Can Hydrogel Crystals be Used as a Sensitive Humidity Indicator?

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Author: Michael Calhoun

Physical Science & Meteorology 

Grade Level
Upper Elementary (Grades 4 and 5)  
Difficulty of Project
Cost (Approximate Cost)

$12.00 - Excluding the cost of the Tri-fold cardboard display board

Safety Issues

Investigator should not ingest the crystals or drink the liquids associated with the activity. The crystals used in the project are extremely slippery when spilled. Never flush or pour these crystals down the drain, crystal swelling could possibly clog drainpipes. The supervising adult should discuss the warnings and safety information with the child or children before commencing the activity. 

Material Availability

The materials required for this project are readily available and inexpensive. 

Approximate Time Required to Complete the Project

1 day to complete the activity once the materials are secured and setup. Also between 1 and 2 hours to prepare the Tri-fold board display. 


Hydrogel superabsorbent polymer crystals are able to soak up as much as 500 times their weight in water! The research aspect of this science fair project is to determine if these crystals will absorb water vapor that results from high atmospheric humidity?

This science fair project focuses on a special kind of polymer called a hydrogel superabsorbent crystal which absorbs water, swelling to many times its original size. This water absorption ability will be tested by determining if these crystals are sensitive enough to remove water vapor from the air and grow. The amount of humidity in the air will be determined based on the size of crystal growth. From the observations made a data table comparing the size of crystal growth to relative humidity will be generated and results displayed in the form of a graph. 

The possible practical application from the results of this investigation is use of hydrogel crystal composite material as an economical alternative to traditional humidity sensors. 

Materials and Equipment / Ingredients

  • Hydrogel superabsorbent crystals
  • 8 oz clear sealable plastic bag or container with a lid
  • Metric ruler
  • Optional: hygrometer or humidity indicator card

With the possible exception of the crystals, all the other items can be purchased from the local hardware, homebuilding or garden supply store. Also, a Tri-fold cardboard display board can be purchased from an art & crafts supply store.

The hydrogel crystals can be purchased either locally from a garden supply store or online. Fortunately the price has been coming down. Science in A Bag charges $10 plus free shipping for a pack of these crystals. They may also be ordered from, or Nasco Science


Relative humidity is a term used to describe the amount of water vapor that exists in the air it depends on the temperature of the air, as warm air can hold more moisture than cold air. A relative humidity of 100 percent indicates that the air is holding all the water it can at a given temperature and any additional moisture at that point will result in condensation. To determine the relative humidity in the air a device called a hygrometer is used. 

A hydrogel crystal (sometimes called a “Disappearing crystal,” “Water Crystal,” “Superabsorbent gel,” etc) is a long chain of molecules bonded together to form a superabsorbent polymer, that does not dissolve, but forms a gel when placed in water and is often used in garden, landscape, and farming applications as a way of retaining moisture. Instead of dissolving, these crystals absorb water, swelling to many times their original size. The crystal is made up almost entirely of water. As they dry, water is slowly released to the soil. Some of these crystals can soak up as much as 500 times their weight in water! This superabsorbent characteristic makes hydrogel crystals useful as a possible humidity indicator. 

As the young investigator will discover hydrogel crystals can absorb water vapor on days of high humidity and although the crystals will not change completely into a gel, the increase in size that they swell can give a good indication of the relative humidity on the day that they are used.

Digital photos can be taken during the experimenting process and/or images of hydrogel crystals can be downloaded from the Science in a Bag website for free and without copyright infringement issues.

The following sites offer down loadable images that can be used on the display board:

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