1b STATION 3 – Reading: Water has a high heat of vaporization
The heat of vaporization is the quantity of heat a liquid must absorb for 1 gram to be converted from a liquid to
a gas. It can take twice as long for water to evaporate then liquids such as ammonia. This property is the result
of hydrogen bonds, which must be broken in order to turn water into a gas. Because of the large amount of
energy required to evaporate water, water exists as a solid, a liquid and a gas naturally on Earth.
As a liquid evaporates, the surface of the liquid that remains behind cools down. This evaporative cooling
occurs because the “hottest” molecules are the most likely to leave as a gas. Evaporative cooling of water
contributes to the stable temperature of lakes and rivers and also provides a mechanism that prevents terrestrial
organisms from overheating. For example, evaporation of sweat from human skin dissipates body heat and
helps prevent overheating on a hot day when excess heat is generated by lots of activity. High humidity on a hot
day makes one feel uncomfortable because the high amount of water vapor in the air prevents the evaporation of
sweat from the body.
1b Station 3: Heat of Vaporization Lab
Problem: Which uses less energy to evaporate water or alcohol?
Hypothesis: Use if- then -because format
Variables: Identify the independent and dependent variables.
Constants: Identify two controlled variables (constants) in the experiment.
Data: Create a data table in your notebook to record the temperature changes of the paper towels when dampened
with water or alcohol.
Materials:
2 thermometers / temperature probes
3 mL water
3 mL isopropyl alcohol
2 pieces of paper towel 4 cm X 4 cm
Timer
2 Graduated cylinders
Procedure:
1. Put on your goggles.
2. Cut out 2 squares of paper towel 4 cm X 4 cm.
3. Fold the paper towels over the bulb of your thermometers or temperature probes.
4. Measure 3 mL of isopropyl alcohol using the graduated cylinder.
5. Measure 3 mL of water using the graduated cylinder.
6. Record the initial temperature of each thermometer.
7. Pour the alcohol on one paper towel and water on the other paper towel.
8. Record the temperature of the thermometers every minute for 5 minutes.
9. Graph your data.
Analysis and Conclusion:
1. Describe the patterns you see in your graph.
2. Which substance cooled more rapidly?
3. At the end of 5 minutes which substance was cooler?
4. Which substance used the most energy to evaporate?
5. Define heat of vaporization.
6. What causes water to have a high heat of vaporization?
7. How does water’s high heat of vaporization help you feel cooler when you sweat?
8. Describe the importance of high heat of vaporization to ecosystems.
9. Based on your knowledge of water and the way it behaves write a scientific explanation using the format ClaimEvidence-Explanation (reasoning) to explain what happened in the lab. (Hint: alcohol is a non-polar molecule
that does not form hydrogen bonds.)
Claim = statement that answers the original question/problem.
Evidence = your lab data or text information.
Explanation = reasoning that links your claim and evidence using your background knowledge.