1. Based on the provided information (text and video) it is inferable that the compress would be at room temperature and that they have cooled after being twisted, tight and agitated. Thus, it can be concluded that these actions were essential for their cooling, which suggests that the compress would have compartments with different substances that, after their rupture trough the twisting and tightening of the compress, allows the mixing of the components. The drop in the temperature must been caused by an endothermic process, and this should be reported by students.
Instant cold compresses are composed of a packaging filled with water (inner bag) and another with a water-soluble solid substance (outer bag), so that these two components cannot mix. When someone wants to use the compress, it must be twisted and tightened to break the inner bag of water and mix it with the solid. The dissolution of the solid substance in water is an endothermic process, in which energy (as heat) absorption occurs. This results in a decrease in the temperature of the compress and surrounding environment, since the system is not isolated. Thus, after the rupture of the inner bag, containing water, the compress is shaken for a faster dissolution. The most common compound used in cold compresses is ammonium nitrate, a crystalline solid that, when dissolved in water, dissociates into ammonium ions and nitrate ions. In this process, occurs the transfer of energy from water, leading to a decrease in its temperature. This dissolution process is an endothermic process so that, according to the Le Chatelier principle, the temperature increase of the system favors dissolution: the balance between the solid and dissolved solute evolves in order to counteract this disturbance (temperature increase), resulting in an increase of the solubility of the solutes. As an alternative to ammonium nitrate, cold compresses may contain urea or ammonium chloride.
Suggested link: https://www.youtube.com/watch?v=A5q0NUDbGp8
2. Although it is possible to carry out an experimental activity with ammonium nitrate, the use of urea is more recommended due to the toxicity of ammonium nitrate. Students will be able to dissolve urea in water (e.g., 100 g urea in 100 mL of deionized water) and, by measuring the initial temperature and final temperature, verify that it is an endothermic process. Students can also investigate the increased solubility of this solute with the increase in temperature: this activity can be done through the preparation of saturated solutions and consequent dissolution, as a result of temperature increase. Observing the solubility values of this solute in water, at different temperatures, it can be verified that the solubility increases with the increase in temperature, which reinforces the completion of its dissolution in water, as a result of being an endothermic process.
3. Students should record data on the initial water temperature and final temperature of the urea solution in water. In the case of the study of urea solubility in water, students should record the amount of solute added (up to saturation) and the final temperature of the solution (suggested temperatures: 20 ºC, 40 ºC and 60 ºC).
4. The amount of energy in the form of heat involved in the dissolution process can be calculated using the equation: q = c m (T2 -T1), where:
q = transferred energy (as heat) (J)
c = heat capacity of water (J.kg-1.K-1 )
T1 = initial temperature (K)
T2 = final temperature (K)
This equation allows the calculation of the energy that is transferred from water to urea, in order to change the temperature of the solution from an initial T1 to a final T2.
5. Macroscopically, students observe that the dissolution of urea in water is a process that results in a decrease in the temperature of the solution. When they explore the variation of solubility with temperature, students will observe, on a microscopic level, that starting from saturated solutions, with undissolved solute) that it "disappears", that is, that dissolves as the temperature increases. The higher the temperature, the more amount of solute will be dissolved.
At a submicroscopic level students will be able to make the representation of the dissolution of urea which, being a molecular substance rather than an ionic one, does not dissociate in ions. Instead, intermolecular interactions (hydrogen bonds) occur between urea molecules and water molecules. In this way, students should make the representation of this type of interactions.
The symbolic representation will be: (NH2)2CO (s) ⇄ (NH2)2CO (aq)
6. Compresses work by dissolving a compound, usually ammonium nitrate, in water. This process is an endothermic process, so it occurs with a temperature decrease.
7. Just as there are instant cold compresses, there are also hot compresses. Instant hot compresses are composed of a solute (e.g., calcium chloride or magnesium sulfate) that dissolves in water in an exothermic process, causing an increase in the temperature of the vicinity in non-isolated systems. In these cases, the release of energy, in the form of heat, caused by the dissociation of the substances, into the water, makes its temperature to rise.
Suggested link: https://sciencing.com/chemicals-used-heat-packs-7441567.html
