What is the relationship between ambient pressure and the melting point of ice?

Humidity meter An instrument used to measure the absolute or relative humidity of air is called a "hygrometer". There are many kinds, such as dry and wet bulb thermometer, hair hygrometer, ventilation hygrometer, self-recording hygrometer, dew point hygrometer and so on.

The dew point makes the water vapor contained in the air saturated, and the temperature at which it condenses is called "dew point". It is one of the ways to express the humidity of the atmosphere. Under the condition that the water vapor does not increase or decrease and the air pressure remains unchanged, the temperature at which the water vapor in the air reaches saturation due to cooling. When the difference between the air temperature and the dew point is smaller, it means that the air is closer to saturation, and the relative humidity of the air is higher. For example, under a certain air pressure, the measured air temperature is 2℃ and the dew point is 12℃. From the table, the saturated vapor pressure at 2℃ is 2328Pa(17.54 mm Hg) and that at 12℃ is 142.3Pa(1.52 mm Hg).

the absolute humidity of air is P=142.3Pa,

the dew point is related to the humidity of the atmosphere. When the relative humidity of the atmosphere is high, the dew point is high, and when the relative humidity is low, the dew point is low. If the dew point is above freezing point, it becomes rain, dew, clouds and fog. If it is below freezing point, frost, snow, hail and so on will be generated.

Dew air condenses into water droplets on the surface of cold objects, which mostly occurs outdoors at night. For example, after dark, plants or rocks will give off heat and cool down. The warm and humid air around it, in contact with the object, reaches saturation state and becomes crystal water, and when it is attached to it, it is dew, which is a liquefaction phenomenon. This phenomenon mostly occurs between summer and autumn, because the temperature difference between day and night is large during this period.

In summer, when the sun is shining in the shower, the water vapor on the ground rises, which is easy to form violent updraft and cumulus clouds. Large water droplets fall, forming a downpour, mixed with thunder, also known as rainstorm.

Fog During the day, the sun shines on the ground, and the ground absorbs and accumulates a lot of heat. At night, heat begins to radiate into the air, which lowers the ground temperature. If it is cooled below the dew point, the water vapor near the ground will reach saturation. These saturated water vapor takes the smoke and dust in the air as the core, and condenses into tiny water droplets, floating in the air, such as white gas, which is fog. The diameter of the droplets is .3mm ~ .4mm.. Fog must be formed under the condition of no wind or extremely weak wind, with condensation nuclei, and the air must be cooled below the fog point. Chongqing in China, Sichuan and London in England are often foggy due to geographical location and environmental factors, so they are called foggy cities.

fog ice is a white opaque small granular ice crystal frozen by fog. In dense fog, when the temperature drops below ℃, the water droplets of fog freeze on the surface of objects or ice. It is not a crystal like frost, but a small granular ice aggregate.

the process that condensed matter changes from gas phase to liquid phase is called condensation, that is, the process of liquefaction. Condense steam into liquid, and give off heat during condensation. If steam condenses alone, droplets, such as fog, often form around the condensation nucleus. If vapor and liquid exist together, condensation usually occurs on the surface of the liquid. See "Liquefaction".

In the process of condensation, condensation nucleus steam usually centers on dust, impurity particles or charged particles in the gas, and starts to condense around them. These condensation particles are called condensation nuclei. If there is no such condensation nucleus in the steam, the steam will not condense, but will become supersaturated steam.

bubble chamber is similar to a cloud chamber, which uses high-pressure superheated liquid to replace the supersaturated steam in the cloud chamber. The liquid used is usually liquid hydrogen or propane. When the liquid is overheated, it will not boil even though the liquid temperature has exceeded the normal boiling point. At this time, if charged particles pass through, the liquid is ionized on the path through which the particles pass. Some small bubbles are generated around these ions, thus showing the tracks of charged particles.

Sublimation is the process in which a solid substance evaporates directly into steam without going through the liquid process. Sublimation is an endothermic process. Generally, under normal temperature and pressure, sublimation will occur on any solid surface. For example, potassium iodide, dry ice, sulfur, phosphorus, camphor and other substances have a very significant sublimation phenomenon. From a microscopic point of view, the process that molecules on the crystal surface break away from the attraction of other molecules and run outside the crystal to become steam molecules is sublimation. When heated below the pressure of the triple point, the solid substance can directly change into the gas phase without passing through the liquid phase. For example, the mothballs gradually become smaller, and clothes that freeze outside in winter will dry, which is the result of sublimation.

sublimation heat is the heat absorbed by a unit mass of a substance during sublimation, which is also equal to the sum of melting heat and vaporization heat of the same substance under the same conditions. Sublimation is actually a phenomenon that the particles in the crystal are directly separated from the crystal lattice structure and transformed into gas molecules. The heat absorbed when one kilogram of substance can be sublimated is called sublimation heat. If r is used to represent the sublimation heat, there is a formula

where m is the mass of the sublimated substance; Q is the heat absorbed during sublimation, and its unit is also Joule/kg.

in the sublimation process, on the one hand, particles must overcome the binding force between particles to do work, on the other hand, they must overcome the external pressure to do work. According to the law of conservation of energy, heat must be absorbed from the outside at this time. Therefore, the sublimation heat is equal to the sum of melting heat and vaporization heat in numerical value. The relationship is

r = λ+l.

dry ice is solid carbon dioxide (CO2), snow white, with a melting point of -78.5℃, which can be directly sublimated from solid to gas. The low temperature of about-8℃ can be obtained when evaporating under normal pressure, and the temperature is even lower when evaporating under reduced pressure. It is mainly used in food industry and as a refrigerant, and can also be used as a chemical agent for artificial rainfall.

the process that a condensed substance changes from a gaseous state to a solid state without passing through a liquid state is called "condensed matter". In this process, the substance releases heat and lowers the temperature. When a unit mass of gaseous substance condenses, the heat released is called sublimation heat. At the same temperature, the sublimation heat of the same substance is equal to the sublimation heat and the sum of its vaporization heat and melting heat under the same conditions. For example, water vapor in the air condenses directly on the surface of an object when it is cold, and becomes frost.

Frost When the temperature drops below ℃, the water vapor in the air condenses into white crystals on the surface of ground objects without liquid, which is called frost. Frost usually appears on a clear and windless night or early morning. Early frost occurs mostly in late autumn, while late frost occurs in early spring. The appearance of frost is generally greatly influenced by local areas, although frost may not be seen everywhere in the same area at the same time. In the frost season, it is often accompanied by frost. Frost is a manifestation of condensation. First frost in the north is generally at the end of October, which is the first frost period. Plants may suffer from freezing injury in the cold and warm transition season because the ambient temperature drops to ℃ or below for a short time. But frost does not necessarily appear when frost appears.

graupel is a white opaque spherical or conical solid precipitation with a diameter of 2 ~ 5 mm larger than that of Michelle. This is caused by supercooled water droplets colliding with ice crystals (or snowflakes) and freezing, which will rebound after landing and be easily broken. Most of the graupel landed in the clouds with a certain convective intensity before the snow fell, and most of them landed in bursts.

Hailstones are spherical, conical or irregular ice cubes with different diameters, ranging from 5 to 5 mm, and some hail with a diameter of about 3 cm. Hail often falls from cumulonimbus clouds with particularly strong updraft. Hail usually rises and falls with the airflow in cumulonimbus clouds for many times, and constantly merges with snowflakes and water droplets along the way to form ice cubes with alternating layers of transparency and opacity. When it increases to a certain extent, the updraft can't support it and falls to the ground, commonly known as hail. Hail is an array, but it is extremely harmful.

Three-phase diagram When a solid sublimes, if the solid and its vapor reach dynamic equilibrium, the vapor at this time is called saturated vapor, and its pressure is called saturated vapor pressure. As shown in Figure 2-12p-T, the curve OS is called sublimation curve. It represents the relationship between the temperature and saturated steam pressure when solid and gas phases exist. Of the two parameters, P and T, as long as any one is determined, the other parameter can be determined, but neither of them can be selected arbitrarily. The P-T diagram shown in Figure 2-12 is a three-phase diagram. It represents the conditions for the existence of solid, liquid and gas phases and their mutual transformation. If solid, liquid and gas exist in equilibrium, then the temperature and pressure are certain, and no parameter can be selected arbitrarily. Therefore, the common * * * intersection o of these three curves represents the existence state of three phases, so it is called the three-phase point. For example, the temperature of the phase point of water is .1℃ (i.e. 273.16 K) and the pressure is 546.84Pa(4.851 mm Hg).

any substance has its unique phase diagram, especially in metallurgical technology, which is an important basis. Master the conditions under which the three-phase diagram can control the phase transition. Because the three-phase existence is an invariable system, and the three-phase point is a state that is not affected by other conditions, the temperature of the three-phase point is a certain temperature. Therefore, the temperature of the triple point is selected as the reference point for setting the temperature scale.

Law of Conservation of Energy In all the processes that take place in nature, energy will neither be destroyed nor created. It can only be transformed from one form to another or from one object to another, while the total amount of energy remains unchanged. This law is called "Law of Transformation and Conservation of Energy". In other words, no matter what happens to any closed system, the total value of its energy remains unchanged. This law includes both qualitative and quantitative aspects. In nature, it determines the variability of energy forms and numerically affirms the conservation of the total energy in nature. A decrease in energy is always accompanied by an increase in energy, and its value is equal when it is decreased and increased. Various forms of motion (mechanical motion, thermal motion, electromagnetic motion, etc.) have corresponding energy, so this law is a summary of human experience in long-term observation and research of natural phenomena.

Thermoengineering is a comprehensive subject that studies the mutual transformation between thermal energy and mechanical energy and how to reasonably apply thermal energy to life and production. It is based on heat transfer and engineering thermodynamics. The main research scope includes the working principle and structure of boilers, steam engines, steam turbines, internal combustion engines, gas turbines and refrigeration equipment. The utilization of thermal energy, solar energy and underground heat of atomic nuclear reactor is also within the scope of thermal engineering research.

short for heat engine. It can continuously convert the heat released by fuel combustion into the internal energy of matter by heat transfer, and then into other forms of energy (such as mechanical energy) by doing work. There are many kinds of them, but their main working principle is to use high temperature and high pressure gas or steam to expand and do work. Such as steam engines, steam turbines, gas turbines, internal combustion engines and jet engines. It is the main source of power needed by industrial and agricultural production, power generation and transportation departments. The sources of heat energy are heat energy released by fuel combustion, atomic energy, solar energy and geothermal energy. The composition of a heat engine must have three components. One is the heater, which is a device to convert the energy released by the fuel into the internal energy of the working medium; The second is the working part, which is a device that makes the working medium consume internal energy to do mechanical work; The third is the condenser, which is a device to accommodate the waste working medium discharged from the working part. When the heat engine works, only a part of the heat obtained by the working medium from the heater is converted into mechanical work, and the rest is transferred to the condenser. The heat obtained by the working medium from the heater is Q1, and a part of Q2 is brought into the condenser by the waste working medium that has done work, and only Q1-Q2 is converted into mechanical work.

working medium heat engines all use the expansion of gas or steam to do work. In technology, gas or steam is often called the working substance of the heat engine, which is called working medium for short. The working medium in steam engines and steam turbines is steam; The working medium of an internal combustion engine is a mixture of gasoline or diesel and air.

boiler, which is a generator of high pressure steam. In the boiler, the chemical energy of fuel is converted into the internal energy of steam. The boiler consists of a fire chamber and a boiler. According to different structures and forms, it can be divided into water tube boilers and flue tube boilers. After water is heated in a water pipe or boiler to become steam, it is sent to the upper part of the boiler by the front water pipe, and the saturated steam in the boiler is sent to the superheater by the steam pipe, and then heated again to become superheated steam. The superheated steam is sent to the cylinder of the steam engine through the gas pipe to push the piston to do work.

The temperature of the flue gas coming out of the fire room is very high, usually around 35℃ to 4℃. Therefore, an economizer is installed in the flue, and water is filled in the economizer, so that the water can be preheated and heated when the flue gas passes through. Injecting such high-temperature water into the boiler can avoid drastic changes in the temperature of the boiler.

Water-tube boilers have large evaporation capacity and large water pipes, steam boilers and fire chambers, which are generally used in fixed positions such as thermal power stations. Smoke tube boilers are used in trains, which are simple in structure and small in size, and are widely used in trains or small factories.

when the pressure of the steam sealed in the boiler by the safety valve exceeds a certain limit, the boiler is in danger of explosion. In order to ensure safe production, all kinds of boilers are equipped with safety valves, which are usually closed. When the steam pressure in the boiler exceeds a certain limit, the steam will open the safety valve and release some steam, so that the pressure in the boiler can be restored to a safe limit and accidents can be avoided. The safety valve is a device which can adjust and control the steam pressure based on the lever principle.

A steam engine uses the circulation of steam to convert thermal energy into mechanical energy. The high-temperature and high-pressure steam is introduced into the cylinder of the steam engine, and the piston in the cylinder is driven to reciprocate by the expansion of the steam. And the reciprocating motion of the piston is converted into the rotation of the flywheel by using the piston rod, crosshead, connecting rod, crank and flywheel. A steam locomotive is a device powered by a steam engine.

Static point When the piston rod, connecting rod and crank of a steam engine are located on the same straight line, the connecting rod can't make the crank rotate. This position is called "static point", and the machine can't run when there is a static point. In order to make the crank continue to rotate at the static point, a heavy flywheel is installed on the machine shaft, and the crank passes through the static point to maintain the continuous rotation of the machine by relying on the inertia of the flywheel rotation. There will be two dead points when the piston reciprocates once. There are also static points called dead points.

after the condenser steam expands in the cylinder to do work, the internal energy has been reduced, and it is often called waste steam or waste working medium. In order to make the steam engine continue to work, it is necessary to discharge the waste working medium from the cylinder and suck in the new working medium. The device for containing waste working fluid is called a condenser. There are different types of condensers according to different needs. The condenser of a train steam engine is the atmosphere. The common condensers are jet type and water tube type. After the waste working medium passes through the condenser, the temperature drops and condenses into water. This part of the water contains few impurities and the water temperature is high. It can be sent to the boiler as water supply after degreasing and other treatments, which can not only save fuel, but also prolong the life of the boiler.

combustion efficiency when the fuel is burned in the heater, it can't be completely burned because the equipment is not perfect, and it is also impossible to convert all the chemical energy released during combustion into the internal energy of the working medium. Suppose that the heat released by the fuel after complete combustion is Q, and the heat transferred to the working medium is only Q1, then the combustion efficiency: < P > Because the combustion process of the fuel is carried out in a boiler, the combustion efficiency is also called boiler efficiency.

The heat Q1 absorbed by the heat efficiency working medium from the heater cannot be completely converted into mechanical work when doing work, and a part of the heat Q2 is always taken out of the working part of the heat engine by the waste working medium. Therefore, the net heat converted into mechanical work is Q1-Q2, and the thermal efficiency of the heat engine: < P > The mechanical efficiency (heat) is converted from heat Q1-Q2.