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

Grade 6IB

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

🔑Concepts

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Matter is made of tiny particles that are constantly moving. The state of matter depends on the kinetic energy of these particles and the strength of the attractive forces between them.

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Melting is the process where a solid turns into a liquid. This occurs when heat energy is absorbed, causing particles to vibrate faster until they break free from their fixed positions. The temperature at which this occurs is the Melting PointMelting\ Point.

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Freezing (Solidification) is the transition from a liquid to a solid. As energy is removed, particles slow down and attractive forces pull them into a rigid, organized structure. For water, this occurs at 0∘C0^\circ C.

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Evaporation is the process where particles at the surface of a liquid gain enough kinetic energy to escape into the gas phase. Unlike boiling, evaporation can happen at any temperature below the Boiling PointBoiling\ Point.

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During a change of state, the temperature of a substance remains constant. The thermal energy added or removed is used to change the bonds/forces between particles rather than increasing the average kinetic energy (TemperatureTemperature).

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Kinetic Molecular Theory states that in a solid, particles are packed tightly; in a liquid, they are close but can slide; in a gas (H2O(g)H_2O(g)), they are far apart and move rapidly.

📐Formulae

Solid+Heat→LiquidSolid + Heat \rightarrow Liquid

Liquid−Heat→SolidLiquid - Heat \rightarrow Solid

H2O(s)→MeltingH2O(l)H_2O(s) \xrightarrow{Melting} H_2O(l)

T(K)=T(∘C)+273.15T(K) = T(^\circ C) + 273.15

Energyinput⇒ΔPhase (Temperature remains constant)Energy_{input} \Rightarrow \Delta Phase \text{ (Temperature remains constant)}

💡Examples

Problem 1:

A beaker contains 100g100g of ice at 0∘C0^\circ C. If you apply heat to the beaker, what happens to the temperature of the mixture until all the ice has turned into H2O(l)H_2O(l)?

Solution:

The temperature remains constant at 0∘C0^\circ C.

Explanation:

During the phase change from solid to liquid, the thermal energy provided is used to overcome the intermolecular forces holding the ice crystals together. This energy is called latent heat. Because the average kinetic energy of the particles does not increase during this time, the temperature does not rise until the melting process is complete.

Problem 2:

Why does a wet sidewalk dry faster on a hot, windy day compared to a cool, still day?

Solution:

Higher temperature and wind increase the rate of evaporation (Liquid→GasLiquid \rightarrow Gas).

Explanation:

Increased temperature means more particles have the kinetic energy required to escape the surface of the H2O(l)H_2O(l). Wind moves the escaped water vapor away from the surface, preventing them from falling back into the liquid, thus speeding up the process.

Problem 3:

Represent the freezing of water using chemical state symbols.

Solution:

H2O(l)→H2O(s)+EnergyH_2O(l) \rightarrow H_2O(s) + Energy

Explanation:

The symbol (l)(l) denotes the liquid state and (s)(s) denotes the solid state. Since freezing is an exothermic process, energy is released into the surroundings as the particles lose kinetic energy and bond together.