HEATING CURVE: WHAT IS IT, HOW IS IT DONE, EXAMPLES - PHYSICAL - 2025

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

Figure 1. The heating curve of a substance is obtained by adding heat and measuring the temperature every certain interval of time. Source: Pixabay.

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: T (t) is obtained.

What is the heating curve?

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

When adding heat to a sample there are two possibilities:

- The substance increases its temperature, given that its particles are agitated with greater intensity.

- The material is going through a phase change, in which the temperature remains constant. Adding heat has the effect of weakening to a certain extent the forces that hold the particles together, making it 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.

Figure 2. The states of matter and processes necessary to pass between one and the other. Source: Wikimedia Commons.

-State changes in a substance

Starting with a sample in solid state, when it melts it goes into a liquid state, when it vaporizes it turns into a gas and through ionization it turns into 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 CO ₂ or dry ice, as well as naphthalene and iodine.

While the sample undergoes 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, the warming curve is expected to be composed of a combination of increasing sections and horizontal sections, where the latter correspond to phase changes. One of these curves is shown in Figure 3 for a given substance.

Figure 3. A heating curve of a given substance, with the typical configuration based on steps and slopes.

Interpretation of the heating curve

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

While in bc it is changing its state from solid to liquid, therefore the two phases coexist. This happens in the section of, in which the sample changes from liquid to 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 to successively adopt other states. In this case we speak of a cooling curve.

The heating curves have the same general appearance for all substances, although 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 the 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 do you make a warm-up curve?

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

Simultaneously, at the beginning of the procedure, a stopwatch is activated and the corresponding temperature-time pairs are noted from time to time.

The heat source can be a gas burner, with a good heating rate, 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.

They compare the temperature difference between the sample under study and another reference sample with a high melting temperature, almost always an aluminum oxide. With these methods it is easy to 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 0 º C, for iron 1500 º C.

Figure 4. Heating curves for water and iron.

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

Melting the ice

When heating the ice sample, according to the graph we are at point A, at a temperature below 0ºC. It is observed that the temperature increases at a constant rate until reaching 0ºC.

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

The energy that arrives is invested in reducing the attractive force 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, it can continue heating 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

1. Atkins, P. Principles of Chemistry: The Paths of Discovery. Editorial Médica Panamericana. 219-221.
2. Chung, P. Heating curves. Recovered from: chem.libretexts.org.
3. Heating curves. Heat of Fusion and Vaporization. Recovered from: wikipremed.com.
4. Hewitt, Paul. 2012. Conceptual Physical Science. 5th. Ed. Pearson. 174-180.
5. University of Valladolid. Degree in Chemistry, Recovered from: lodging.uva.es.