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Endothermic reaction. View of a digital thermometer display during the mixing of barium hydroxide and ammonium chloride. A beaker containing the two chemicals is set on a block of wood dampened with water. As the chemicals are mixed, there is a dramatic drop in temperature. Here, during the mixing, the temperature drops from 8C to -11C over about 25 seconds. This freezes the water beneath the beaker so that the block of wood sticks to it.
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