例1(2008年黄冈)用蒸笼蒸馒头,是上层还是下层蒸格中的馒头先熟?小明仔细观察后发现:高温的水蒸气经过多层蒸格向上升,遇到冷的蒸笼盖时,大量水蒸气发生现象,很多热量,使层蒸格中的馒头先熟.有经验的师傅拿刚出笼的馒头前,先将手沾点水,这样做主要是利用,使手不会被烫伤.解析:从物质发生物态变化的前后两种状态和结合物态变化的定义,来确定是发生了哪种物态变化,然后再确定是吸热还是放热. Example 1 (Huanggang in 2008) steamed steamed buns in steamers, is steamed steamed buns in the upper or lower steamer? After careful observation, Xiaoming found that high temperature steam rises upwards through multiple layers of steaming, and when it encounters a cold steamer cover, A lot of water vapor occurs, a lot of heat, so that steamed bread steamed first cooked. Experienced masters get out of the basket just before the first hand touched the water, this is mainly used, so that the hand will not be burned. Analysis: From the definition of the two states before and after the physical state of matter changes and the definition of the change of the bound state, it is determined which kind of state change has occurred, and then it is determined whether it is endothermic or exothermic.