HKDSE Physics Core
Chapter 2: Change of State | 物態變化
Billy Sir’s Smart Notes: Master Latent Heat, Phase Changes, Evaporation, and Condensation.
由中大物理系碩士 Billy Sir 編寫，助你極速掌握潛熱、物態轉變、蒸發及凝結。
1. Temperature Variation During Change of State | 物態變化時的溫度變化
When a substance changes its state (e.g., melting or boiling), energy is absorbed or released, but the temperature remains constant. 當物質改變狀態（如熔解或沸騰）時，會吸收或釋放能量，但溫度保持不變。
📈 The Heating Curve | 加熱曲線
- Solid phase: Temperature increases as heat is added. Kinetic energy of particles increases. (固態：加熱時溫度上升，粒子動能增加)
- Melting (Solid to Liquid): Temperature stays constant at the melting point. Energy is used to overcome intermolecular forces (increasing potential energy, not kinetic). (熔解：溫度停留在熔點。能量用於克服分子間的引力，增加勢能而非動能)
- Boiling (Liquid to Gas): Temperature stays constant at the boiling point. Energy is used to completely separate the molecules. (沸騰：溫度停留在沸點。能量用於完全分離分子)
2. Latent Heat & Specific Latent Heat | 潛熱與比潛熱
Latent Heat is the energy transferred during a change of state without a change in temperature. 潛熱是在沒有溫度變化的情況下，物態變化時轉移的能量。
⚙️ Formula for Specific Latent Heat | 比潛熱公式
Specific Latent Heat (\( l \)) is the energy required to change the state of 1 kg of a substance without temperature change. Unit: \( \text{J kg}^{-1} \).
| Type (類型) | Definition (定義) | Phase Change (物態轉變) |
|---|---|---|
| Specific Latent Heat of Fusion (\( l_f \)) 熔解比潛熱 |
Energy to change 1 kg of solid to liquid at melting point. (在熔點將 1 kg 固體變成液體所需的能量) | Solid \( \rightleftharpoons \) Liquid (固體 \( \rightleftharpoons \) 液體) |
| Specific Latent Heat of Vaporization (\( l_v \)) 汽化比潛熱 |
Energy to change 1 kg of liquid to gas at boiling point. (在沸點將 1 kg 液體變成氣體所需的能量) | Liquid \( \rightleftharpoons \) Gas (液體 \( \rightleftharpoons \) 氣體) |
3. Experiment: Measuring Specific Latent Heat | 實驗：量度比潛熱
To measure the specific latent heat of fusion of ice, we use an immersion heater to melt crushed ice. 為了量度冰的熔解比潛熱，我們使用浸沒式電熱器來熔化碎冰。
🔬 Experimental Setup & Calculation | 實驗裝置與計算
- Setup: Two identical funnels with crushed ice. One has a heater turned ON (Experimental), the other is turned OFF (Control). (裝置：兩個裝有碎冰的漏斗。一個開啟電熱器（實驗組），另一個關閉（對照組）。)
- Why a Control? To account for the ice melted by the heat from the surrounding room temperature. (為何需要對照組？為了補償因室溫環境熱量而熔化的冰。)
- Calculation: Let \( m_1 \) be the mass of water collected in the experimental setup, and \( m_2 \) be the mass in the control setup. The mass melted purely by the heater is \( m = m_1 – m_2 \).
(Where \( P \) is power of heater, \( t \) is time. \( P \) 為電熱器功率，\( t \) 為時間。)
4. Evaporation and Condensation | 蒸發與凝結
💨 Evaporation | 蒸發
Evaporation is the process where a liquid turns into a gas at temperatures below its boiling point. It only occurs at the surface of the liquid. 蒸發是液體在低於沸點的溫度下變成氣體的過程。它只發生在液體的表面。
Cooling Effect: Faster-moving particles (higher kinetic energy) escape the surface, leaving behind slower particles. The average kinetic energy drops, so the temperature of the liquid decreases. (冷卻效應：移動較快（動能較高）的粒子逃離表面，留下較慢的粒子。平均動能下降，導致液體溫度降低。)
Factors increasing evaporation rate: (增加蒸發速率的因素：)
- Higher temperature (較高溫度)
- Larger surface area (較大表面積)
- Presence of wind / air movement (有風 / 空氣流動)
- Lower humidity (較低濕度)
💧 Condensation | 凝結
Condensation is the process where a gas turns into a liquid when it cools down. During condensation, latent heat is released to the surroundings. 凝結是氣體冷卻時變成液體的過程。在凝結過程中，潛熱會釋放到周圍環境中。
Example: Water droplets forming on the outside of a cold glass of water. (例子：冷水杯外壁形成的水滴。)
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