Investigation One introduced students to the concept of heat as the transfer of energy. Students learned that thermometers register temperature by undergoing energy (heat) transfer, and that temperature is a measurement of the kinetic energy of molecules. Students continued their exploration of heat in Investigation Two using experimentation to define the difference between an insulator and a conductor of heat. Students tested their discoveries about the differing abilities of materials to transfer heat by constructing a thermos from a beaker of water and comparing its rate of heat transfer to an uninsulated beaker of water. Students found that wrapping the beaker in insulating materials such as paper and plastic resulted in a decrease in the amount of heat absorbed by the water in the beaker, and thus slowed the average rate of heat transfer into the water from the beaker’s surroundings. Students will use the knowledge gained about the rate of heat transfer to explore how heat is transferred during chemical reactions in Investigation Three.

The Law of Conservation of Energy

Scientists are able to classify physical and chemical processes by the direction of heat transfer. In order to classify processes based on heat transfer, it is important to remember that heat is the transfer of kinetic energy and thus is governed by the Law of Conservation of Energy. The Law of Conservation of Energy states that energy can neither be created nor destroyed, it simply changes form. Therefore, heat represents energy that has changed form during a physical or chemical process. The CAP for Investigation Three briefly addresses the Law of Conservation of Energy in somewhat more detail.

Physical processes involving changes in states of matter result in heat transfer. For example, ice melting and water freezing are two different physical processes involving changes in the state of water between solid and liquid. Both processes involve changes in temperature. When ice melts, it does so because it has been exposed to heat in some fashion. In most cases, this is simply due to the ice being placed in an environment with a greater temperature than that of the ice, such as a glass of soda. Heat is transferred from the soda into the ice, causing the temperature of the ice to rise and the ice to change from a solid to a liquid. In other words, ice has absorbed heat from the soda. Scientists refer to a process in which heat is absorbed as being endothermic. Therefore, an endothermic process is one that absorbs heat from its surroundings.

The opposite of an endothermic process is an exothermic process. As the term exothermic suggests, heat exits or is released from the process. The freezing of water is an exothermic process. When a sample of water is placed into an environment that is colder than the water (a freezer, for example), heat will be transferred from the water to its environment. Again, this is because heat is transferred from areas of higher kinetic energy (the water) to areas of lower kinetic energy (the freezer). In other words, heat is released by the water to its surroundings. Thus, exothermic processes are those processes that release heat.

Heat and Chemical Reactions

Heat transfer is not limited to changes in states of matter. Chemicals can react with other chemicals to form new substances. The formation of new substances is a chemical process because there is a change in the chemical composition of the substances involved. As with physical processes, chemical processes (reactions) can be either endothermic or exothermic. Each substance in a reaction has a specific amount of kinetic energy, based on its chemical composition and the amount of substance available for reaction.

A chemical reaction that releases heat to its surroundings is an exothermic reaction. A chemical reaction that absorbs heat from its surroundings is an endothermic reaction. In general, endothermic processes or reactions are signaled by a decrease in temperature, and exothermic reactions or processes are signaled by an increase in temperature. 

Temperature change is one of the most common signs of a chemical reaction. When determining if the change in temperature indicates the occurrence of a chemical reaction, scientists also look for additional signs of chemical change as proof that a chemical reaction occurred. Other signs of chemical change include an unexpected color change, a change in pH, a change in electrical conductivity, a change in physical or chemical properties, a change in melting or boiling point, a change in volume, formation of a precipitate, production of a gas and a change in odor. In the caramelization example, an unexpected color change also occurred.

In Investigation Three, students will observe two chemical reactions for heat transfer. Students will first soak steel wool in vinegar, and observe the increase in temperature that occurs as the steel begins to rust. The increase in temperature is accompanied by an unexpected color change, indicating that the temperature increase is due to a chemical reaction. This reaction is exothermic. Students will then observe a decrease in temperature resulting from the addition of baking soda to vinegar. The decrease in temperature is accompanied by the production of a gas (seen as bubble formation) that indicates that the temperature decrease is due to a chemical reaction. This reaction is endothermic.