📚 Melting, Freezing, and Subliming | 熔化、凝固与升华
Melting, freezing, and subliming are three important physical changes. They help us explain how a solid turns into a liquid, how a liquid turns into a solid, and how some solids can change directly into gases. In this article, we will use the particle model to understand these changes and link them to energy transfer.
熔化、凝固和升华是三种重要的物理变化。它们帮助我们解释固体如何变成液体、液体如何变成固体，以及某些固体如何直接变成气体。本文将使用粒子模型来理解这些变化，并将它们与能量传递联系起来。
1. States of Matter and the Particle Model | 物质的三种状态与粒子模型
All substances are made of tiny particles. In a solid, the particles are packed closely together in a regular arrangement. They vibrate in fixed positions and cannot move around freely. This is why a solid keeps its own shape and volume.
所有物质都由微小粒子组成。在固体中，粒子紧密地排列在一起，形成规则的排列方式。它们在固定位置上振动，不能自由移动。这就是固体能够保持自身形状和体积的原因。
In a liquid, the particles are still close together, but they can slide past one another. A liquid takes the shape of its container but keeps a fixed volume. In a gas, the particles are far apart and move quickly in all directions. A gas fills any container completely.
在液体中，粒子仍然彼此靠近，但能够相互滑动。液体会呈现容器的形状，但体积保持不变。在气体中，粒子相距很远，并朝各个方向快速运动。气体会完全充满任何容器。
solid → liquid → gas
固体 → 液体 → 气体
2. What Is Melting? | 什么是熔化？
Melting is the change of state from a solid to a liquid. It happens when a solid is heated to its melting point. For example, ice melts at 0 °C under normal atmospheric pressure. During melting, particles gain energy, vibrate faster and break free from their fixed positions.
熔化是指物质从固态变为液态的过程。当固体被加热到其熔点时，就会发生熔化。例如，在标准大气压下，冰在 0 °C 时熔化。在熔化过程中，粒子获得能量，振动加快，并脱离原来的固定位置。
The particles in a liquid are still close together, so the volume does not change greatly during melting. However, the regular arrangement of the solid is lost. Melting is endothermic because it takes in energy from the surroundings.
液体中的粒子仍然彼此靠近，因此熔化时体积不会发生很大变化。但固体的规则排列会被破坏。熔化是吸热过程，因为它从周围环境吸收能量。
H₂O(s) → H₂O(l)
3. What Is Freezing? | 什么是凝固？
Freezing is the change of state from a liquid to a solid. It happens when a liquid is cooled to its freezing point. For water, this is 0 °C. During freezing, particles lose energy, move more slowly and arrange themselves into a regular pattern.
凝固是指物质从液态变为固态的过程。当液体被冷却到其凝固点时，就会发生凝固。对于水来说，凝固点是 0 °C。在凝固过程中，粒子失去能量，运动减慢，并排列成规则的图案。
Freezing releases energy to the surroundings, so it is an exothermic process. The temperature of a pure substance stays constant while it is freezing. The liquid and solid exist together at the freezing point until all the liquid has become solid.
凝固会向周围环境释放能量，因此它是一个放热过程。纯物质在凝固过程中温度保持不变。在凝固点，液体和固体会同时存在，直到所有液体都变为固体。
H₂O(l) → H₂O(s)
4. What Is Subliming? | 什么是升华？
Subliming, also called sublimation, is the change of state from a solid directly to a gas. The substance does not become a liquid first. This happens when particles at the surface of a solid gain enough energy to escape directly into the gas phase.
升华也称为升化，是指物质从固态直接变为气态的过程。物质不会先变成液体。当固体表面的粒子获得足够能量直接逃逸到气相时，就会发生升华。
Sublimation is an endothermic process. A small amount of a solid may sublime slowly at room temperature, but heating usually makes it faster. Common examples include solid carbon dioxide, known as dry ice, and iodine crystals.
升华是一个吸热过程。少量固体在室温下可能会缓慢升华，但加热通常会使升华加快。常见的例子包括固体二氧化碳（干冰）和碘晶体。
CO₂(s) → CO₂(g)
5. Energy Changes During State Changes | 状态变化中的能量变化
State changes need either an input or an output of energy. Melting, boiling and subliming all require energy to break forces between particles. These are endothermic changes. The substance takes energy from the surroundings, which often makes the surroundings cooler.
状态变化需要输入或输出能量。熔化、沸腾和升华都需要能量来破坏粒子之间的作用力。这些变化属于吸热变化。物质从周围环境吸收能量，这通常会使周围环境变冷。
Freezing, condensing and deposition release energy as particles move closer together and form stronger bonds or arrangements. These are exothermic changes. The surroundings gain energy, so they may become warmer.
凝固、凝结和凝华会释放能量，因为粒子彼此靠近并形成更强的作用力或更规则的排列。这些变化属于放热变化。周围环境获得能量，因此可能会变暖。
solid + energy → liquid
liquid → solid + energy
6. Melting Point and Freezing Point | 熔点与凝固点
The melting point is the temperature at which a solid turns into a liquid. The freezing point is the temperature at which a liquid turns into a solid. For a pure substance, the melting point and freezing point are the same temperature.
熔点是固体变成液体的温度。凝固点是液体变成固体的温度。对于纯物质来说，熔点和凝固点是同一个温度。
For example, pure water melts at 0 °C and freezes at 0 °C. Impure substances or mixtures usually do not have one sharp melting point. Instead, they may melt or freeze over a range of temperatures.
例如，纯水在 0 °C 熔化，也在 0 °C 凝固。不纯的物质或混合物通常没有单一的固定熔点，而是可能在一个温度范围内熔化或凝固。
| Substance | 物质 | Melting point | 熔点 | Freezing point | 凝固点 |
|---|---|---|
| Pure water | 纯水 | 0 °C | 0 °C |
| Pure ethanol | 纯乙醇 | -114 °C | -114 °C |
| Pure iron | 纯铁 | 1538 °C | 1538 °C |
7. Heating and Cooling Curves | 加热曲线与冷却曲线
A heating curve shows how temperature changes while a solid is heated. At first the temperature rises. When the substance reaches its melting point, the temperature stops rising for a while. The energy being supplied is used to separate particles, not to increase their temperature.
加热曲线显示固体受热时温度的变化情况。一开始温度会上升。当物质达到熔点时，温度会在一段时间内保持不变。此时供给的能量被用来分离粒子，而不是用来升高温度。
Once all the solid has melted, the temperature starts to rise again. On a cooling curve, the opposite happens. The temperature stops falling at the freezing point while the liquid changes into a solid.
当所有固体都熔化后，温度会再次开始上升。在冷却曲线上则相反。当液体变成固体时，温度会在凝固点停止下降。
Temperature stays constant at the melting point and freezing point.
温度在熔点和凝固点保持恒定。
8. Sublimation in Everyday Life | 生活中的升华现象
Dry ice is solid carbon dioxide. At ordinary room temperature it does not melt into a liquid, but sublimes directly into carbon dioxide gas. This makes it useful for creating fog effects and for keeping items cold during transport.
干冰是固态二氧化碳。在普通室温下，干冰不会熔化成液体，而是直接升华成二氧化碳气体。这使得它常用于制造烟雾效果以及在运输过程中保持物品低温。
Iodine is another solid that sublimes. When grey iodine crystals are warmed gently, they form a purple vapour. Mothballs and some solid air fresheners also become smaller over time because they sublime slowly at room temperature.
碘是另一种会升华的固体。当灰色碘晶体被缓慢加热时，会形成紫色蒸气。樟脑丸和一些固体空气清新剂也会随着时间慢慢变小，因为它们会在室温下缓慢升华。
9. Reversibility and Physical Changes | 可逆性与物理变化
Melting, freezing and subliming are examples of physical changes. No new substance is made. The particles themselves do not change; only their arrangement and movement change. These changes are usually easy to reverse.
熔化、凝固和升华都是物理变化的例子。这些过程中不会生成新物质，粒子本身不会改变，只有粒子的排列和运动方式发生改变。这些变化通常很容易逆转。
For example, water can freeze into ice and then melt back into water. The reverse of subliming is called deposition. In deposition, a gas changes directly into a solid without becoming a liquid first. Frost forming on a cold surface can be an example of deposition.
例如，水可以凝固成冰，然后又熔化成水。升华的逆过程称为凝华。在凝华过程中，气体不经过液态直接变成固态。寒冷表面上霜的形成就可以作为凝华的一个例子。
solid ⇌ liquid ⇌ gas
固体 ⇌ 液体 ⇌ 气体
10. Key Vocabulary Check | 关键词汇检查
Melting: the change from solid to liquid when heat is supplied.
熔化：加热时物质从固态变为液态的过程。
Freezing: the change from liquid to solid when heat is removed.
凝固：移除热量时物质从液态变为固态的过程。
Subliming: the change from solid directly to gas without passing through the liquid state.
升华：物质不经过液态，直接从固态变为气态的过程。
Melting point: the temperature at which a solid becomes a liquid.
熔点：固体变成液体的温度。
Freezing point: the temperature at which a liquid becomes a solid.
凝固点：液体变成固体的温度。
Endothermic: a change that takes in energy from the surroundings.
吸热：从周围环境吸收能量的变化。
Exothermic: a change that releases energy to the surroundings.
放热：向周围环境释放能量的变化。
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