Evaporative Cooling with Water vs Alcohol

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required

Categories: Heat

Alternative titles: Comparing Evaporation Rates of Liquids

Summary

This experiment compares the cooling effect of water and methylated spirits (ethanol) as they evaporate from cotton wool wrapped around the bulb of thermometers. Because alcohol evaporates faster than water, it causes a greater and quicker drop in temperature.

Procedure

Obtain two thermometers and ensure they are dry.
Wrap a small piece of cotton wool around the bulb of each thermometer.
Soak the cotton wool on the first thermometer with water.
Soak the cotton wool on the second thermometer with methylated spirits.
Place both thermometers in a well-ventilated area, ideally in front of a fan to encourage evaporation.
Record the temperature readings from both thermometers at one-minute intervals.
Continue for several minutes and compare how the temperature drops over time.

Links

Evaporative cooling experiment - Rachel Peach:

Variations

Test with additional liquids such as acetone or isopropyl alcohol.
Repeat the experiment in different environments (humid vs dry air).
Try using digital data loggers to automatically collect readings.
Compare results at room temperature vs outdoors on a hot day.

Safety Precautions

Methylated spirits are flammable; keep away from flames, sparks, and hot surfaces.
Use in a well-ventilated space to avoid inhaling fumes.
Avoid skin or eye contact with methylated spirits.
Dispose of alcohol-soaked cotton safely in a sealed container.
Handle thermometers carefully to avoid glass breakage.

Questions to Consider

Why does methylated spirits cool the thermometer faster than water? (Because it evaporates more quickly, requiring more heat energy from the surroundings.)
How does air movement affect the rate of cooling? (Faster airflow increases evaporation, causing quicker cooling.)
How would high humidity influence the results? (High humidity slows evaporation, reducing the cooling effect.)
What real-life examples use evaporative cooling? (Sweating, cooling in clay water jars, cooling towers.)
How could students present their results to highlight differences between liquids? (By plotting a graph of temperature vs time for water and methylated spirits.)