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Chemical Refrigerator
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Welcome to the eighth issue of Teacher Connection, a free online monthly newsletter to provide teachers with effective teaching techniques and helpful ideas and tools for explaining chemistry and mathematics concepts to students of all levels.

Chemical Refrigerator
When most people think of chemical reactions they think of heat. Fuels burning, fireworks exploding or even drain cleaner being poured down the drain. In each case, heat production is both expected and desirable. However, endothermic reactions, reactions which consume heat, should also be expected and can be just as exciting. The most exciting example of this that I have found is when solid barium hydroxide and solid ammonium thiocyanate are mixed together in a small flask.

Caution: This activity requires the use of hazardous substances and has the potential for hazardous reactions. Carefully review the Safety Precautions section and relevant Materials Safety Data Sheets.


  • 10 g, Ammonium thiocyanate, NH4SCN
  • 20 g, Barium hydroxide octahydrate, Ba(OH)2 · 8H20
  • Laboratory weighing paper
  • 125 mL Erlenmeyer flask
  • Thermometer, if you have one graduated to at  least -30oC
  • Stirring rod


  1. Weigh out 10 g of ammonium thiocyanate and 20 g of Barium hydroxide using laboratory weighing paper.
  2. Transfer both to the 125 mL Erlenmeyer flask and mix with a glass or plastic stirring rod.
  3. Within two or three minutes, the solids become liquid. A thermometer placed in the mixture reveals the temperature falling far below freezing. (Don't allow the temperature to fall below the last graduation on your thermometer.) The irritating odor of ammonia will be apparent but not overwhelming. Loosely stopper the flask with a cork stopper to keep the odor down.
  4. Place a small six inch by six inch square of cardboard or light wood on the demonstration desk. Pour one or two milliliters of water on the square and sit the flask in the puddle. Now the students will see just how "cool" this demonstration is; the water will freeze the flask to the square.

This demonstration is a good way to introduce the concept of heat of reaction. Chemical reactions may be either endothermic or exothermic. This reaction is strongly endothermic absorbing so much heat that the water freezes.

The reaction between the solids is:

NH4SCN(s) + Ba(OH)2 · 8H2O(s) + heat → Ba(SCN)2(aq) + 2NH3(aq) + 10H2O(l)

Safety Precautions:
Barium salts are toxic by ingestion. Ammonium thiocyanate is also toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron when working with these chemicals.

Use caution when handling the cold flask. Use tongs if available. The temperatures involved are cold enough to freeze skin. Ammonia vapor is very irritating to the eyes and respiratory tract. Do not inhale the gas.

Consult a reliable reference manual for appropriate disposal methods.

Shakhashiri, B. Z., Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, WI. 1983; Vol. 1, pp. 10-12

More Teaching Tips from Quantum coming soon!


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