📚 Changes of State | 物态变化
In CIE A Level Physics, changes of state are studied under thermal physics and the particle model. When a substance is heated or cooled, its temperature may change, or its phase may change. Energy transfers during melting, boiling, evaporation, condensation and freezing are described using specific latent heat. A key idea is that temperature stays constant during a change of state even though energy is still being supplied or removed.
在 CIE A Level 物理中，物态变化属于热物理学和粒子模型的重要内容。物质受热或冷却时，温度可能改变，也可能发生相变。熔化、沸腾、蒸发、凝结和凝固过程中的能量转移用比潜热描述。关键思想是：发生物态变化时，即使仍然供热或放热，温度也保持不变。
1. States of Matter and the Particle Model | 物质状态与粒子模型
Matter usually exists in three common states: solid, liquid and gas. The particle model explains the macroscopic properties of these states by considering particle arrangement, particle motion and intermolecular forces.
物质通常以三种常见状态存在：固态、液态和气态。粒子模型通过粒子的排列方式、运动方式以及分子间作用力来解释这些状态的宏观性质。
In a solid, particles are closely packed in a regular lattice and mainly vibrate about fixed positions. In a liquid, particles are still close together but can slide past one another. In a gas, particles are far apart and move rapidly in random directions.
在固体中，粒子紧密排列成规则晶格，主要在固定位置附近振动。在液体中，粒子仍然彼此靠近，但可以相互滑动。在气体中，粒子相距很远，并沿随机方向快速运动。
| State 状态 | Arrangement 排列 | Motion 运动 | Intermolecular forces 分子间作用力 |
|---|---|---|---|
| Solid 固体 | Regular lattice 规则晶格 | Vibration about fixed positions 在固定位置附近振动 | Strong 强 |
| Liquid 液体 | Close but disordered 靠近但无序 | Particles slide past each other 粒子相互滑动 | Moderate 中等 |
| Gas 气体 | Far apart 相距很远 | Fast random motion 快速随机运动 | Very weak 非常弱 |
2. Internal Energy | 内能
Internal energy is the sum of the random kinetic energy of all particles and the total potential energy due to intermolecular forces. It does not include the macroscopic kinetic energy or gravitational potential energy of the object as a whole.
内能是所有粒子的随机动能与由分子间作用力产生的总势能之和。它不包括物体整体的宏观动能或重力势能。
For an ideal gas, internal energy is entirely kinetic because intermolecular potential energy is negligible. For real solids and liquids, potential energy is significant because particles are close enough to exert strong forces on each other.
对于理想气体，内能完全是动能，因为分子间势能可以忽略。对于真实的固体和液体，势能非常重要，因为粒子相距足够近，彼此之间会产生较强的作用力。
3. Kinetic and Potential Energy during Phase Change | 相变过程中的动能与势能
During a temperature rise, the average kinetic energy of particles increases. The internal energy therefore increases, but the intermolecular potential energy may remain almost unchanged if the state does not change.
温度升高时，粒子的平均动能增大。因此内能增加，但如果状态没有改变，分子间势能几乎不变。
During a change of state, the temperature remains constant. The average kinetic energy does not change, but the intermolecular potential energy changes because bonds are broken or formed. This explains why energy can be supplied during melting or boiling without any temperature rise.
发生物态变化时，温度保持不变。平均动能不变，但由于化学键断裂或形成，分子间势能发生变化。这就解释了为什么在熔化或沸腾过程中不断供热，温度却不上升。
4. Key Terms for Changes of State | 物态变化的关键术语
The table below summarises the main changes of state and the direction of energy transfer. Each process occurs at a specific temperature for a pure substance, although evaporation can occur over a range of temperatures.
下表总结了主要的物态变化以及能量转移方向。对于纯净物质，每种过程在特定温度下发生，但蒸发可以在一定温度范围内发生。
| Process 过程 | Change 变化 | Energy transfer 能量转移 |
|---|---|---|
| Melting / fusion 熔化 | Solid → Liquid 固态 → 液态 | Absorbed 吸收 |
| Freezing / solidification 凝固 | Liquid → Solid 液态 → 固态 | Released 释放 |
| Boiling / evaporation 沸腾 / 蒸发 | Liquid → Gas 液态 → 气态 | Absorbed 吸收 |
| Condensation 凝结 | Gas → Liquid 气态 → 液态 | Released 释放 |
| Sublimation 升华 | Solid → Gas 固态 → 气态 | Absorbed 吸收 |
| Deposition 凝华 | Gas → Solid 气态 → 固态 | Released 释放 |
5. Specific Latent Heat | 比潜热
Specific latent heat L is the energy required per unit mass to change the state of a substance at constant temperature. Its SI unit is J kg⁻¹.
比潜热 L 是单位质量的物质在恒定温度下发生物态变化所需的能量。其 SI 单位是 J kg⁻¹。
The specific latent heat of fusion Lf refers to melting or freezing. The specific latent heat of vaporisation Lv refers to boiling or condensation. The value of Lv is usually much larger than Lf because gas particles must be separated almost completely from each other.
熔化比潜热 Lf 与熔化或凝固有关。汽化比潜热 Lv 与沸腾或凝结有关。Lv 的值通常远大于 Lf，因为气态粒子需要几乎完全彼此分离。
6. Latent Heat Formula and Calculations | 潜热公式与计算
The energy Q transferred during a change of state is calculated using the mass m and the specific latent heat L:
物态变化过程中传递的能量 Q 可由质量 m 和比潜热 L 计算：
Q = mL
For example, the energy needed to melt 0.50 kg of ice at 0 °C, given Lf = 3.34 × 10⁵ J kg⁻¹, is:
例如，在 0 °C 时熔化 0.50 kg 冰所需能量，已知 Lf = 3.34 × 10⁵ J kg⁻¹：
Q = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J
The temperature does not change during this process. All the supplied energy increases the potential energy of the particles rather than their kinetic energy.
该过程中温度不发生变化。所有供给的能量都增加了粒子的势能，而不是粒子的动能。
7. Heating and Cooling Curves | 加热与冷却曲线
A heating curve shows how temperature changes with time when a solid is heated at a constant rate. A pure substance shows two flat plateaus: one at the melting point and one at the boiling point.
加热曲线表示固体在恒定速率加热时温度随时间的变化。纯净物质的加热曲线有两个水平平台：一个在熔点，一个在沸点。
- Solid warms up: temperature rises, Q = mcΔθ
- Melting plateau: temperature is constant, Q = mLf
- Liquid warms up: temperature rises, Q = mcΔθ
- Boiling plateau: temperature is constant, Q = mLv
- Gas warms up: temperature rises, Q = mcΔθ
- 固体升温：温度上升，Q = mcΔθ
- 熔化平台：温度恒定，Q = mLf
- 液体升温：温度上升，Q = mcΔθ
- 沸腾平台：温度恒定，Q = mLv
- 气体升温：温度上升，Q = mcΔθ
A cooling curve is the reverse process. The flat plateaus appear at the condensation point and freezing point, where latent heat is released without a temperature drop.
冷却曲线是相反的过程。水平平台出现在凝结点和凝固点，此时释放潜热但温度不下降。
8. Melting and Freezing at the Molecular Level | 分子层面的熔化与凝固
As a solid is heated, its particles vibrate more vigorously. At the melting point, many particles have enough energy to break free from their fixed lattice positions. The absorbed latent heat increases the potential
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