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States of Matter - Changes of State (Melting, Boiling, Evaporation)

Grade 6Cambridge (IGCSE)

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

🔑Concepts

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Matter is made of tiny particles. The three states of matter are SolidSolid, LiquidLiquid, and GasGas.

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Melting (Fusion) is the process where a SolidSolid turns into a LiquidLiquid. This occurs when heat energy is absorbed, causing particles to vibrate faster until they break free from their fixed positions.

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The Melting Point is the specific temperature at which a solid becomes a liquid, such as 0∘C0^\circ C for pure ice (H2OH_2O).

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Boiling is the rapid change from LiquidLiquid to GasGas occurring throughout the liquid at a specific temperature called the Boiling Point (TbT_b). For water, Tb=100∘CT_b = 100^\circ C at standard pressure.

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Evaporation is the change from LiquidLiquid to GasGas that occurs only at the surface of a liquid. Unlike boiling, it can happen at any temperature below the boiling point.

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Factors affecting evaporation include surface area (AA), temperature (TT), and wind speed. Increasing these factors increases the rate of evaporation.

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During a change of state (e.g., melting or boiling), the temperature of the substance remains constant (ΔT=0\Delta T = 0) because the energy is used to overcome intermolecular forces rather than increasing the kinetic energy of the particles.

📐Formulae

Q=m×LQ = m \times L

Rate of Evaporation∝Surface Area (A)Rate \ of \ Evaporation \propto Surface \ Area \ (A)

H2O(s)→MeltingH2O(l)→BoilingH2O(g)H_2O_{(s)} \xrightarrow{Melting} H_2O_{(l)} \xrightarrow{Boiling} H_2O_{(g)}

💡Examples

Problem 1:

Compare Boiling and Evaporation for a sample of H2OH_2O. State two key differences.

Solution:

  1. Boiling occurs at a fixed temperature (100∘C100^\circ C for water), while evaporation occurs at all temperatures. 2. Boiling happens throughout the liquid (bubbles form), while evaporation only happens at the surface.

Explanation:

Boiling is a bulk phenomenon requiring a specific energy threshold, whereas evaporation is a surface phenomenon where only high-energy molecules escape.

Problem 2:

A beaker of ice is heated. A thermometer shows the temperature stays at 0∘C0^\circ C even though heat is being added. Why?

Solution:

The heat energy is being used as 'Latent Heat' to break the bonds between the H2OH_2O molecules.

Explanation:

In a state change, the energy provided does not increase the temperature (Average Kinetic EnergyAverage \ Kinetic \ Energy). Instead, it increases the Potential EnergyPotential \ Energy of the particles to change the state from solid to liquid.