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 -5C to -8C over about 18 seconds. This freezes the water beneath the beaker so that the block of wood sticks to it.