Endothermic and exothermic processes
Endothermic and exothermic processes are two types of heat-related changes that matter undergoes, affecting its physical or chemical properties. An endothermic process absorbs thermal energy from the environment, resulting in changes such as melting, evaporation, and sublimation. For example, when ice melts into water, it absorbs heat but retains its chemical identity. In contrast, exothermic processes release thermal energy, leading to changes like freezing, condensation, and deposition. When water freezes into ice, it loses heat without changing its chemical composition. Both types of processes play significant roles in everyday phenomena, such as the evaporation of puddles or the formation of frost on cold surfaces. Understanding these concepts is essential for grasping how energy transfer influences the states of matter and the transformations substances undergo.
Endothermic and exothermic processes
An endothermic process involves the absorption of thermal energy, or heat, from the surrounding environment to cause a change in matter. An exothermic process involves the release of thermal energy into the environment to cause a change in matter. The changes in matter caused by endothermic and exothermic processes may be physical or chemical. Physical changes cause substances to change only their physical properties; their chemical properties remain unchanged. Chemical changes, also called chemical reactions, cause substances to change their chemical properties and result in entirely new substances. Chemical changes that result from the absorption or release of heat are known as endothermic reactions and exothermic reactions, respectively. The following discussion focuses on endothermic and exothermic processes that cause physical changes in matter but do not involve chemical reactions.
Changes in Matter
Matter generally exists in three physical states: solid, liquid, or gas. A solid has a distinct volume and shape. The molecules that make up a solid are arranged in a regular geometric pattern and are packed together very closely. Although the molecules in a solid move, they do so only by vibrating slightly in a relatively stationary position. An ice cube is an example of a solid.
A liquid, like a solid, has a distinct volume, but it lacks a distinct shape; a liquid actually changes its shape to fit its container. The molecules that make up a liquid are not arranged in a regular geometric pattern. They can move more easily and slip past one another. Room-temperature water is an example of a liquid.
A gas has no distinct volume or shape; however, gases contained within a particular vessel take the volume and shape of that vessel. Gas molecules have a random arrangement and move freely at high speeds. Water vapor (steam) is an example of a gas.
Changes that transform matter from one physical state to another, such as from a solid to a liquid or from a liquid to a gas, are called phase changes. The six possible phase changes are melting (also known as fusion), evaporation, sublimation, freezing, condensation, and deposition.
Phase changes affect the physical properties of substances without altering their chemical compositions. Phase changes generally occur as a result of changes in temperature or pressure. Changes in temperature result from the absorption or release of thermal energy, known as endothermic processes and exothermic process, respectively.
Endothermic Processes
The prefix endo- means "inside" or "within," while the root word therm means "heat." Therefore, endothermic literally means "inside heat" or "heat within." An endothermic process takes in heat to cause a physical change in matter. Melting (fusion), evaporation, and sublimation are endothermic processes.
Melting (Fusion)
Melting is an endothermic process that changes a solid to a liquid. Ice, or frozen water, is a solid. When ice absorbs heat from its environment, it begins to melt. Over time, melting causes ice to change from a solid to a liquid. Although the physical properties of the ice change, the liquid water that results remains unchanged chemically.
Evaporation
Evaporation is an endothermic process that changes a liquid to a gas. Consider a puddle of water on the ground. As water in the puddle absorbs thermal energy from its environment (i.e., heat from the Sun), it begins to heat up. Eventually, the liquid water becomes hot enough that it changes into a gas known as water vapor and escapes into the atmosphere. Although the physical properties of the water change, the water vapor that results still has the same chemical composition as liquid water.
Sublimation
Sublimation is an endothermic process that changes a solid into a gas. A good example of sublimation is dry ice, which is a solid form of carbon dioxide. As dry ice absorbs heat from its surrounding environment, its temperature increases. At room temperature, dry ice readily changes from a solid to a gas and escapes into the atmosphere. Although the physical properties of the dry ice change, the carbon dioxide gas that results from sublimation is chemically the same as the dry ice.
Exothermic Processes
In contrast to endothermic processes, exothermic processes release heat. The prefix exo- means "outside," so exothermic literally means "outside heat." Exothermic processes release heat to cause physical changes in matter. Examples of exothermic processes are freezing, condensation, and deposition.
Freezing
Freezing is an exothermic process that changes a liquid to a solid. Water, at room temperature, is a liquid. In cold temperatures, water loses heat to its environment and eventually freezes. Over time, the freezing process causes water to change from a liquid to a solid. Although the physical properties of the water change, the ice that results remains unchanged chemically.
Condensation
Condensation is an exothermic process that changes a gas to a liquid. Water droplets that form on the outside of a cold glass on a hot day result from condensation. When water vapor in the atmosphere makes contact with the cool surface of the glass, it loses some of its heat to the surrounding environment. As a result, the water vapor cools and condenses into a liquid. Although the physical properties of the water vapor change, the liquid water that results still has the same chemical composition as the water vapor.
Deposition
Deposition is an exothermic process that changes a gas into a solid. A good example of sublimation is frost that forms on a car windshield on a cold day. Frost is a type of ice, which is a solid form of water. In subfreezing temperatures, water vapor in the atmosphere, a gas, loses heat so quickly that it freezes and turns to ice, a solid, without first condensing into liquid water. Although the physical properties of the water vapor change, the frost that results from deposition is chemically the same as the water vapor.
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