Making a Humidity Monitor

INTRODUCTION

Humidity cannot be seen, but it has important effects on the weather. Leonardo da Vinci was the first to invent an instrument for measuring humidity, the amount of water in air. Around 1500 he developed the hygrometer to monitor the moisture in the air. A hygrometer uses temperature measurements from wet and dry thermometers to calculate humidity. This humidity monitor uses a chemical reaction to produce color changes that indicate changes in humidity.

MATERIALS

• White drawing paper or white coffee filter paper
• 10% cobalt chloride in water (Approximately 5 grams in 50 milliliters of water.)
• Scissors
• Goggles
• Double sided tape
• Tongs or large tweezers
• Hair dryer
• Spray bottle
• Water
• Shallow disposable foil baking pan

WHAT TO DO

1. Cut the filter paper into a desired shape.
2. Put on goggles and gloves. Pour a small amount of cobalt chloride solution into the shallow pan. Dip the cutout into the solution of 10% cobalt chloride (This must be done by an adult.)
3. Allow the dipped paper to dry. Continue using gloves when handling the paper until it is completely dry. Use a hair dryer to make sure that the filter paper shape is completely dry. Watch the color changes as the paper dries. Try spraying a little water on the dried paper to show the blue to pink color change.
4. Place the humidity monitors in a couple of places around the classroom or home. Monitor color changes. Try placing one on mirror of the bathroom. Place one monitor outside on a window.

CAUTION: Cobalt Chloride could be harmful if swallowed and students should not taste! Cobalt chloride could also cause skin irritation. If skin contact occurs, wash thoroughly. Once the humidity monitors are completely dry, it is safe for students to handle them.

QUESTIONS

1. What happens to the color of the filter paper as heat is applied?
2. Which color corresponds to high humidity and which corresponds to low humidity?
3. What changes are observed from day to day for the monitor that is attached on the outside of the school or your house?

SUMMARY

Cobalt chloride is blue. When cobalt chloride reacts with water forming cobalt chloride hexahydrate (CoCl₂ • H₂O), this product is pink. For low humidity conditions the filter paper should appear blue, for moderate humidity the paper should appear purple and for high humidity it should change to pink.

SOURCE

"Teaching Chemistry with Toys." Sarquis, Sarquis and Williams, Terrific Science Press, 1995.

Grade Level: This experiment is expected to be appropriate for grades 1 and above with proper adult supervision.

© S. Olesik, WOW Project, Ohio State University, 2002.