Quick Lab
Observing Phase Changes
Materials
250-mL Erlenmeyer flask, graduated cylinder, thermometer, dry ice
Procedure 
Pour 150 milliliters of water into a 250-mL Erlenmeyer flask. Place a thermometer in the flask. CAUTION Wipe up any spilled water right away to avoid slips and falls.
Observe what happens after your teacher adds a small piece of dry ice to the flask. (Dry ice is solid carbon dioxide.) CAUTION Dry ice can damage skin on contact. Do not touch the dry ice.
Record the temperature of the water just after the dry ice is added and again after it is no longer visible.
Analyze and Conclude
Observing What happened when the dry ice was added to the water?
Analyzing Data Did adding the dry ice cause the water to boil? Explain your answer.
Inferring What was the source of the bubbles in the water?
Formulating Hypotheses What caused a cloud to form above the flask?
Applying Concepts What phase changes occurred in the flask?
The boiling point of a substance depends on the atmospheric pressure. The normal boiling point of water at sea level is 100°C. At higher elevations, the atmospheric pressure is lower. Do you know that Denver, Colorado, is called the mile-high city? This nickname is based on Denver's location at one mile above sea level. In Denver, the vapor pressure of water will equal atmospheric pressure at temperatures below 100°C. The boiling point of water in Denver can be as low as 95°C. Food does not cook as quickly at 95°C as it does at 100°C. Pasta takes longer to cook in Denver than in New Orleans, Louisiana, a city that is located near sea level.
Condensation
Have you ever come out of a shower to find your bathroom mirror clouded over? The “cloud” on the mirror is caused by water vapor that cooled as it came in contact with the mirror. The water vapor transferred heat to the mirror and condensed into liquid water. Condensation is the phase change in which a substance changes from a gas or vapor to a liquid. This process is also responsible for the morning dew on the blades of grass in Figure 23. Condensation is an exothermic process.
Figure 23 Water vapor from the air condensed into drops of liquid water on these blades of grass.
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