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Matter and its Composition - Change of State

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 (mm) and occupies volume (VV). It is composed of tiny particles called atoms or molecules.

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The Kinetic Theory of Matter states that particles of matter are in continuous motion and possess Kinetic Energy (K.E.K.E.). The K.E.K.E. is directly proportional to the temperature (TT).

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Change of State is a physical change where matter moves from one state (solid, liquid, or gas) to another. This occurs by the absorption or release of heat energy.

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Melting (Fusion) is the process where a solid changes into a liquid at a fixed temperature called the Melting Point. For ice, this is 0∘C0^\circ C.

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Vaporization (Boiling) is the process where a liquid changes into a gas at its Boiling Point. For pure water at 1 atm1\text{ atm}, this is 100∘C100^\circ C.

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Condensation is the change of state from gas to liquid on cooling, e.g., water vapor turning into droplets of H2OH_2O.

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Freezing (Solidification) is the change from liquid to solid. The freezing point of a substance is numerically equal to its melting point.

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Sublimation is the direct change of a solid into a gas without passing through the liquid state. Examples include Naphthalene (C10H8C_{10}H_8) and Ammonium Chloride (NH4ClNH_4Cl).

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During a change of state, the temperature remains constant even though heat is supplied. This heat is used to overcome the intermolecular forces of attraction.

📐Formulae

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

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

Melting Point of Ice=0∘C=273.15 K\text{Melting Point of Ice} = 0^\circ C = 273.15\text{ K}

Boiling Point of Water=100∘C=373.15 K\text{Boiling Point of Water} = 100^\circ C = 373.15\text{ K}

💡Examples

Problem 1:

Explain why the temperature of a beaker containing ice and water remains at 0∘C0^\circ C until all the ice has melted, even when heat is being supplied?

Solution:

The heat supplied is used as Latent Heat of Fusion. Instead of increasing the temperature (kinetic energy), the energy is used to break the strong intermolecular forces holding the water molecules in the solid lattice of ice.

Explanation:

In terms of the kinetic theory, the energy goes into increasing the potential energy (distance between particles) rather than the average kinetic energy (K.E.K.E.) of the molecules.

Problem 2:

If a sample of matter has a mass m=500 gm = 500\text{ g} and occupies a volume V=250 cm3V = 250\text{ cm}^3, calculate its density (ρ)(\rho).

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 mass per unit volume of a substance, which helps determine if a substance is a solid, liquid, or gas under specific conditions.

Problem 3:

Identify the process when dry ice (CO2CO_2) is exposed to room temperature and disappears without leaving any liquid.

Solution:

The process is called Sublimation.

Explanation:

Dry ice is solid carbon dioxide (CO2CO_2). At standard atmospheric pressure, it changes directly from the solid state to the gaseous state at −78.5∘C-78.5^\circ C.

Change of State - Revision Notes & Key Formulas | ICSE Class 7 Science