How to find the specific heat of fusion of a substance. Quantity of heat

We have seen that a vessel of ice and water brought into a warm room does not heat up until all the ice has melted. At the same time, water is obtained from ice at the same temperature. At this time, heat flows to the ice-water mixture and, consequently, the internal energy of this mixture increases. From this we must conclude that the internal energy of water at is greater than the internal energy of ice at the same temperature. Since the kinetic energy of molecules, water and ice at is the same, the increment in internal energy during melting is an increment in the potential energy of molecules.

Experience shows that what has been said is true for all crystals. When a crystal melts, it is necessary to continuously increase the internal energy of the system, while the temperature of the crystal and the melt remains unchanged. Usually, an increase in internal energy occurs when a certain amount of heat is transferred to the crystal. The same goal can be achieved by doing work, for example by friction. So, the internal energy of the melt is always greater than the internal energy of the same mass of crystals at the same temperature. This means that the ordered arrangement of particles (in the crystalline state) corresponds to a lower energy than the disordered arrangement (in the melt).

The amount of heat required to transfer a unit mass of a crystal into a melt of the same temperature is called the specific heat of fusion of the crystal. It is expressed in joules per kilogram.

When a substance solidifies, the heat of fusion is released and transferred to surrounding bodies.

Determining the specific heat of fusion of refractory bodies (bodies with a high melting point) is not an easy task. The specific heat of fusion of such a low-melting crystal as ice can be determined using a calorimeter. Having poured into the calorimeter, a certain amount of water of a certain temperature and throwing into it a known mass of ice that has already begun to melt, i.e., having a temperature, we wait until all the ice has melted and the temperature of the water in the calorimeter takes a constant value. Using the law of conservation of energy, we will compose the heat balance equation (§ 209), which allows us to determine the specific heat of ice melting.

Let the mass of water (including the water equivalent of the calorimeter) be equal to the mass of ice - , specific heat of water - , initial water temperature - , final - , specific heat of ice melting - . The heat balance equation has the form

In table. 16 shows the values of the specific heat of fusion of some substances. Noteworthy is the high heat of melting ice. This circumstance is very important, since it slows down the melting of ice in nature. If the specific heat of fusion were much lower, spring floods would be many times stronger. Knowing the specific heat of fusion, we can calculate how much heat is needed to melt any body. If the body is already heated to the melting point, then heat must be expended only to melt it. If it has a temperature below the melting point, then it is necessary to spend heat on heating.

Table 16

Substance		Substance

Everyone knows that water can be found in nature in three states of aggregation - solid, liquid and gaseous. On melting, the transformation solid ice into a liquid, and upon further heating, the liquid evaporates, forming water vapor. What are the conditions for melting, crystallization, evaporation and condensation of water? At what temperature does ice melt or steam form? We will talk about this in this article.

It cannot be said that water vapor and ice are rare in Everyday life. However, the most common is the liquid state - ordinary water. Experts have found that our planet is more than 1 billion cubic kilometers of water. However, no more than 3 million km 3 of water belong to fresh water bodies. A fairly large amount of fresh water "rests" in glaciers (about 30 million cubic kilometers). However, melting the ice of such huge blocks is far from easy. The rest of the water is salty, belonging to the seas of the oceans.

Water surrounds modern man everywhere, during most daily procedures. Many believe that water resources are inexhaustible, and humanity will always be able to use the resources of the Earth's hydrosphere. However, this is not the case. The water resources of our planet are gradually depleted, and in a few hundred years, fresh water on Earth may not remain at all. Therefore, absolutely every person needs to take care of fresh water and save it. After all, even in our time there are states in which water supplies are catastrophically small.

Water properties

Before talking about the melting temperature of ice, it is worth considering the main properties of this unique liquid.

So, water has the following properties:

Lack of color.
Lack of smell.
Lack of taste (but quality drinking water tastes good).
Transparency.
Fluidity.
The ability to dissolve various substances (for example, salts, alkalis, etc.).
Water does not have its own permanent shape and is able to take the shape of the vessel into which it enters.
The ability to be purified by filtration.
Water expands when heated and contracts when cooled.
Water can evaporate to become steam and freeze to form crystalline ice.

This list presents the main properties of water. Now let's figure out what are the features of the solid state of aggregation of this substance, and at what temperature ice melts.

Ice is a solid crystalline substance that has a rather unstable structure. It, like water, is transparent, colorless and odorless. Ice also has properties such as brittleness and slipperiness; it is cold to the touch.

Snow is also frozen water, but has a loose structure and is white in color. It snows every year in most countries of the world.

Both snow and ice are extremely unstable substances. It doesn't take much effort to melt the ice. When does it start melting?

In nature, solid ice exists only at temperatures of 0 °C and below. If the temperature environment rises and becomes more than 0 °C, the ice begins to melt.

At the melting temperature of ice, at 0 ° C, another process occurs - freezing, or crystallization, of liquid water.

This process can be observed by all inhabitants of the temperate continental climate. In winter, when the temperature outside drops below 0 °C, it often snows and does not melt. And the liquid water that was on the streets freezes, turning into solid snow or ice. In the spring, you can see the reverse process. The ambient temperature rises, so the ice and snow melt, forming numerous puddles and mud, which can be considered the only disadvantage of spring warming.

Thus, we can conclude that at what temperature the ice begins to melt, at the same temperature the process of water freezing begins.

Quantity of heat

In a science such as physics, the concept of the amount of heat is often used. This value shows the amount of energy required for heating, melting, crystallization, boiling, evaporation or condensation of various substances. Moreover, each of these processes has its own characteristics. Let's talk about how much heat is required to heat ice under normal conditions.

To heat the ice, you must first melt it. This requires the amount of heat needed to melt the solid. Heat equals the product of the mass of ice and the specific heat of its melting (330-345 thousand Joules / kg) and is expressed in Joules. Suppose we are given 2 kg of solid ice. Thus, in order to melt it, we need: 2 kg * 340 kJ / kg = 680 kJ.

After that, we need to heat the resulting water. The amount of heat for this process will be a little more difficult to calculate. To do this, you need to know the initial and final temperature of the heated water.

So, let's say that we need to heat the water resulting from the melting of ice by 50 ° C. That is, the difference between the initial and final temperatures = 50 °C (initial water temperature - 0 °C). Then you should multiply the temperature difference by the mass of water and its specific heat capacity, which is equal to 4,200 J * kg / ° C. That is, the amount of heat required to heat water = 2 kg * 50 °C * 4,200 J*kg/°C = 420 kJ.

Then we get that for the melting of ice and the subsequent heating of the resulting water, we need: 680,000 J + 420,000 J = 1,100,000 Joules, or 1.1 Megajoules.

Knowing at what temperature ice melts, you can solve many difficult problems in physics or chemistry.

Finally

So, in this article, we learned some facts about water and its two states of aggregation - solid and liquid. Water vapor, however, is an equally interesting object to study. For example, our atmosphere contains approximately 25*10 16 cubic meters water vapor. In addition, unlike freezing, the evaporation of water occurs at any temperature and is accelerated when it is heated or in the presence of wind.

We learned at what temperature ice melts and liquid water freezes. Such facts will always be useful to us in everyday life, since water surrounds us everywhere. It is important to always remember that water, especially fresh water, is a finite resource of the Earth and needs to be treated with care.

In order to melt any substance in the solid state, it is necessary to heat it. And when any body is heated, one curious feature is noted

The peculiarity is this: the temperature of the body rises up to the melting point, and then stops until the entire body passes into a liquid state. After melting, the temperature begins to rise again, if, of course, heating is continued. That is, there is a period of time during which we heat the body, but it does not heat up. Where does the heat energy that we use go? To answer this question, we must look inside the body.

In a solid, the molecules are arranged in a certain order in the form of crystals. They practically do not move, only slightly oscillating in place. In order for a substance to pass into a liquid state, the molecules must be given additional energy so that they can escape from the attraction of neighboring molecules in the crystals. By heating the body, we give the molecules this necessary energy. And until all the molecules receive enough energy and all the crystals are destroyed, the body temperature does not rise. Experiments show that for different substances one mass requires a different amount of heat to completely melt it.

That is, there is a certain value on which depends, how much heat must be absorbed by a substance to melt. And this value is different for different substances. This value in physics is called the specific heat of fusion of a substance. Again, as a result of experiments, the values \u200b\u200bof the specific heat of fusion for various substances were established and collected in special tables from which this information can be gleaned. The specific heat of fusion is denoted by the Greek letter λ (lambda), and the unit of measurement is 1 J / kg.

Specific heat of fusion formula

The specific heat of fusion is found by the formula:

where Q is the amount of heat required to melt a body of mass m.

Again, it is known from experiments that, during solidification, substances emit the same amount of heat that was required to be spent on their melting. Molecules, losing energy, form crystals, being unable to resist the attraction of other molecules. And again, the temperature of the body will not decrease until the moment when the whole body solidifies, and until all the energy that was expended on its melting is released. That is, the specific heat of fusion shows how much energy must be expended to melt a body of mass m, and how much energy will be released during the solidification of this body.

For example, the specific heat of fusion of water in the solid state, that is, the specific heat of fusion of ice is 3.4 * 105 J / kg. These data allow us to calculate how much energy is required to melt ice of any mass. Knowing also the specific heat capacity of ice and water, it is possible to calculate exactly how much energy is required for a particular process, for example, to melt ice with a mass of 2 kg and a temperature of -30 ° C and bring the resulting water to a boil. Such information for various substances is very necessary in industry to calculate the real energy consumption in the production of any goods.

Specific heat of fusion (also: enthalpy of fusion; there is also an equivalent concept of specific heat of crystallization) - the amount of heat that must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid (same the amount of heat released during the crystallization of a substance).
Unit of measurement - J/kg. The heat of fusion is a special case of the heat of a thermodynamic phase transition.

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In physics, melting is the transition of a substance from a solid to a liquid state. classic examples melting processes are the melting of ice and the transformation of a solid piece of tin into liquid solder when heated with a soldering iron. The transfer of a certain amount of heat to the body leads to a change in its state of aggregation.

Why does solid become liquid?

Heating a solid body leads to an increase in the kinetic energy of atoms and molecules, which at normal temperature are clearly located at the nodes crystal lattice, which allows the body to maintain a constant shape and size. When certain critical speeds are reached, atoms and molecules begin to leave their places, bonds are broken, the body begins to lose its shape - it becomes liquid. The melting process does not occur abruptly, but gradually, so that for some time the solid and liquid components (phases) are in equilibrium. Melting refers to endothermic processes, that is, to those that occur with the absorption of heat. The opposite process, when a liquid solidifies, is called crystallization.

Rice. 1. The transition of a solid, crystalline, state of matter into a liquid phase.

It was found that until the end of the melting process, the temperature does not change, although heat is supplied all the time. There is no contradiction here, since the incoming energy during this period of time is spent on breaking the crystalline bonds of the lattice. After the destruction of all bonds, the influx of heat will increase kinetic energy molecules, and consequently, the temperature will begin to rise.

Rice. 2. Graph of body temperature versus heating time.

Determination of specific heat of fusion

The specific heat of fusion (designation - the Greek letter "lambda" - λ), is called physical quantity equal to the amount of heat (in joules) to be transferred solid body weighing 1 kg to completely transfer it to the liquid phase. The formula for the specific heat of fusion is:

$$ λ =(Q \over m)$$

m is the mass of the melting substance;

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

Values for different substances are determined experimentally.

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

In what units is the specific heat of fusion measured?

Specific heat of fusion in SI (International System) is measured in joules per kilogram, J / kg. For some tasks, an off-system unit of measurement is used - kilocalorie per kilogram, kcal / kg. Recall that 1 kcal = 4.1868 J.

Specific heat of fusion of some substances

Information on specific heat values for a particular substance can be found in book references or in electronic versions on Internet resources. They are usually presented in the form of a table:

Specific heat of fusion of substances

One of the most refractory substances is tantalum carbide - TaC. It melts at a temperature of 3990 0 C. TaC coatings are used to protect metal molds in which aluminum parts are cast.

Rice. 3. Metal melting process.

What have we learned?

We learned that the transition from solid to liquid is called melting. Melting occurs by transferring heat to a solid. The specific heat of fusion shows how much heat (energy) is needed for a solid substance weighing 1 kg to convert it into a liquid state.

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