.E.K.E.). The K.E.K.E. increases with an increase in temperature.

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Solids have a definite shape and a definite volume because the particles are closely packed with negligible intermolecular space and very strong intermolecular forces.

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Liquids have a definite volume but no definite shape. They take the shape of the container because the intermolecular forces are weaker than in solids, allowing particles to slide over each other.

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Gases have neither a definite shape nor a definite volume. They are highly compressible because of the large intermolecular spaces between particles.

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Change of State: Matter can change from one state to another by changing temperature or pressure. For example, Solid→MeltingLiquid→BoilingGasSolid \xrightarrow{Melting} Liquid \xrightarrow{Boiling} Gas.

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Sublimation is the process where a solid directly changes into a gas without passing through the liquid state (e.g., Camphor, NH4ClNH_4Cl).

📐Formulae

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

K.E.=12mv2K.E. = \frac{1}{2}mv^2

Total Energy=Potential Energy+Kinetic Energy\text{Total Energy} = \text{Potential Energy} + \text{Kinetic Energy}

H2O(s)→HeatH2O(l)→HeatH2O(g)H_2O_{(s)} \xrightarrow{\text{Heat}} H_2O_{(l)} \xrightarrow{\text{Heat}} H_2O_{(g)}

💡Examples

Problem 1:

If a substance has a mass of 500 g500\text{ g} and occupies a volume of 250 cm3250\text{ cm}^3, calculate its density.

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 the ratio of mass to volume. By dividing the given mass by the volume, we find the density of the substance.

Problem 2:

Explain why H2OH_2O (water) exists as a solid at 0∘C0^\circ C and as a gas at 100∘C100^\circ C based on the Kinetic Theory.

Solution:

At 0∘C0^\circ C, the K.E.K.E. of molecules is low, and intermolecular forces bind them into a fixed lattice (Ice). At 100∘C100^\circ C, molecules gain enough K.E.K.E. to overcome all intermolecular forces of attraction, moving independently as steam.

Explanation:

Temperature is directly proportional to the average kinetic energy of the particles. Higher temperature leads to state changes by overcoming attractive forces.

Problem 3:

Why can a gas be compressed easily while a solid cannot?

Solution:

Gases have very large intermolecular spaces compared to solids. Under pressure, these spaces can be reduced significantly.

Explanation:

In solids, particles are already so close together that there is no space to move them further inward. In gases, the distance between molecules is many times their own size.

States of Matter - Revision Notes & Key Formulas | ICSE Class 7 Science