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an exothermic process

We can all appreciate that water does not spontaneously boil at room temperature; instead we must heat it. Because we must add heat, boiling water is a process that chemists call endothermic. Clearly, if some processes require heat, others must give off heat when they take place. These are known as exothermic. For purposes of this discussion, processes that require or give off heat will be limited to changes of state, known as phase changes, and changes in chemical constitution, or chemical reactions.

Changes of state involve a solid melting, a liquid freezing, a liquid boiling or a gas condensing. When steam, which is gaseous water, condenses, heat is released. Likewise when liquid water freezes, heat is given off. In fact heat must be continually removed from the freezing water or the freezing process will stop. Our experience makes it easy for us to realize that to boil water or any liquid and thereby convert into a gas, heat is required and the process is endothermic. It is less intuitive to grasp that when a gas condenses to a liquid, heat is given off and the process is exothermic.