August 2006


Cobalt chloride paper is used to test for the presence of water leaking through tiny cracks in pipes or porcelain. When water is present, the test paper goes from blue to pink.

Heat and humidity are two factors that help determine our outdoor comfort level in summer. The unwelcome combination of high temperature and high humidity can drive us to the nearest pool or air-conditioned building, seeking relief.

Cobalt chloride, CoCl2, is a fascinating compound that changes color in response to humidity. As humidity increases, cobalt chloride changes color from sky blue to purple to pink. Such striking changes in color make cobalt chloride useful as a humidity indicator in weather instruments.

Coordinating with Water

Cobalt chloride
hexahydrate, CoCl2·H2O
(photo from
Cobalt chloride, CoCl2
(photo from

If we could examine blue cobalt chloride solid with an extremely powerful microscope, we would see a repeating, three-dimensional pattern of cobalt and chlorine atoms, known as a crystal structure. This regular, repeating internal arrangement of atoms is the reason that individual grains viewed through a magnifying lens, for example, look like tiny crystals.

As the humidity increases, and water is absorbed by CoCl2, the crystal structure rearranges itself to make room for water molecules. First, two water molecules surround each cobalt atom, forming the dihydrate, which is "chemistry speak" for "two water molecules." Cobalt chloride dihydrate is purple. The hydration reaction may be represented by the following chemical reaction:

As the humidity increases further, the crystal structure again changes, this time rearranging itself to let four more water molecules in to surround each cobalt atom, forming the hexahydrate:

Chemists use the raised dot symbol before the H2O to indicate the number of water molecules that have become incorporated into a compound at the atomic level. Heating the hydrated forms of cobalt chloride reverses the reactions above, returning cobalt chloride to the blue, water-free, or anhydrous, state. Water is "liberated" in these reactions, known as dehydration reactions.

Cobalt glass bottle
for Bromo- Seltzer
(photo from Wikipedia)

Chemical compounds of the element cobalt have been used for thousands of years as coloring agents in paint, ink, ceramics and glass. Cobalt glass, for example, owes its attractive deep blue color to a compound of cobalt and oxygen, cobalt oxide, CoO.

Relative Humidity: The Basics

Humid air, especially combined with high summer temperatures, feels clammy and uncomfortable. At any given temperature, air can contain a certain amount of water vapor (gaseous water). The more water vapor held in the air, the higher the humidity, and the stickier we feel. That's because our own perspiration does not evaporate readily when the air is already somewhat humid. The body cools off when sweat evaporates from the skin, and humidity slows that process.

The term "relative humidity" explains this concept in numbers. The relative humidity of air on any given day is the ratio of the amount of water vapor absorbed in air, to the maximum amount of water vapor that can be absorbed at that temperature. Relative humidity is reported as a percentage. For example, if the relative humidity is 38 percent, the air contains 38 percent of the water vapor it is capable of containing at the current temperature. On a recent day in Miami, Florida, the temperature at noon was 85 degrees Fahrenheit, and the relative humidity was 72 percent, meaning the air contained 72 percent of the water vapor it is capable of holding at 85 degrees.

Apparent Temperature: The Temperature We Feel

Heat and humidity can be combined to give an "apparent temperature," or "heat index," a measure of how it really feels outside. The apparent temperature is used to determine when outdoor activities may be dangerous.

The graph below shows that when the air temperature was 85 degrees and the relative humidity was 72 percent in Miami, the apparent temperature, or heat index was about 94 degrees. That combination of heat and humidity comes with an "extreme" caution for outdoor activities. The U.S. Centers for Disease Control and Prevention has posted a prevention guide for promoting personal health and safety during periods of extreme heat.

Chart from:

Follow-up Activities

  1. What is the apparent temperature where you are today? Is there a warning for outdoor activities?

  2. Design a use for cobalt chloride that takes advantage of its color-changing properties.

  3. Make a humidity monitor by following the instructions at:

For a list of previous "Chlorine Compound of the Month" features, click here.


News & Resources

View our resource center to find press releases, testimonies, infographics and more.