7 junio, 2024

Heating curve: what it is, how it is done, examples

A heating curve It is the graphical representation of how the temperature of a sample varies as a function of time, keeping the pressure constant and adding heat uniformly, that is, at a constant rate.

To build a graph of this type, pairs of temperature and time values ​​are taken, which are later graphed by placing the temperature on the vertical axis (ordinate) and the time on the horizontal axis (abscissa).

Then the most appropriate curve is fitted to these experimental points and finally a graph of the temperature T as a function of time t is obtained: T

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What is the heating curve?

As a substance is heated, it goes through different states successively: from being a solid it can go to a vapor, almost always going through a liquid state. These processes are called changes of state, in which the sample increases its internal energy while heat is added, as indicated by the kinetic molecular theory.

When adding heat to a sample there are two possibilities:

– The substance increases its temperature, since its particles are agitated with greater intensity.

– The material is undergoing a phase change, in which the temperature remains constant. Adding heat has the effect of weakening to some extent the forces that hold the particles together, which is why it is easy to go from ice to liquid water, for example.

Figure 2 shows the four states of matter: solid, liquid, gas and plasma, and the names of the processes that allow the transition between them. The arrows indicate the direction of the process.

-Changes of state in a substance

Starting with a sample in a solid state, when it melts it becomes a liquid state, when it vaporizes it becomes a gas and through ionization it becomes plasma.

The solid may be converted directly to a gas by a process known as sublimation. There are substances that easily sublimate at room temperature. The best known is CO2 or dry ice, as well as naphthalene and iodine.

While the sample goes through a change of state, the temperature remains constant until it reaches the new state. This means that if, for example, you have a portion of liquid water that has reached its boiling point, its temperature remains constant until all the water has turned into steam.

For this reason, it is expected that the heating curve is composed of a combination of increasing sections and horizontal sections, where the latter correspond to phase changes. Figure 3 shows one of these curves for a given substance.

Interpretation of the heating curve

In the intervals of growth ab, CD and eff the substance is found as solid, liquid and gas respectively. In these regions the kinetic energy increases and with it the temperature.

Meanwhile in bc it is changing its state from solid to liquid, therefore the two phases coexist. This is what happens in the section of, in which the sample changes from a liquid to a gas. Here the potential energy is changing, and the temperature remains constant.

The reverse procedure is also possible, that is, the sample can be cooled so that it successively assumes other states. In such a case, one speaks of cooling curve.

Heating curves have the same general appearance for all substances, though certainly not the same numerical values. Some substances take longer than others to change state, and they melt and vaporize at different temperatures.

These points are known respectively as melting point and boiling point, and are characteristics of each substance.

That is why heating curves are very useful, since they indicate the numerical value of these temperatures for millions of substances that exist as solids and liquids in the range of temperatures considered normal and at atmospheric pressure.

How is a heating curve made?

In principle it is very simple: just place a sample of substance in a container fitted with a stirrer, insert a thermometer and heat evenly.

Simultaneously, when starting the procedure, a stopwatch is activated and from time to time the corresponding temperature-time pairs are recorded.

The heat source can be a gas burner, with good heating speed, or an electrical resistance that emits heat when heated, which can be connected to a variable source to achieve different powers.

For greater precision there are two techniques widely used in the chemistry laboratory:

– Differential thermal analysis.

– Differential scanning calorimetry.

In them, the temperature difference between the sample under study and another reference sample with a high melting temperature, almost always an aluminum oxide, is compared. With these methods it is sought to easily find the melting and boiling points.

Examples (water, iron…)

Consider the heating curves for water and iron shown in the figure. The time scale is not shown, however it is immediate to distinguish the melting temperatures for both substances that correspond to point B of each graph: for water 0ºC, for iron 1500ºC.

Water is a universal substance, and the range of temperatures needed to see its changes of state is easy to achieve in the laboratory. Rather higher temperatures are required for iron, but as indicated above, the shape of the graph does not change substantially.

melting the ice

When heating the ice sample, according to the graph we find ourselves at point A, at a temperature below 0º C. It is observed that the temperature increases at a constant rate until it reaches 0º C.

The water molecules inside the ice vibrate with greater amplitude. Once the melting temperature is reached (point B), the molecules can already move in front of each other.

The energy that arrives is invested in decreasing the force of attraction between the molecules, so the temperature between B and C remains constant until all the ice has melted.

turning water into steam

Once the water is completely in a liquid state, the vibration of the molecules increases again and the temperature increases rapidly between C and D up to the boiling point of 100º C. Between D and E the temperature remains at that value while The energy that arrives ensures that all the water in the container evaporates.

If all the water vapor can be contained in a container, heating can continue from point E to point F, the limit of which is not shown on the graph.

An iron sample can go through these same changes. However, given the nature of the material, the temperature ranges are very different.

References

Atkins, P. Principles of Chemistry: The Pathways of Discovery. Panamerican Medical Editorial. 219-221. Chung, P. Heating curves. Retrieved from: chem.libretexts.org. Heating curves. Heat of Fusion and Vaporization. Retrieved from: wikipremed.com.
Hewitt, Paul. 2012. Conceptual Physical Science. 5th. Ed. Pearson. 174-180.
University of Valladolid. Degree in Chemistry, Retrieved from: alojamientos.uva.es.

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