Because ice is less dense than water. So for this guy, however, the solid is more dense than the liquid, and therefore, this slope is positive. So if you place a solid carbon dioxide into the liquid, it's going to sink straight down. Now, one more effect because of this negative slope is the following Because the slope is negative. If we keep our temperature constant, say, somewhere right here. So if we keep this temperature constant, we see that we can actually make the solid become a liquid by simply increasing our pressure.