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Chemistry - Temperature and changes of state (melting, freezing, boiling)

Grade 4IGCSE

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

🔑Concepts

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Matter exists in three primary states: solid, liquid, and gas. A change of state occurs when thermal energy is either added to or removed from a substance.

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Melting is the process where a solid turns into a liquid. This occurs at a specific temperature called the melting point. For pure water (H2OH_2O), the melting point is 0∘C0^\circ\text{C}.

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Freezing (or solidification) is the reverse of melting, where a liquid turns into a solid. For a pure substance, the freezing point is the same as the melting point (0∘C0^\circ\text{C} for water).

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Boiling is the process where a liquid turns into a gas throughout the entire volume of the liquid. This happens at the boiling point. For pure water (H2OH_2O), the boiling point is 100∘C100^\circ\text{C} at standard pressure.

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Evaporation is different from boiling; it only occurs at the surface of a liquid and can happen at temperatures below the boiling point.

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During a change of state, the temperature of a substance remains constant. For example, while ice is melting, the temperature remains at 0∘C0^\circ\text{C} until all the ice has turned into liquid water.

📐Formulae

Solid+Energy→MeltingLiquidSolid + \text{Energy} \xrightarrow{\text{Melting}} Liquid

Liquid+Energy→BoilingGasLiquid + \text{Energy} \xrightarrow{\text{Boiling}} Gas

Liquid−Energy→FreezingSolidLiquid - \text{Energy} \xrightarrow{\text{Freezing}} Solid

Tmelting(H2O)=0∘CT_{\text{melting}}(H_2O) = 0^\circ\text{C}

Tboiling(H2O)=100∘CT_{\text{boiling}}(H_2O) = 100^\circ\text{C}

💡Examples

Problem 1:

A student heats a beaker of ice that is initially at −10∘C-10^\circ\text{C}. Describe what happens to the temperature when the ice starts to melt.

Solution:

The temperature stays at 0∘C0^\circ\text{C}.

Explanation:

Even though heat is still being added, the temperature does not rise above 0∘C0^\circ\text{C} until every bit of ice has melted into liquid water. The energy is being used to overcome the forces holding the solid particles together.

Problem 2:

At what state of matter will H2OH_2O be if the temperature is 25∘C25^\circ\text{C}?

Solution:

Liquid.

Explanation:

Since 25∘C25^\circ\text{C} is higher than the melting point (0∘C0^\circ\text{C}) but lower than the boiling point (100∘C100^\circ\text{C}), the water exists in the liquid state.

Problem 3:

What is the name of the process when a gas turns back into a liquid, such as steam hitting a cold window?

Solution:

Condensation.

Explanation:

Condensation is the change of state from a gas to a liquid. It occurs when gas particles lose thermal energy and move closer together.

Temperature and changes of state (melting, freezing, boiling) Revision - Grade 4 Science IGCSE