1. Pure substance A is a solid at room temperature. Upon being heated to about 250°C, it gradually liquefies. Upon being cooled to room temperature, the liquid cannot be induced to solidify.
a. Is substance A probably an element or a compound? Explain
b. Has a chemical change occurred? If so, can it be said that the change is endothermic or exothermic?
c. Can it be said that the resulting liquid is an element, based on the information given?
(a) The substance can be an element or a compound. Both possibilities exist an element or a compound can be heated to melting and in the process, it reacts with either oxygen or nitrogen in the air to form a compound that has the properties described.
(b) For a substance to change its physical properties, a chemical change must have occurred. The change can be thought of to have occurred while heating was being done, and therefore the change occurred by consuming some of the heat. Therefore in that case the change is exothermic.
(c) Based on the information given, the final substance can be a compound. However, there is also a possibility the starting substance was a compound that underwent thermal decomposition while it was being heated. Both possibilities also do exist.