What is the melting point of sand please?
- Melting temperature
From Wikipedia, the free encyclopedia
Jump to: navigation, searchTemperaturémelting&solidificatiońtemperature is the temperature at which a solid crystalline body makes a transition to a liquid state and vice versa. At the melting point, a substance can be in both a liquid and a solid state. When adding additional heat, the substance will go into a liquid state, and the temperature will not change until all the substance in the system under consideration has melted. When removing excess heat (cooling), the substance will go into a solid state (freeze) and until it solidifies completely, the temperature will not change.
Melting/solidification point and boiling/condensing point are considered important physical properties substances. The solidification temperature coincides with the melting point only for a pure substance. Special thermometer calibrators are based on this property for high temperatures. Since the pour point of a pure substance, such as tin, is stable, it is enough to melt and wait until the melt begins to crystallize. At this time, under the condition of good thermal insulation, the temperature of the solidified ingot does not change and exactly coincides with the reference temperature indicated in the reference books. Mixtures of substances do not have a melting / solidification temperature at all, and make a transition in a certain temperature range (the temperature of the appearance of the liquid phase is called the solidus point, the temperature of complete melting is the liquidus point). Since it is impossible to accurately measure the melting point of such substances, special methods are used (GOST 20287 and ASTM D 97). But some mixtures (of eutectic composition) have a certain melting point, as pure substances.
Amorphous (non-crystalline) substances, as a rule, do not have a clear melting point, the viscosity of such substances decreases with increasing temperature, and the lower the viscosity, the more liquid the material becomes.
For example, ordinary window glass is a supercooled liquid. Over several centuries, it becomes clear that at room temperature the glass on the window slides down under the influence of gravity and becomes thicker at the bottom. At a temperature of 500-600, the same effect can be observed for several days.Since the volume of the body changes insignificantly during melting, the pressure has little effect on the melting point. The dependence of the phase transition temperature (including melting and boiling) on pressure for a one-component system is given by the Clausius-Clapeyron equation. The melting point at normal atmospheric pressure (1013.25 hPa, or 760 mmHg) is called the melting point.
Melting points of some important substances:
sand (melting point (tmelt) = 1710 C), clay (tmelt from 1150 to 1787 C),
melting point C
hydrogen #8722;259.2
oxygen #8722;218.8
nitrogen #8722;210.0
ethyl alcohol #8722;114.5
ammonia #8722;77.7
mercury #8722;38.87
ice (water) +0
benzene +5.53
cesium +28.64
sucrose +185
saccharin +225
tin +231.93
lead +327.5
aluminum +660.1
silver +960.8
gold +1063
iron +1535
platinum +1769.3
corundum +2050
tungsten +3410 - 17131728C if the sand is pure quartz
http://ru.wikipedia.org/wiki/Sand
http://ru.wikipedia.org/wiki/Quartz
fulgurites(eng. Fulgurite) - hollow tubes in the sand, consisting of remelted silica, and melted surfaces on rock outcrops, formed under the action of a lightning discharge. The inner surface is smooth and melted, and the outer surface is formed by grains of sand and foreign inclusions adhering to the melted mass. The diameter of tubular fulgurite is not more than a few centimeters, the length can reach up to several meters; individual finds of fulgurites 5-6 meters long have been noted.
During a lightning discharge, 10 9 -10 10 joules of energy are released. Lightning can heat up the channel through which it travels up to 30,000°C, five times the temperature on the surface of the Sun. The temperature inside the lightning is much higher than the melting temperature of sand (1600-2000°C), but whether the sand melts or not depends on the duration of the lightning, which can range from tens of microseconds to tenths of a second. The amplitude of the lightning current pulse is usually equal to several tens of kiloamperes, but sometimes it can exceed 100 kA. The most powerful lightning and cause the birth of fulgurites - hollow cylinders from melted sand.
The appearance of a glass tube in the sand during a lightning discharge is due to the fact that there is always air and moisture between the grains of sand. The electric current of lightning in a fraction of a second heats up the air and water vapor to enormous temperatures, causing an explosive increase in air pressure between the grains of sand and its expansion. The expanding air forms a cylindrical cavity inside the molten sand, and subsequent rapid cooling fixes fulgurite, a glass tube, in the sand.
Often carefully excavated from the sand, fulgurite is shaped like a tree root or a branch with numerous branches. Such branched fulgurites are formed when a lightning strike hits wet sand, which is known to have a higher electrical conductivity than dry sand. In these cases, the lightning current, entering the soil, immediately begins to spread to the sides, forming a structure similar to the root of a tree, and the resulting fulgurite only repeats this shape. Fulgurite is very brittle, and attempts to remove adhering sand often lead to its destruction. This is especially true for branched fulgurites formed in wet sand.
Fulgurite is sometimes also called the melting of solid rocks, marble, lava, etc. ( petrofulgurites) formed by a lightning strike; such melting is sometimes found in large numbers on the rocky peaks of certain mountains. So, for example, andesite, which forms the top of Small Ararat, is penetrated by numerous fulgurites in the form of green glassy passages, which is why it received the name fulgurite andesite from Abikh.
The longest of the excavated fulgurites went underground to a depth of more than five meters. Fulgurite is also the name given to melting of hard rocks formed by a lightning strike; they are sometimes found in large numbers on the rocky peaks of the mountains. Fulgurites, composed of remelted silica, are usually cone-shaped tubes as thick as a pencil or a finger. Their inner surface is smooth and melted, and the outer surface is formed by grains of sand adhering to the melted mass. The color of fulgurites depends on the mineral impurities in the sandy soil. Most of them are reddish brown, gray or black, but greenish, white or even translucent fulgurites are also found.
“A strong thunderstorm has passed, and the sky above us has already cleared up. I went through the field that separates our house from my sister-in-law's house. I had walked about ten yards down the path when suddenly my daughter Margaret called me. I stopped for about ten seconds and had barely moved on, when suddenly a bright blue line cut through the sky, with the roar of a twelve-inch gun, hitting the path twenty paces in front of me and raising a huge pillar of steam. I went further to see what trace the lightning had left. Where the lightning had struck was a spot of burnt clover five inches in diameter, with a half-inch hole in the middle.... I returned to the laboratory, melted eight pounds of tin, and poured it into the hole... What I dug out, when the tin had solidified, looked like a huge, slightly curved dog's harp, heavy, as it should be, in the handle and gradually converging towards the end. He was a little longer than three feet ”(quoted by W. Seabrook. Robert Wood. - M .: Nauka, 1985, p. 285).
Employees of the Autonomous University of Mexico City have revealed new details about the history of the appearance of the Sahara Desert. According to them, 15 thousand years ago, the Sahara (at least that part of it that is located in southwestern Egypt) was in a temperate climate and could please the eye not with sand dunes, but with a variety of vegetation. For their research, a team of chemists led by Dr. Rafael Navarro-Gonzalez found "frozen" lightning, or fulgurite.
Fulgurites (pictured) are sand caked from a lightning strike. The melting point of sand is about 1700 ° C, relics electric charge enough to melt it. Therefore, hollow branched glass tubes are formed in the thickness. Their inner surface is smooth, but the outer one is rough, because it is formed by grains of sand adhering to the melted mass. In addition, such lightning bolts frozen into the sand also record many other natural inclusions characteristic of a particular stage of geological history.
The fulgurite discovered by Navarro-Gonzalez was different from the usual traces of lightning. Egyptian fulgurite contained small bubbles.
Using a laser, scientists opened the bubbles and found in them a gas mixture of carbon oxides, carbon monoxide and nitrogen oxides. As the chemist noted, these substances could be formed as a result of the oxidation of organic substances during heating.
An analysis of the ratio of carbon isotopes in the compounds showed Navorro-Gonzalez and his colleagues that at the time of the lightning strike, grass, shrubs and other vegetation characteristic of a semi-arid area should have been in the affected area. It is worth noting that now in this area of \u200b\u200bthe Sahara desert, such plants can by no means grow. And scientists decided to calculate the time in order to understand when grass grew on the site of the Sahara.
To establish the date of occurrence of an electric discharge, a member of the research team, geochronologist Shannon Megan from the Geological Research Center in Denver (USA), used the thermoluminescence method - he heated fulgurite to 500 ° C and estimated the energy of electrons “heated” by natural radiation, which was released in the form of light during heat treatment. Its quantity directly indicates the moment of the last heating. In this case, it happened at the moment of a lightning strike, which occurred 15,000 years ago.
The analysis of fulgurite once again confirmed the theory that the Sahara was not so long ago quite a habitable region with a temperate climate.
According to Steve Foreman, a geochronologist at the University of Illinois at Chicago, scientists from Mexico City have demonstrated new approach to the study of the ecological situation of that period and drew the attention of other researchers to the previously unexplored possibilities of fulgurites.
With regard to the representatives' comments Russian science, then, as Sergei Tikhotsky, an employee of the Research Institute of Physics of the Earth of the Russian Academy of Sciences, noted in a conversation with a Gazeta.Ru correspondent, Sergey Tikhotsky, from the point of view of physics, the Navarro-Gonzalez team acted competently: “Everything that has been done by scientists is included in the classical model for determining the composition and the age of matter,” he said. Accordingly, no falsifications and hoaxes can be noted during this isotope analysis - rather, it is quite traditional way research.
Employees of the Institute of Atmospheric Physics of the Russian Academy of Sciences also confirmed to Gazeta.Ru the legitimacy of the theory of the international team of scientists. According to Sergei Demchenko, senior researcher at the Climate Theory Laboratory, 15,000 years ago, vegetation could well have existed in Southwestern Egypt.
Moreover, even during the Holocene period (about 6 thousand years ago), this area could be within the temperate climate zone.
As Demchenko's colleague, PhD Aleksey Eliseev, clarified, vegetation in various areas of the Sahara desert was present in different time, and, for example, on the Arabian Peninsula, vegetation persisted until the era of Alexander the Great.
As for the figure of 15 thousand years, here scientists noted that the end of the last ice age belongs to this time. Which indirectly confirms the theory of Navarro-Gonzalez, so that in general the discovery of Mexican scientists can be classified as verifiable.
Details of the study by Dr. Navarro-Gonzalez's team can be found in the journal of the Geological Society of America.
Apparently, the first description of fulgurites and their connection with lightning strikes was made in 1706 by pastor D. Herman ( David Hermann). Subsequently, many found fulgurites near people struck by lightning. Charles Darwin during world travel on the ship "Beagle" found on sandy shore near Maldonado (Uruguay) there are several glass tubes that go vertically down more than a meter into the sand. He described their size and connected their formation with lightning discharges. Renowned American physicist Robert Wood got an 'autograph' of the lightning bolt that nearly killed him
Each of us has met glass more than once. What is this fragile and transparent material every student knows. We see it every day in mirrors, windows, dishes and furniture, but do we know it well? How is it produced, what is it and what are the properties of glass?
What does this word mean
There are many reference materials which can help in this matter. What is the meaning of the word "glass" according to one of the most popular sources? Ozhegov's dictionary characterizes this substance as solid material, obtained from quartz sand mixed with oxides of certain metals. Even the definition gives some idea of the mode of production this material. But we will come to this topic later.
Surely everyone is used to the fact that glass is a transparent material. But pay attention - Ozhegov's dictionary does not give such a clarification. Glass can be not only transparent, but also colored or frosted. But the composition of the material differs insignificantly.
What is glass made of
The standard composition of glass is a mixture of pure lime and soda. Various additives can be used to change the properties of the material. But still, the main component is precisely pure river sand. Its amount is approximately 75% of the entire mixture. Soda allows you to reduce the sand by almost 2 times. Lime protects the glass from most chemicals and adds strength and shine.
Additional impurities:
- Manganese. It is added to glass to obtain a specific green tint. Nickel or chrome can be used for other colors.
- Lead gives the glass extra shine and a characteristic ringing. The material is colder to the touch. Glass with an admixture of lead is called crystal.
- Oxide boric acid also gives the material additional gloss and transparency, while lowering the coefficient of thermal expansion of products.
History of glass production
Even 6000 years ago, people were able to create this beautiful and fragile material. Of course it appearance somewhat different from modern glass, because in Ancient Egypt and Mesopotamia did not have equipment for high-quality sand cleaning and other tools. Nevertheless, the production of glass began there. Due to impact resistance environment this material gave historians an idea of the culture and technical capabilities ancient peoples.
In Russia, the first glass factory appeared in 1636. It was located near Moscow. Crockery was created there and this branch of industry received great development under Peter I.
Only in 1859 the invention of the pump high pressure made it possible to create glass without the participation of glassblowers. This greatly simplified production. At the beginning of the 19th century, it was discovered interesting property material - if ready product heat up to a certain temperature mechanical properties glasses will rise by 400%.
Modern production
Technology has stepped far forward, which made it possible to create any materials in huge quantities and with least cost human strength. Currently, there are many factories where glass is created using a standard, well-established technology. What modern material, obtained from molten quartzite sand, we learn by familiarizing ourselves with the technology. Let's take sheet metal as an example.
Glass production by stages:
- All the necessary ingredients are loaded into the oven and heated until a liquid homogeneous mass is formed.
- In a special homogenizer, this alloy is mixed until a homogeneous state.
- The resulting mass is poured into a flat container, at the bottom of which is molten tin. There, the glass is distributed, forming a uniform thin layer.
- The cooled and hardened material is sent to the conveyor. There, glass thickness control and cutting are carried out. material, not verified, as well as defective parts are sent for remelting.
- The last quality check is carried out, after which the glass is delivered to the finished product warehouse.
Glass types
Currently, this material is one of the most common. It is not surprising that there are Various types glasses that differ both in appearance and in physical properties. Here are some of them:
- Crystal glass. It is a material containing lead. We talked about it above.
- Contains the purest sand, which makes it different high strength. Able to withstand temperature fluctuations, therefore it is used to create optical instruments, laboratory glassware and windows.
- Foam glass. Lightweight building material that can be used both for decoration and for laying walls and floors. It contains a large number of voids, due to which it has high heat and sound insulation properties.
- Glass wool. Volume air material, consisting of thin and very strong threads. It is fire resistant, therefore it is used not only in construction, but also in tailoring firefighters and welders.
Glass Application
Depending on the properties and appearance, almost any application can be found for this material. The main consumer of glass produced in our time is the construction industry. It uses more than half of the material produced. Its purpose can be the most diverse - wall cladding, window glazing, building walls from hollow bricks, thermal insulation, etc. To construction area can be attributed and What is a Gothic window, everyone knows for sure. Typically, it is made from a large number colored glasses. Nowadays, stained-glass windows have not lost their relevance and are used both in construction and in furniture production.
In second place in popularity are glass vessels for various purposes. Slightly less tableware is produced. It should be noted that glass is an indispensable material in the chemical industry, since it is resistant to most reagents.
Physical properties
Like any other material, glass has a number of qualities that you need to know before using it in a particular area.
- Density. It may vary depending on the composition of the mixture and the method of manufacture. The glass density value can vary from 220 to 650 kg/m 3 .
- Fragility. This feature is distinctive feature glass and limits its use in the construction industry. Currently, scientists are creating more complex alloys that maximize the strength of the material.
- Thermal resistance. Ordinary glass withstands temperatures up to 90 ° C. After processing, the thermal properties of the material increase significantly. For example, industrial glass can withstand temperatures in excess of 200°C.
We learned a lot about glass - what it is, how it is produced and what properties it has. It's time to take a break and get to know the most interesting facts about this very common material. Few people know that:
- The speed of crack movement along is 4828 km/h.
- The decay time of this material is approximately one million years.
- Glass can be remelted repeatedly with virtually no loss of quality. In this regard, he has almost no analogues.
- Being an amorphous material, molten glass will not solidify when cooled rapidly. This requires special conditions.
Glass is not in vain so actively used in construction and other areas of human life. It will surely remain one of the most popular materials. This statement is supported by the strength, durability and relative ease of manufacturing glass, due to the fact that the components for its creation are present on Earth in large quantities.
Majority basis building materials are natural ingredients that have necessary properties and are in sufficient quantity for industrial production. Quartz sand is one of the most common natural minerals and is used in all areas of construction.
What provides the chemical properties of the material
The main component of quartz sand is silicon dioxide (quartz). Its formula is SiO2. It may also contain organic impurities, clay, oxides of iron and a number of other metals. The content of quartz in the original mineral is usually not less than 93-95%.
The principle of operation of building mixtures used to obtain building blocks and slabs is based on the chemical interaction of the components. The resulting inorganic chains provide the required parameters of the material.
Silicon dioxide is an acidic oxide and therefore reacts with calcium and aluminum compounds found in limestone and clay. The interaction can proceed both during the drying of the wet mixture and during thermal baking.
Varieties of quartz sand and its extraction
Separate natural and artificial variety sand, differing in the method of extraction.
natural natural
This type of sand is ubiquitous in nature and is found at the bottom of water basins and in the composition of the soil. Most of its grains are 0.2 to 1 mm in size.
There are several ways to extract quartz sand:
- quarrying- is the main way. If mining is carried out above sea level, then the resulting sand is called mountain sand. Varieties of soil sand are extracted during the development of soil on the plains. Appearance quarry sand characterized by pointed shapes and often a rough surface, which makes it a valuable building material. The extracted sand may be subjected to additional processing- sifting, washing and drying. The more stringent the requirements for sand properties in any industry, the more thorough the preparation required. In the construction of small structures, sand is usually unaffected and supplied directly from the place of its extraction;
- development of water basins– the sand is washed out by the dredger and is characterized by high purity provided by natural washing. Mining is carried out in riverbeds, lakes, as well as marine areas. Sea sand somewhat less valuable due to the higher content of mineral impurities. River sand has a smooth shape - under a microscope, the grains of sand resemble sea pebbles. The use of smooth (rounded) sands is common in self-leveling mixtures - sand grains do not cling to each other.
This is how natural quartz sand looks like in the photo
artificial sand
Despite the name, the mineral has a natural origin, but initially it is in the form of large crystals. To turn quartz crystals into sand, mechanical action (explosion) is used, after which the fragments are crushed.
Ways to classify quartz sand
The starting point in the classification system is the properties of the material and how it is prepared. There are the following areas of classification of quartz sand:
By size (fractional composition)
The numerical expression is the average value of particle sizes or their size range (fraction):
- pulverized quartz - represents a fraction of less than 0.1 mm (sifted into a sieve with a pore diameter of 0.1 mm) and is usually found when crushing quartz crystals;
- fine-grained sand - a fraction of 0.1-0.25 mm;
- medium sand - fraction 0.25-0.5 mm;
- coarse sand - a fraction of 0.5-1 (rarely up to 3) mm.
For enrichment
Quartz sand is divided into unenriched and enriched sand:
- raw sand is the original mineral that has not been treated to increase the silica content;
- enriched sand contains a few percent increased quartz content, obtained by removing most of the impurities. So, white quartz sand is cleaned from organic compounds, iron oxides and clay impurities by screening, washing and drying.
Due to the nature of production, the main specifications received material. This, in turn, affects the possibility of further .
enrichment technology
High purity of the quartz mixture is a necessary requirement in a number of technological processes. The initial enrichment stage includes fractionation and washing - with their help, the coarsest impurities are removed.
The next step is to use special technologies, such as:
- gravity enrichment- the main method, the essence of which is to separate the components of the mixture by density. The lighter particles are carried away by the water flow, while the heavier particles settle to the bottom of the apparatus. The gravitational effect can be enhanced by centrifugation or the addition of chemicals that change the wettability of the sand components;
- electrical and magnetic separation- is an impact electric shock and magnetic field leading to the separation of some impurities. So, the magnetic effect is especially effective when cleaning from iron particles with magnetic properties.
The parameters of the enriched sand fundamentally affect the quality of the work performed. Sand mixtures with the best properties are produced only by certified enterprises using standard technologies.
By color
It is natural and dyed. Natural quartz sand is pale yellow to brownish yellow in color. Artificial dyeing is carried out with stable paints based on synthetic binders, allowing you to create original multi-color ornaments from sand. Such sand can be colored and white.
According to the degree of preparation
Depending on technological requirements, sand can be produced in the following varieties:
- fractionated- represents a specific fraction of sand, the size of which is limited by technical regulations;
- dry- air-dried. Together with fractionated sand, it can be used as a working medium for sandblasting machines;
- calcined sand- completely dehydrated by calcination. Heating significantly above 100 °C ensures the desorption of moisture even from the deep pores of quartz. Such sand is used in finished building mixtures stored long time- even a slight moisture content can render the entire mixture unusable;
- rounded quartz sand- has less abrasive properties, therefore it is suitable for delicate applications, for example, sandboxes on playgrounds;
- molding quartz sand- used to produce cast quartz products and is characterized by a high degree of enrichment.
Production and extraction of quartz sand
On the territory of Russia there are a significant number of large deposits of quartz sand. The most famous are the Chulkovskoe (Moscow region), Kozlovskoe (Bryansk region), Elshanskoe (Volgograd region), Berezichskoe (Kaluga region) deposits and a number of others.
Differences of quartz construction sand mined from these places lie in the initially high quality parameters and greater cost. It is important to understand that the properties of sand from the nearest quarry will be quite sufficient for the construction of small summer cottages so don't overpay. If the goal is to build a large mansion, then saving on the quality of sand can negatively affect the durability of the house.
This is how quartz sand is mined on a special production line:
What are the characteristics of sand
The main regulatory document is GOST 2138-91, there are also others regulations (GOST 22551 77, GOST 51641 2000, 8736 93). They reflect the requirements for the main quality parameters and properties, namely:
- the content of the clay component. Allocate 5 groups with established amounts of clay from 0.2 to 2.0%;
- silicon dioxide content - from 99% to 93%, corresponding to groups from K1 to K5;
- coefficient of uniformity, reflecting the variation in particle size relative to the average (in %). The larger the value, the more uniform sand mix. In total, there are five groups (from O1 to O5), differing in the coefficient of uniformity (from 80 to 50%);
- fraction composition. This parameter reflects the average particle size of quartz sand: up to 0.14 mm; 0.14-0.18 mm, 0.19-0.23 mm, 0.24-0.28 mm, more than 0.28 mm;
- humidity. Dry sands contain no more than 0.5% moisture, wet - no more than 4.0%, wet - no more than 6.0%;
- in the composition of the sand, the content of metal oxides, the surface area of the grains, their shape, gas permeability, as well as the weight loss upon ignition are also normalized.
Quality sand must have a certificate of compliance with the specified standards.
Operational properties of quartz sand
Material parameters that affect the quality of work and determine the scope of application include:
- bulk density - is about 1300-1500 kg / m3;
- true density - is in the range of 2600-2700 kg / m3. The value of true density is used in calculating the volume of cement or concrete mortar obtained by mixing the components;
- the thermal conductivity of quartz sand is about 0.30 W / (m? ° C). The shape and dimensions of sand granules have a significant effect on the heat-insulating properties - the denser their arrangement and the smaller the gaps, the higher the thermal conductivity coefficient;
- melting temperature - maximum working temperature quartz sand is estimated at 1050 ° C, which is quite enough for any construction works. When casting quartz products, temperatures of 1700 °C and above are used.
- ordinary quartz sand in a loose state has volume weight 1,500 kg/m3 and volumetric weight is 1,600 kg/m3.
Advantages and disadvantages of the material - general assessment
Quartz sand is an almost indispensable component in many applications, and technologies with its participation have been worked out to perfection. For practicality, the material receives a "5".
The appearance of the sand is familiar from childhood, and sand playgrounds are often associated with the beach and relaxation - we also put a solid "5" for the appearance.
Despite the massive use of sand, its fine dust can lead to chronic diseases at the builders. For environmental friendliness, the material receives a "4".
The cost of sand is comparable to the cost of other building materials. Lacking special advantages in price, quartz sand deserves a rating of "4".
The approximate cost of various fractions of quartz sand is shown in the table:
| Name | Cost, rub |
| Quartz sand VS-050-1 | 3000 |
| Quartz sand fr.0.1-0.63 | 3200 |
| Quartz sand fr.0.5-0.8 | 3750 |
| Quartz sand fr.0.5-1.0 | 3750 |
| Quartz sand fr.0.8-1.4 | 3950 |
| Quartz sand fr.0.8-2.0 | 3950 |
| Quartz sand fr.1,2-3,0 | 3950 |
Along with clay and limestone, quartz sand is one of the most important and necessary components for production and everyday life. Variety of material properties provides wide range applications. Availability sand pits near the construction site significantly reduces the cost of building a house.
The melting and solidification temperature is the temperature at which a solid crystalline body makes a transition to a liquid state and vice versa. At the melting point, a substance can be in both a liquid and a solid state. When additional heat is added, the substance will go into a liquid state, and the temperature will not change until all the substance in the system under consideration has melted. When removing excess heat (cooling), the substance will go into a solid state (harden) and until it solidifies completely, the temperature will not change.
The melting/solidification point and the boiling/condensing point are considered to be important physical properties of a substance. The solidification temperature coincides with the melting point only for a pure substance. Special thermometer calibrators for high temperatures are based on this property. Since the pour point of a pure substance, such as tin, is stable, it is enough to melt and wait until the melt begins to crystallize. At this time, under the condition of good thermal insulation, the temperature of the solidified ingot does not change and exactly coincides with the reference temperature indicated in the reference books. Mixtures of substances do not have a melting / solidification temperature at all, and make a transition in a certain temperature range (the temperature of the appearance of the liquid phase is called the solidus point, the temperature of complete melting is the liquidus point). Since it is impossible to accurately measure the melting point of such substances, special methods are used (GOST 20287 and ASTM D 97). But some mixtures (of eutectic composition) have a certain melting point, as pure substances.
Amorphous (non-crystalline) substances, as a rule, do not have a clear melting point, the viscosity of such substances decreases with increasing temperature, and the lower the viscosity, the more liquid the material becomes.
For example, ordinary window glass is a supercooled liquid. Over several centuries, it becomes clear that at room temperature the glass on the window slides down under the influence of gravity and becomes thicker at the bottom. At a temperature of 500-600, the same effect can be observed for several days.
Since the volume of the body changes insignificantly during melting, the pressure has little effect on the melting point. The dependence of the phase transition temperature (including melting and boiling) on pressure for a one-component system is given by the Clausius-Clapeyron equation. The melting point at normal atmospheric pressure (1013.25 hPa, or 760 mmHg) is called the melting point.
Melting points of some important substances:
Sand (melting point (tmelt) = 1710 °C), clay (tmelt from 1150 to 1787 °C),
melting point °C
hydrogen −259.2
oxygen −218.8
nitrogen −210.0
ethyl alcohol −114.5
ammonia −77.7
mercury −38.87
ice (water) +0
benzene +5.53
cesium +28.64
sucrose +185
saccharin +225
tin +231.93
lead +327.5
aluminum +660.1
silver +960.8
gold +1063
iron +1535
platinum +1769.3
corundum +2050
tungsten +3410