Do Cobalt Chloride Turn Blue on Absorbing Water? Exploring the Color-Changing Property and Applications

Introduction

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. This property is due to the color change when cobalt(II) chloride absorbs water and becomes hydrated, indicating the presence of moisture.

The Chemistry Behind the Color Change

Cobalt(II) chloride, or CoCl2, is a fascinating compound that changes color in response to humidity. In its anhydrous form, it is a blue solid. When it absorbs water, it forms a pink hydrated form, CoCl2·6H2O. This color change makes cobalt chloride a useful indicator for moisture.

Understanding the Crystal Structure and Hydration Process

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. As humidity increases, water molecules are absorbed by CoCl2, rearranging the crystal structure to accommodate them.

Color Change Process

The color change of cobalt chloride from blue to pink is a result of the hydration process. First, two water molecules surround each cobalt atom forming the dihydrate phase. This is often referred to as hydration. As the humidity increases further, the crystal structure rearranges to accommodate four more water molecules, forming the hexahydrate phase:

Chemical reactions involved in this process can be represented as:

1H2O → CoCl2

2H2O → CoCl2·2H2O

6H2O → CoCl2·6H2O

Heating the hydrated forms of cobalt chloride reverses these reactions, returning cobalt chloride to the blue, anhydrous state.

Applications of Cobalt Chloride

Cobalt chloride has been used for thousands of years as a coloring agent in paints, inks, ceramics, and glass. For example, cobalt glass gets its deep blue color from a compound of cobalt and oxygen, cobalt oxide (CoO). In addition to color-changing properties, cobalt chloride is also used as a moisture indicator in weather instruments.

Relative Humidity: The Basics

Relative humidity is a measure of how much water vapor is in the air in comparison to how much the air can hold at a specific temperature. High relative humidity makes us feel uncomfortable because our sweat does not easily evaporate, preventing effective cooling.

Apparent Temperature: The Temperature We Feel

Heat and humidity can be combined to give an apparent temperature, which is the temperature we feel outside. The heat index is used to determine when outdoor activities may be dangerous. For instance, on a 85-degree day in Miami with 72 percent relative humidity, the apparent temperature (heat index) would be about 94 degrees.

Follow-up Activities

What is the apparent temperature where you are today? Is there a warning for outdoor activities? Design a use for cobalt chloride that takes advantage of its color-changing properties. Follow the instructions provided to make a humidity monitor.

Conclusion

Cobalt chloride's unique property of changing color in response to moisture makes it a valuable tool in various applications. Understanding this chemistry can help us better address issues related to humidity and moisture in daily life, enhancing comfort and safety.