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Endothermic reaction. High view down into a beaker containing a mixture of barium hydroxide and ammonium chloride, being stirred using the probe of a digital thermometer. The beaker is set on a block of wood dampened with water. As the chemicals are mixed, there is a dramatic drop in temperature down to around -20C. This freezes the water beneath the beaker so that the block of wood sticks to it.
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