Dry Ice, solid carbon dioxide, is useful because it is very cold and because, under room pressure, it evaporates directly from the solid to the gaseous state leaving no messy liquid residue. (Evaporation from the solid to the gaseous state is called sublimation. Dry Ice sublimes.)

Purpose

To observe the melting of dry ice from solid to liquid at high pressure.

Materials

Safety goggles

Gloves

BeralÔ pipet, 4 mL with wide stem (3-5 mm bore) (use only a polyethylene pipet)

0.5 lb Dry ice

Scoopula with handle

Towel, hammer, Styrofoam Ò box Scissors

Pliers (with flat surface)

Safety and Disposal

Handle the dry ice with care to avoid contact. Wear gloves. The teacher should powder the dry ice. Wrap a 0.5-1.0 lb piece of dry ice in a towel or canvas cloth. Smash the wrapped ice against a hard surface such as a concrete floor. Place the powdered ice in a Styrofoam or other insulated container. Dispose of excess dry ice by allowing the solid to sublime in a well-ventilated area.

Procedure

1. Use scissors to cut the stem of a wide-bore tip 4-mL transfer pipet (Beral pipet) to a length of 3 cm.

2. Wear eye protection. Protect hands with gloves or towel, or use scoop with handle. Repeatedly squeeze the pipet bulb, place it into powdered dry ice, release quickly to draw in powder, and tilt to trap powder until about 1-2 cm 3 of dry ice power are in bulb.

3. Wear eye protection. Use pliers to squeeze the pipet tip shut. Observe.

4. Once the solid melts, release the seal of the pliers on the bulb. Observe.

5. Wear eye protection. Use pliers to squeeze the pipet tip shut. Observe until the pipet ruptures.

Remarks

Use only a polyethylene pipet. As the dry ice sublimes, it builds up pressure. When the triple point pressure is reached inside the bulb, the solid melts to form liquid carbon dioxide. Releasing the grip of the pliers drops the pressure and cooling due to expansion of the gas is sufficient to refreeze much of the remaining carbon dioxide. Holding the bulb sealed usually leads to an increase of gas pressure that exceeds the strength of the pipet and causes it to rupture. This is usually a gentle process as the polymer in the bulb stretches and distorts rather than shatters. Note: If you find that the pipet ruptures before the triple point is reached, try Tygon TM tubing, clamping a 6-in. piece at both ends.

The observed changes can be interpreted in terms of a phase diagram for carbon dioxide. When solid dry ice is exposed to the atmosphere, the solid sublimes and the pressure never exceeds one atmosphere. When the bulb tip is clamped, the pressure can rise. Adding heat to the clamped system causes the temperature of the solid dry ice to rise. When the temperature reaches about ­56 °C, the pressure is just over 5 atmospheres and the triple point is reached. Under these conditions, the liquid phase of carbon dioxide is stable. Releasing the pressure permits cooling by expansion. The solid reforms, the pressure goes to 1 atmosphere, and the temperature drops back to ­78 °C. Keeping the bulb sealed permits the pressure to rise until the bulb breaks. Room temperature is around the critical temperature of carbon dioxide, 31 °C. A tank of carbon dioxide at room temperature is filled with gas or a mixture of gas and liquid.

Reference

Adapted from Becker, R. (1991). Wet dry ice. Journal of Chemical Education, 68, 782-783.