Part 1:
a. Is it possible to add heat to a pure substance and not observe a temperature change? If so, provide examples.
b. Describe, on a molecular level, what happens to the heat being added to a substance just before and during melting. Do any of these molecular changes cause a change in temperature?
Part 2: Consider two pure substances with equal molar masses: substance A, having very strong intermolecular attractions, and substance B, having relatively weak intermolecular attractions. Draw two separate heating curves for 0.25-mol samples of substance A and substance B in going from the solid to the vapor state. You decide on the freezing point and boiling point for each substance, keeping in mind the information provided in this problem. Here is some additional information for constructing the curves. In both cases, the rate at which you add heat is the same. Prior to heating, both substances are at –50°C, which is below their freezing points. The heat capacities of A and B are very similar in all states.
a. As you were heating substances A and B, did they melt after equal quantities of heat were added to each substance? Explain how your heating curves support your answer.
b. What were the boiling points you assigned to the substances? Are the boiling points the same? If not, explain how you decided to display them on your curves.
c. According to your heating curves, which substance reached the boiling point first? Justify your answer.
d. Is the quantity of heat added to melt substance A at its melting point the same as the quantity of heat required to convert all of substance A to a gas at its boiling point? Should these quantities be equal? Explain.