Time-cooling curves

Heating curves are graphical representations of the temperature changes of a substance as it changes state from solid to gas, plotted as a function of energy absorbed (or time). Cooling curves show the reverse of this process. 

Hg. 2-12. Time-cooling curve for a mixture, showing supercooling. 

Fig. 2. Time—temperature—transformation (TTT) diagram where A represents the cooling curve necessary to bypass crystallization. The C-shaped curve separates the amorphous soHd region from the crystalline soHd region. Terms are defined ia text.

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Complicated problems of transient heat flow can be resolved by computer. Typical time-temperature curves for non-steady cooling are shown in Figures 16.1 and 16.2, and the subject is met again in Section 26.2. 

On a sheet of graph paper, make a graph of time versus temperature for your after cooling data. You should have four points each for sand and water. This graph is called a cooling curve. Make sure you place the independent variable on the x-axis. 

Using graph paper and your data from Part A, construct a temperature (y-axis) versus time (x-axis) graph for the cooling of the naphthalene. Do not connect the data points. Label this graph Cooling Curve of Pure Naphthalene. ... 

The value of the calorimeter time constant r (= n), may be determined from the cooling curve which is recorded, for instance, when a Joule heating, which produced a constant deviation A0 of the base line (Fig. 11), is suddenly stopped (16). The comparison of Eqs. (14) and (15) shows that the cooling curve is represented by... 

This material shows a long flat at 23.2° cor. in a time-temperature cooling curve. It can be stored in a refrigerator in paraffin-covered stoppered bottles for long periods of time without discoloration. 

P14.3 Draw time against temperature cooling curves showing the temperature halts that occur when liquid mixtures with xz = 0.4, 0.5, and 0.6, are cooled to a temperature where only solid remains for the systems shown in Figures 14.16, 14.23, 14.24, and 14.25. 

PI4.4 Draw time against temperature cooling curves showing the temperature halts when a liquid mixture of (X1H2O + X2CH3CN) with xz = 0.5 is cooled from 273.15 K to 233.15 K at p = 100, 124, 140, and 175 MPa. (See Figure 14.20 for the phase diagram.)... 

Phase diagrams can either be calculated [15] or determined experimentally. On the experimental side, cooling curves have often been used in which a molten mixture at sufficiently high temperature is slowly cooled and its temperature recorded as a function of time. At the transformation temperature, the sample temperature will remain invariant until the transformation is complete. By comparison, cooling traces using DTA/DSC provide greater phase equilibria sensitivity, since signals from only transformation events are detected and amplified. 

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