Physics abstract. Solving problems on the topic: “Melting and crystallization” (grade 8)

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In solid crystalline bodies, molecules are arranged in an orderly manner, forming a crystal lattice, the structure of which is reproduced throughout the entire volume - this arrangement of particles is called long-range order. When a body is heated, the kinetic energy of the molecules increases, and when the melting temperature is reached, the lattice structure begins to collapse, the solid body loses its shape - the melting process begins. When cooled, solidification occurs - a transition from the liquid phase to the solid phase.

Why does melting happen?

In the solid state, molecules and atoms are located at lattice sites, performing continuous vibrations near a fixed position. Such vibrations do not disturb the crystal structure. The strength of the lattice is provided by intermolecular bonds. In the process of heating a body, thermal energy is transferred, which is converted into the internal energy of molecules, increasing their speed and vibration frequency. When a certain critical value of temperature Tmelt (melting temperature) is reached, intermolecular bonds are broken, molecules leave their places, which leads to a change in the shape of the body, which begins to transform into a liquid state.

Rice. 1. Examples of the structure of crystal lattices: graphite, diamond, NaCl.

So, melting is the process of transition from a solid to a liquid state.

Melting of amorphous bodies

Amorphous bodies do not have a specific melting point. The structure of amorphous bodies is more like a very viscous liquid than a crystalline solid. When heated, they will become more fluid, increasingly exhibiting the property of a liquid. At the same time, the fragility inherent in the solid state will disappear. Simultaneously with melting, the temperature of amorphous bodies will increase.

Important! Simultaneously with melting, the temperature of amorphous bodies will continuously increase. Because such bodies do not have a specific melting point.

Examples of amorphous bodies

• rosin (coniferous tree resin);
• glass;
• ebonite;
• sealing wax;
• various plastics;

Note: Ebonite (“Ebenos” in ancient Greek - ebony) is vulcanized rubber with the addition of a large amount of sulfur, up to 50% of the mass of the rubber. The color of ebonite is usually dark brown or black. This material does not conduct electrical current - that is, it is a good insulator.

What is hardening

Observations show that if the molten substance is cooled, then when the temperature Tts (solidification temperature) is reached, the reverse process of transition from the liquid to the solid state begins. This phase transition is called solidification or crystallization. It has been experimentally proven that for crystalline solids Tmelt = Tres. “Hot” molecules lose speed when cooled and give off heat to the environment. The internal energy decreases, and particles, under the influence of molecular interaction forces, begin to “occupy” permanent positions, restoring the lattice structure.

The processes of melting and solidification do not occur abruptly, but gradually, so that solid and liquid components can coexist simultaneously. Experiments show that until the melting (or solidification) of the entire mass of a substance is completed, its temperature remains constant.

Metals whose melting point is greater than 16500C are called refractory. For example, the melting point of tungsten is 33700C. Therefore, long-lasting filaments for lamps are made from it. Refractory metals and their alloys are indispensable in rocket science, nuclear energy, metallurgy, space technology - wherever high heat-resistant properties are required.

Melting of crystalline bodies

In order for a crystalline body to begin to melt, it must be heated to a certain temperature. Some crystalline bodies will melt at low temperatures, while others will melt at high temperatures. That is, each substance has its own melting point. It can be found in the physics reference book. At the same time, until the substance melts, its temperature will not change.

Important! Crystalline solids have a specific melting point. Until the crystalline substance is completely melted, its temperature will not change!

Notes:

1. Crystalline substances melt at the same temperature at which they will turn into a solid (crystallize).
2. In order for a liquid substance to begin to crystallize, it must first cool to a certain temperature.
3. The melting point and the crystallization temperature are the same temperature.

Examples of crystalline solids

• ice;
• lead;
• aluminum;
• mercury;
• iron;
• gold;
• silver;

Graphical representation of melting and solidification processes

A graph of the melting and solidification of crystalline solids provides a visual representation of the time dependence of these phase transitions.

Rice. 2. Water-ice melting and solidification graph.

Ordinary water is a good example to illustrate the phenomena discussed. In the presented graph, time t is plotted along the abscissa axis, and temperature is plotted along the ordinate axis. Let initially, at the moment of time t = 0, when the temperature of the ice (crystal) was equal to -400C, the heat supply - heating - will begin. Let us next consider the time dependence of the temperature dependence T(t):

• In section AB, from -400C to 00C (melting temperature of ice), ice exists in crystalline form;
• Section BC - the melting stage occurs, both ice and water are present. The temperature remains constant, equal to 00C;
• CD - at point C melting has ended, there is only a liquid phase - water;
• DE - at point D heating has stopped, cooling occurs up to point E, i.e. up to a temperature of 00C. Only liquid water is present;
• EF - at point E, solidification begins, ice crystals appear, but at the same time there is also a liquid phase. The temperature remains constant, equal to 00C;
• FK - at point F complete solidification has occurred, only ice remains in crystalline form, the temperature of which gradually decreases.

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$ λ ={Q \over m}$ (1)

Where:

m is the mass of the melting substance, kg;

Q is the amount of heat transferred to the substance during melting, J.

The values ​​of λ for different substances are determined experimentally. The dimension λ follows from formula (1):

$ [λ] = { [J]\over [kg] } $ (2)

Knowing λ, we can calculate the amount of heat Q that must be imparted to a body of mass m for its complete melting:

$Q={ λ * m}$ (3)

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Rice. 3. Formation of frost patterns on glass.

Melting point table

Is metal melting physical or chemical?

Is the melting of graphite and metal a physical or chemical phenomenon? Why?

Answers and explanations 1

Physical phenomena, because melting under fire causes deformation. These are physical phenomena.

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