Change of State

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1Summary 重點摘要

During a change of state the temperature stays constant: melting and vaporisation absorb energy, freezing and condensation release it. Use Q = ml with water's specific latent heats of 3.34 × 10⁵ (fusion) and 2.26 × 10⁶ J kg⁻¹ (vaporisation); the plateau of a heating curve marks a change of state.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Three states in particle termsDescribe changes of state with the particle picture. Particles in a solid are tightly packed and orderly, vibrating about fixed positions, so solids keep both shape and volume; liquid particles stay close but can slide past each other, so liquids keep volume but not shape; gas particles are far apart and fly freely, filling the whole container. A change of state is a change of particle arrangement and separation. For example, when ice melts, particles go from orderly vibration to sliding about; when water boils into steam, the particles break apart completely and the volume expands enormously.
2. Step 2: Melting and freezing — a pair of oppositesMelting turns a solid into a liquid and absorbs energy; freezing turns a liquid into a solid and releases energy. Both happen at the same temperature — the melting point (which equals the freezing point). For ice it is 0°C. For example, leave an ice-water mixture in a warm room and the ice keeps melting and absorbing heat; while any ice remains, the mixture stays at exactly 0°C. Conversely, water freezing releases latent heat — orchard farmers spray water on frosty nights, and the heat released as the water freezes protects the fruit from frost damage.
3. Step 3: Vaporisation and condensation — the energetic pairVaporisation turns liquid into gas and absorbs a great deal of energy; condensation turns gas into liquid and releases exactly as much. At standard atmospheric pressure water boils at 100°C, with vigorous vaporisation throughout the liquid and at the surface. For example, steam scalds are often worse than scalds from boiling water at the same temperature: the steam first condenses on the skin, releasing its specific latent heat of 2.26 × 10⁶ J kg⁻¹, then cools — so the total energy delivered far exceeds that of the same mass of boiling water. This is also why pressurised steam carries such enormous energy.
4. Step 4: Specific latent heat and Q = mlThe specific latent heat l is the energy needed to change the state of 1 kg of a substance completely, in J kg⁻¹: use the specific latent heat of fusion l_f for melting and of vaporisation l_v for boiling, both with Q = ml. For water, l_f = 3.34 × 10⁵ J kg⁻¹ and l_v = 2.26 × 10⁶ J kg⁻¹. For example, turning 0.5 kg of water at 100°C fully into steam takes Q = ml = 0.5 × 2.26 × 10⁶ = 1.13 × 10⁶ J, while heating the same 0.5 kg of water from 0°C to 100°C needs only 0.5 × 4200 × 100 = 210000 J — vaporisation costs more than five times as much.
5. Step 5: The heating curve and its plateausHeat ice steadily and record temperature against time to obtain a heating curve: first the ice warms up, then at 0°C comes the first plateau — the ice is melting, and all the latent heat goes into raising particle potential energy, so the temperature holds; once melted, the water warms again until 100°C brings the second plateau — the water is boiling. For example, an exam question shows a curve with two plateaus: the plateau temperatures are the melting and boiling points, and the plateau lengths reflect how much latent heat is involved. Remember: plateau = change of state, temperature constant — the heating has not stopped.
6. Step 6: Evaporation and coolingEvaporation is vaporisation below the boiling point: only particles at the surface with exceptionally high kinetic energy can break free and fly away. These escapees take extra energy with them, so the average kinetic energy of the liquid left behind falls and its temperature drops — the cooling effect of evaporation. Evaporation speeds up with higher temperature, larger surface area, faster air movement and lower humidity. For example, sponging a feverish patient with warm water cools the body as the water evaporates; a dog pants on a hot day to lose heat as its saliva evaporates.
7. Step 7: Combined calculations — warming and changing state togetherHarder questions chain Q = mcΔT with Q = ml: warm first, change state, then warm again, adding the energies of each stage. For example, turning 0.2 kg of ice at 0°C into water at 25°C takes two stages — melting first: Q₁ = ml_f = 0.2 × 3.34 × 10⁵ = 66800 J; then warming the water from 0°C to 25°C: Q₂ = mcΔT = 0.2 × 4200 × 25 = 21000 J; total Q = 66800 + 21000 = 87800 J. Listing each stage with its own formula and adding them up is the safe strategy for such questions.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• During changes of state (melting, freezing, vaporisation, condensation) the temperature stays constant
• Melting and vaporisation absorb energy; freezing and condensation release it — latent heat changes particle potential energy
• Q = ml; for water l_f = 3.34 × 10⁵ J kg⁻¹ and l_v = 2.26 × 10⁶ J kg⁻¹
• A plateau on a heating curve means a change of state; its temperature is the melting or boiling point
• Evaporation happens below the boiling point; escaping high-energy particles cool the liquid — used for cooling