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)..
Then the most appropriate curve is fitted to these experimental points and finally a graph of the temperature T is obtained as a function of time t: T (t).
Article index
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 changes of state, in which the sample increases its internal energy while heat is added, as indicated by the molecular kinetic 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 a certain extent the forces that hold the particles together, so 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. Arrows indicate the direction of the process.
Starting with a sample in solid state, when it melts it goes to the liquid state, when it vaporizes it turns into gas and through ionization it turns into plasma.
The solid may be converted directly to gas by the process known as sublimation. There are substances that easily sublimate at room temperature. The best known is the COtwo or dry ice, as well as naphthalene and iodine.
While the sample undergoes a change of state, the temperature remains constant until the new state is reached. 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. Figure 3 shows one of these curves for a given substance.
In the growth intervals ab, CD Y ef the substance is found as a 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 from, 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 such a case we speak of cooling curve.
The heating curves have the same general appearance for all substances, although of course 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..
In principle, it is very simple: simply place a sample of the substance in a container fitted with a stirrer, insert a thermometer and heat evenly..
Simultaneously, when starting 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 sought to easily find the melting and boiling points.
Consider the heating curves for water and for 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.
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..
When heating the ice sample, according to the graph we are at point A, at a temperature below 0º C. It is observed that the temperature increases at a constant rate until reaching 0º C.
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 force of attraction between the molecules, that is why the temperature between B and C remains constant until all the ice has melted..
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, whose limit is not shown in the graph.
An iron sample can undergo these same changes. However, given the nature of the material, the temperature ranges are very different..
Yet No Comments