Melting facts for kids
Melting is a fascinating process where a solid substance turns into a liquid. Think about an ice cube turning into water on a warm day, or chocolate softening in your hand. That's melting in action. It's a type of phase transition, which is just a fancy way of saying a substance is changing its physical form or state.
This change happens when the tiny particles (like atoms or molecules) that make up the solid get enough energy to break free from their fixed positions and start moving around more freely, like they do in a liquid. The term "melting" is also used in other fields, like physics and genetics, to describe similar processes of breaking down structures.
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From solid to liquid
Melting, also known as fusion, is a fundamental physical process. It's the journey a substance takes from being a rigid solid to a flowing liquid.
Imagine the particles in a solid. They are usually arranged in a very neat, organized pattern, like bricks in a wall. They vibrate a little, but they stay in their spots.
When you add energy to a solid, usually by heating it up, these particles start vibrating faster and faster. This added energy is called internal energy. As the internal energy increases, the substance's temperature goes up.
There comes a point where the particles have enough energy to overcome the forces holding them in their fixed positions. This special temperature is called the melting point. At the melting point, the organized structure of the solid starts to break down. The particles gain enough freedom to slide past each other, and the solid transforms into a liquid.
What happens at the melting point?
At the exact temperature of the melting point, something interesting happens from a scientific point of view. Even though you are still adding heat, the temperature of the substance stops rising for a little while. All the energy being added is being used to break the bonds holding the solid structure together, rather than making the particles move faster (which would increase the temperature).
This energy needed to change a solid into a liquid at its melting point is called the enthalpy of fusion, or sometimes the latent heat of fusion. "Latent" means hidden, because the heat is being absorbed without causing a temperature change.
Once all the solid has turned into a liquid, adding more heat will then cause the temperature of the liquid to rise.
Liquids after melting
Once a substance has melted into a liquid, its properties can change further as you continue to heat it. One important property is viscosity. Viscosity is a measure of how "thick" or resistant to flow a liquid is. Honey is more viscous than water, for example.
Generally, as you heat a liquid, its viscosity decreases. The particles move faster and are less likely to stick together, making the liquid flow more easily.
However, science is full of exceptions. When sulfur melts and is heated between 130°C and 190°C, its viscosity actually increases. This happens because the sulfur atoms start linking up into long chains, a process called polymerization, which makes the liquid thicker.
Some complex substances, especially certain organic compounds, don't just go straight from a solid to a simple liquid. They might pass through intermediate states called mesophases. These states have some properties of both solids (like some order in how particles are arranged) and liquids (like being able to flow). Liquid crystals, used in many electronic displays, are a famous example of a mesophase.
Melting as a "First-Order Phase Transition"
Scientists classify phase transitions based on how certain properties change. Melting is considered a first-order phase transition.
This classification comes from looking at things like Gibbs free energy, enthalpy, and entropy. These are concepts from thermodynamics, which is the study of energy and how it relates to matter.
- Gibbs Free Energy: Think of this as a measure of the total useful energy in a system. At the melting point, the Gibbs free energy of the solid and the liquid forms of the substance are equal.
- Enthalpy: As we discussed, this is related to the heat absorbed or released during a process. Melting absorbs heat (enthalpy of fusion is positive).
- Entropy: This is a measure of disorder or randomness. Liquids are more disordered than solids, so entropy increases during melting.
In a first-order phase transition like melting, the enthalpy and entropy change suddenly at the transition point (the melting point), even though the temperature and pressure stay constant.
The temperature at which melting occurs can be affected by the surrounding pressure. For most substances, higher pressure slightly increases the melting point.
An interesting exception is low-temperature helium (Helium-3 and Helium-4). At very, very low temperatures, these substances have a negative enthalpy of fusion. This means you actually have to remove heat to make them melt at certain pressures! This is very unusual and shows how different substances can behave under extreme conditions.
Why do things melt?
Scientists have developed theories to explain why the solid structure breaks down at the melting point. Two well-known ideas are the Lindemann criterion and the Born criterion.
- The Lindemann Criterion: This idea focuses on the vibrations of the particles in the solid. Even in a solid, particles are always vibrating around their fixed positions. The Lindemann criterion suggests that melting happens when these vibrations become so large that the average distance the particles vibrate from their spots is a significant fraction (about 20-25%) of the distance between particles. It's like the vibrations become so strong they shake the structure apart. This idea works well for many crystalline materials (solids with a very ordered structure).
- The Born Criterion: This idea looks at the rigidity of the solid. Solids are rigid because they can resist forces that try to change their shape (they have a high shear modulus). The Born criterion suggests that melting occurs when the solid loses its rigidity – specifically, when its elastic shear modulus drops to zero. At this point, the solid can no longer mechanically hold itself together and becomes a liquid.
These criteria provide different ways of thinking about the fundamental reasons behind the solid-to-liquid transition.
Supercooling and superheating: bending the rules
Under normal conditions, a substance melts exactly at its melting point and freezes exactly at its freezing point (which is usually the same temperature). However, under very specific and carefully controlled conditions, you can sometimes trick a substance into staying in one state even when its temperature is past the transition point.
- Supercooling: This happens when a liquid is cooled below its freezing point but doesn't turn into a solid. For example, very pure water in a very clean container can sometimes be cooled several degrees below 0°C without freezing. The liquid is in a metastable state, meaning it's not the most stable form at that temperature, but it needs a little "nudge" to change. This nudge is often called nucleation, which is the start of the formation of the new phase (like a tiny ice crystal forming). If there are no impurities or rough surfaces for crystals to start forming on, and no disturbances like shaking, supercooling can occur. However, the supercooled liquid is unstable and will usually freeze suddenly if disturbed or if nucleation finally happens.
- Superheating: This is the opposite – a solid is heated above its melting point but doesn't melt. This is less common than supercooling but can also happen under special conditions.
These phenomena show that while the melting point is a characteristic property, the actual transition can sometimes be delayed if the conditions are just right.
Melting and glasses
Not all solids have a neat, ordered crystalline structure like ice or salt. Some solids are amorphous, meaning their particles are arranged more randomly, like in a liquid, but they are stuck in place, like in a solid. Glasses are a common example of amorphous solids (think of window glass).
Glasses are usually made by cooling a molten material very quickly. The particles don't have enough time to arrange themselves into a regular crystal lattice before they get stuck in a solid-like state.
Amorphous solids don't have a single, sharp melting point like crystalline solids. Instead, they gradually become softer as they are heated, passing through a range of temperatures where they are rubbery or plastic-like. This gradual transition happens around a temperature called the glass transition temperature (Tg).
Melting, whether of crystalline or amorphous solids, can also be thought of in terms of breaking the connections between the particles. In amorphous materials, melting (or softening around Tg) happens when enough connections between particles are broken that they can start moving past each other, forming a network of broken connections that allows flow.
Pre-melting
Even below a substance's main melting point, interesting things can happen on its surface. Sometimes, a very thin layer of liquid, called a quasi-liquid film, can form on the surface of a crystalline solid. This is called pre-melting.
The thickness of this thin liquid film depends on the temperature – it gets thicker as the temperature gets closer to the melting point.
Pre-melting is thought to play a role in several natural phenomena:
- Frost heave: When water in soil freezes, the formation of ice crystals can push the soil upwards. Pre-melting might be involved in how water moves through the soil towards the growing ice.
- Growth of snowflakes: The intricate shapes of snowflakes might be influenced by pre-melting on the surfaces of the ice crystals as they grow.
- Movement of glaciers: The ability of glaciers to slide might be partly due to a thin layer of water or quasi-liquid film at the bottom, possibly related to pre-melting or melting caused by pressure.
Other types of "melting"
The word "melting" is also used in other scientific contexts to describe processes that involve breaking apart structures, even if they don't involve turning a solid into a liquid in the traditional sense.
In genetics, "melting" DNA refers to separating the two strands of the double helix. DNA is a double-stranded molecule, and the two strands are held together by weak bonds. By heating DNA or using certain chemicals, these bonds can be broken, causing the two strands to separate. This process is crucial in techniques like the polymerase chain reaction (PCR), which is used to make many copies of a specific DNA sequence.
Related pages
See also
In Spanish: Fusión (cambio de estado) para niños
