Solid water (ice) is slowly warmed from a very low temperature. (a) What minimum external pressure p1 must be applied to the solid if a melting phase transition is to be observed? Describe the sequence of phase transitions that occur if the applied pressure p is such that p < p1.

Video transcript

Welcome back everybody. We are taking a look at dry ice which is simply just another name for solid C. 02. And we are tasked with finding some pressure, which I'm going to name, p not uh and its minimum in order to facilitate the melting of dry ice. So we need to find this X. Which will be a minimum value for this pressure to facilitate melting. Well, if we're looking at phase changes it might be good for us to take a look at the P. T. Diagram here. So let me go go ahead and draw that out. We're gonna of course have our positive Y axis be pressure, positive X. Axis, be temperature. We're gonna start at the origin at point A. Now R. P. T diagram kind of swerves up a little bit like this and then it allows us to arrive at point B. And then from there you'll have this go this way and this line actually won't be line, it will be more of a curve kind of curve upward. And what does all of this mean? Well, you can't really make sense of it until you fill in the phases of matter. So over here on the left we're gonna have that. This is the solid area in the middle, we're going to have liquid and in the bottom right here we're going to have gas. So what does all of this mean? Well, you can see that at a fairly low pressure and temperature going on the path from A to B. You are moving between a solid and a gas only. Same thing at higher pressures and moderate temperatures. You're gonna be moving between a solid and a liquid here and then at moderate pressures and high temperatures you're moving between a liquid and a gas Now. What about this point right here? Well, point B. We actually call the triple point because it is between all three of our phases of matter. But what are we looking for? Well, as a reminder, we are looking for melting of dry ice melting involves going from a solid to a liquid and we want the minimum pressure here. Well, in order to reach kind of this boundary, our minimum pressure has to be the pressure located at our triple point. Because above the triple point you're just going between a solid and a lick, meaning that X. Is equal to our pressure at the triple point. Which if we look up for carbon dioxide, we get that. The pressure at the triple point on the P. T diagram is 5.1 80 M. Meaning that R. P not should be greater than or equal to 5. 80 M. Giving us a final answer choice of B. Thank you all so much for watching. Hope this video helped. We will see you all in the next one