## 1 Introduction  
Thermal behavior, e.g., the melting point and the thermal stability of materials, is of great importance and is involved in many applications, including material processing, food and drug preparation, etc. Recently, it has been reported that various substances, such as polymers [1], flavonoids [2] and gallic acid [3], upon heating, do not exhibit the typical thermophysical transition of melting, and instead, they decompose. In many cases, the decomposition and \"melting\" (material softening or fluidization) overlap and occur simultaneously. The term \"thermochemical transition\" was proposed for this effect [1]. In some cases, e.g., gallic acid [3], in addition to the solid-liquid thermochemical transition at temperatures above 200 \({}^{\circ}\)C, a solid-solid thermochemical transition was observed to occur at temperatures below 100 \({}^{\circ}\)C. This behavior was explained based on the comparison of the Gibbs free energies of decomposition and melting and was put in the broader frame of \"melting inability\" [4]. In addition to the term melting inability, the term \"desolvation inability\" was introduced in order to describe the occurrence of decomposition of a solute prior to being able to accomplish the solvent's full removal upon the heating of a solute/hydrate [5]. An explanation, similar to that for melting inability, has been provided for the desolvation inability; that is, the Gibbs free energy of decomposition is lower than the one of -desolvation/dehydration [5]. The desolvation inability is the actual cause of the above-mentioned solid-solid thermochemical transition of gallic acid below 100 \({}^{\circ}\)C. In substances like flavonoids and gallic acid, which exhibit antioxidant properties, it would be interesting to examine if and how the antioxidant activity is influenced by the minor decomposition that occurs during the solid-solid thermochemical transition. For substances with melting and/or desolvation inability, a confusion regarding their thermal behavior has been recognized. For example, the solid-solid thermochemical transition of gallic acid has been interpreted as water evaporation or solid-solid crystallographic (thermophysical) transformation [3]. Also, some confusion was reported for its infrared (IR) absorbance bands [3]. Of course, the knowledge of the IR bands for a given a substance is very useful on many occasions. The primary decomposition pathway for gallic acid was found to be an esterification reaction. Also, it has been reported that the thermal behavior of cellulose esters is governed to great extent by the esterification reaction between the polymer and its acid impurities, which can be produced by the hydrolysis of the polymer [1]. The easiness of the occurrence of the esterification reaction was reported to be related to the low value of the enthalpy of esterification [1]. As mentioned above, a thermodynamic explanation has been reported for the melting inability, and also this behavior has been predicted for various organic substances including various hydroxyl-acids, e.g., citric acid [4]. Citric acid is a naturally occurring hydroxyl-acid that is widely used as an antioxidant, a preservative, a stabilizer, etc., in the Food [6; 7] and Pharmaceutical [8; 9] industry. Citric acid is believed to exist in anhydrous and monohydrate forms. Confusion regarding the thermal behavior of citric acid can be recognized in the literature. Although it has been reported that the melting and decomposition of anhydrous citric acid occur at very close, practically overlapping, temperature ranges [10], in various cases [11; 12; 13; 14], including the NIST Chemistry Webbook [12] and the Handbook of Chemistry and Physics [11], typical melting points are reported in the range of 150-160 \({}^{\circ}\)C. In the Handbook of Chemistry and Physics, a melting point of 135 \({}^{\circ}\)C is reported for citric acid monohydrate and a melting point of 153 \({}^{\circ}\)C is reported for anhydrous citric acid [11].