Phase Diagrams © Van Der Sluys, 2004 1 a Phase Diagram Is a Plot Of

Name ______Phase Diagrams

A phase diagram is a plot of temperature versus pressure in which these conditions produce the three phases of matter for a particular substance. Figure 1 represents a phase diagram for water. Generally, heating a solid will cause it to melt to a liquid (at the melting or freezing point). Continued heating (to the boiling point) produces a gas. If you follow the horizontal line beginning at the pressure axis at 1 atm, the substance begins as a solid, passes the normal freezing (or melting) point into a liquid, then boils to a gas. For most substances, at higher pressures, the freezing and boiling points are higher. Once boiling begins the temperature remains fixed at the boiling point until all of the liquid is converted to gas. Thus, at the boiling point liquid and gas coexist in equilibrium. Similarly, solid and liquid coexist in equilibrium at the melting or boiling point. The triple point is the temperature and pressure at which all three phases can exist in equilibrium. Above the critical point, the substance forms a unique material referred to as a supercritical fluid, which is a substance that has properties of both a liquid and gas.

Figure 1. Phase diagram for water.

The diagram for water shows the characteristic negative slope on the solid-liquid equilibrium line, which accounts for why water floats. The hydrogen bonds which form in liquid water make the liquid unusually dense, even more dense than the solid, hence the ice floats. Looking at the graph upward from the temperature axis one notices that increasing the pressure on a solid will change it to a liquid.

Questions. Choose the best response for the following based on the phase diagram for water, Figure 1..

1. The phase of matter when the pressure is 1.0 atm and the temperature is 400 K is

(a) solid (b) liquid (c) gas

2. The phase of matter when the pressure is 0.5 atm and the temperature is 250 K is

(a) solid (b) liquid (c) gas

3. The phase of matter when the pressure is 2.0 atm and the temperature is 50 K is

(a) solid (b) liquid (c) gas

4. The phases of matter that are in equilibrium when the pressure is 1.0 atm and the temperature is 273 K is

(a) solid and liquid (b) liquid and gas (c) solid and gas

5. The phases of matter that are in equilibrium when the pressure is 101.3 kPa and the temperature is 373 K is

(a) solid and liquid (b) liquid and gas (c) solid and gas

6. The triple point for water occurs at

(a) 374 °C and 218 atm (b) 280 K and 0.30 atm (c) 373 K and 1.0 atm

7. What phases of matter are in equilibrium at the triple point?

(a) solid and liquid (b) liquid and gas (c) solid, liquid and gas

8. A supercritical fluid is formed at

(a) the boiling point (b) the triple point (c) the critical point

9. Substances tend to be in the gas phase under conditions of

(a) high pressure and low temperature (b) low pressure and high temperature (c) high pressure and high temperature (d) low pressure and low temperature

10. Substances tend to be solids under conditions of

(a) high pressure and low temperature (b) low pressure and high temperature (c) high pressure and high temperature (d) low pressure and low temperature

Answers

1. c 2. a 3. a 4. a 5. b 6. b 7. c 8. c 9. b 10. a