Question

a pure solid sample of substance x is put into an evacuated flask. the flask is heated at a steady rate and the temperature recorded as time passes. here is a graph of the results: graph of temperature (°c) vs heat added (kj/mol) shown. use this graph to answer the following questions: what is the melting point of x? \boxed{} ^\circ\text{c} what phase (physical state) of x would you expect to find in the flask after 12 kj/mol of heat has been added? (check all that apply) \square solid \square liquid \square gas

Explanation:

First Question: Melting Point of X

Step1: Identify Melting Region

In a heating curve, the flat segment (constant temperature) during heating corresponds to a phase change. Melting is the phase change from solid to liquid, so the temperature at the first flat segment is the melting point.

Step2: Read Temperature from Graph

Looking at the graph, the first flat segment (where temperature doesn't change as heat is added) occurs at 150 °C. So the melting point is 150 °C.

Second Question: Phase after 12 kJ/mol Heat

Step1: Analyze Heat Added vs Phase Change

After 12 kJ/mol of heat, we check the graph. The first flat segment (melting) starts and ends around certain heat values. At 12 kJ/mol, we are in the region where the substance is transitioning from solid to liquid (since the first flat segment is the melting process, and 12 kJ/mol is within the range of the first flat segment's heat addition? Wait, no—wait, the x - axis is heat added (kJ/mol). Let's see: the first flat part (melting) is from, say, around 5 to 10 kJ/mol? Wait, no, looking at the graph, the first flat segment (where temperature is constant at 150 °C) is the melting. Then after that, temperature rises until the next flat segment (boiling). Wait, the x - axis: when heat added is 12 kJ/mol, we are after the first flat segment (melting) has ended? Wait, no, maybe I misread. Wait, the first flat segment (melting) is at 150 °C, and the heat added for that segment: let's see the x - axis. The first flat part is from, say, heat added of ~5 to ~10 kJ/mol? Then after that, temperature rises to the next flat segment (boiling) at around 200 °C? Wait, no, the graph: the first flat (melting) is at 150 °C, then temperature rises, then the next flat (boiling) at around 200 °C? Wait, no, the y - axis is temperature. Wait, the first flat segment (constant temperature) is at 150 °C (melting: solid to liquid). Then, after that, as heat is added, temperature increases (liquid heating up) until the next flat segment (boiling: liquid to gas) at around 200 °C? Wait, no, the graph shows: first, temperature rises, then a flat at 150 (melting), then temperature rises, then a flat at ~200 (boiling), then temperature rises. Wait, no, the initial part: starts at low temp, rises to 150, stays flat (melting: solid to liquid), then rises to ~200, stays flat (boiling: liquid to gas), then rises. So when heat added is 12 kJ/mol: let's see the x - axis. The first flat (melting) is from, say, heat added of 5 to 10 kJ/mol? Then after 10 kJ/mol, temperature starts rising (liquid phase). Wait, maybe the first flat segment (melting) is at 150 °C, and the heat added for that is from, say, 0 - 10 kJ/mol? No, the graph: the first flat is at 150, then after that, temperature goes up. So when 12 kJ/mol is added, we are in the liquid phase (since melting is complete, and we are heating the liquid before boiling). Wait, no—wait, the first flat is melting (solid to liquid), so during the flat segment, both solid and liquid exist. After the flat segment, it's all liquid. Wait, the x - axis: let's assume the first flat (melting) is from heat added = 5 to 10 kJ/mol (approx). Then, at 12 kJ/mol, we are after the melting flat, so the substance is liquid (since melting is done, and we are heating the liquid). Wait, but maybe I made a mistake. Wait, the first flat is melting: solid + liquid. Then, after the flat, it's liquid. So if 12 kJ/mol is after the melting flat, then the phase is liquid. Wait, but let's check the graph again. The first flat (melting) is at 150 °C. Then, as heat is added beyond the melting region, the temperature rises (liquid heating). So when 12 kJ/mol is added, we are in the liquid phase (because the melting process (solid to liquid) is complete, and we are now heating the liquid). So the phases present after 12 kJ/mol: liquid (and maybe? No, after melting is complete, it's liquid. Wait, no—during melting, solid and liquid coexist. After melting (when the flat segment ends), it's all liquid. So if 12 kJ/mol is after the melting flat, then the phase is liquid.

Answer:

(First Question):
150