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States of Matter - Boiling, melting, and freezing points

Grade 5IGCSE

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. Changes between these states are physical changes caused by the gain or loss of thermal energy.

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The Melting Point is the specific temperature at which a solid gains enough energy to turn into a liquid. For pure water (H2OH_2O), the melting point is 0∘C0^\circ C.

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The Freezing Point is the temperature at which a liquid loses energy and turns into a solid. For most substances, the freezing point is the same as the melting point (0∘C0^\circ C for water).

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The Boiling Point is the temperature at which a liquid turns into a gas throughout the bulk of the liquid. For pure water, the boiling point is 100∘C100^\circ C at standard atmospheric pressure.

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During a change of state (like melting or boiling), the temperature of the substance remains constant even if heat is still being added. This energy is used to break the bonds between particles rather than raising the temperature.

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Particle Theory: In a solid, particles vibrate in fixed positions. In a liquid, particles move around each other. In a gas, particles move rapidly and are far apart. Increasing temperature increases the kinetic energy of these particles.

📐Formulae

Tmelting=TfreezingT_{melting} = T_{freezing}

H2O(s)→0∘CH2O(l)H_2O_{(s)} \xrightarrow{0^\circ C} H_2O_{(l)}

H2O(l)→100∘CH2O(g)H_2O_{(l)} \xrightarrow{100^\circ C} H_2O_{(g)}

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

💡Examples

Problem 1:

A beaker containing ice and water is placed on a heater. A thermometer shows a reading of 0∘C0^\circ C. If more heat is added but some ice still remains, what will the thermometer read?

Solution:

0∘C0^\circ C

Explanation:

As long as both the solid and liquid phases of a substance coexist during melting, the temperature will remain at the melting point (0∘C0^\circ C for water). The added heat energy is used to change the state from solid to liquid rather than increasing the temperature.

Problem 2:

Pure water is being heated in a laboratory at sea level. At what temperature will the water begin to turn into steam throughout the liquid?

Solution:

100∘C100^\circ C

Explanation:

The boiling point of pure water (H2OH_2O) at standard pressure is 100∘C100^\circ C. This is the point at which the liquid has enough energy to overcome atmospheric pressure and turn into a gas (water vapor).

Problem 3:

A substance has a melting point of 45∘C45^\circ C. If the current room temperature is 25∘C25^\circ C, what state of matter is the substance in?

Solution:

Solid

Explanation:

Since the current temperature (25∘C25^\circ C) is lower than the melting point (45∘C45^\circ C), the substance has not yet reached the energy level required to turn into a liquid, so it remains a solid.