Investigation Three demonstrated the fundamentals of solutions and solubility. Students discovered that solids interact with other solids to form mixtures, and with liquids to form mixtures and solutions. Students also discovered that liquids can interact to form mixtures and solutions. Students learned that solids that change phase in liquids to form solution are called solutes, and the liquids in which they dissolve are called solvents. Students further learned that when liquids form solutions they are miscible in one another and that the liquid in the smaller amount is called the solute, while the liquid present in the larger amount is the solvent. These concepts serve as a prelude to the study of physical and chemical changes.

When salt dissolves in water, it changes from solid to liquid as the sodium and chloride ions separate from each other. However, when the water evaporates, these ions rejoin each other in the same one-to-one ratio that they had before dissolution, and the salt is recovered. Scientists refer to the dissolution of salt in water as a physical change, because both salt and water can be recovered in their original forms and because no new substance was created as a result of their interaction. Other examples of physical change include ice melting, sugar dissolving in water, and dry ice (a solid) converting back to carbon dioxide’s preferred state as a gas. Although the ice, dry ice, and sugar all change forms, their chemical structures do not change. However, if sugar is spread on top of a dish of crème Brulee and heated with a torch it melts, turns brown, and hardens into caramel.

The caramelization of sugar is an example of a chemical change. A chemical change is one in which a new substance is formed. As observed in the caramelization example, some chemical changes are the result of physical action on a substance, as when the sugar was subjected to high heat. The heat caused irreversible changes in the chemical structure of sugar resulting in the formation of the brown liquid that solidified into a solid sheet of caramel.

Another example of a chemical change that produces a color change is the interaction of oxygen and iron to form rust. Oxygen is colorless and iron is black or dark gray, yet rust has a distinctive reddish-brown color. For this reason, metal surfaces are typically painted as a means of preventing the metal from being exposed to atmospheric oxygen. In the photograph shown here, you can see the chemical reaction between iron and oxygen where the paint has peeled off.

Not all chemical changes are signified by a color change. Some chemicals cause the formation of gas when they react. Other reactions yield an increase or decrease in temperature, and still others may result in changes in odor. Scientists have a list of ten common signs that a chemical change may have occurred:

Evidence of a Chemical Change

In each case, the signs result because new substances are formed.

However, one of the most frustrating aspects of chemistry is that some of these changes can also accompany a physical change. For example, as ice melts, a temperature change occurs, and the resulting water is warmer than solid ice. As students begin investigations of physical and chemical changes they are presented with these types of challenges. To simply say that the appearance of one of these signs is a definite indication that a chemical change has taken place and the absence of these signs is an indication of a physical change can result in student misconceptions in this area of science.

This Investigation approaches the concepts of physical and chemical changes from the viewpoint of comparison, by providing an opportunity for students to view both physical and chemical changes as well as three of the common signs of a chemical change: color change, temperature change, and production of a gas. As Trials are conducted, students are told whether the experiment is an example of a chemical or a physical change and then asked to determine whether they observe any of the three signs of a chemical change. The Investigation has been designed so that the students observe these three signs in both common physical and chemical changes in order to promote a discussion about how practicing scientists determine whether a physical or chemical change has occurred when confronted with a new reaction. What students will discover is that, in most cases, scientists first determine whether the reaction is an example of a known physical change such as boiling, melting, or dissolution. If the reaction is not a known physical change, scientists then place more emphasis on the presence of one of the signs of chemical change and conduct additional experiments to determine whether more than one sign of a chemical change has occurred.

In this Investigation, students will observe temperature changes in water resulting from the physical changes of boiling and melting and compare these changes with a temperature change due to the addition of non-chlorine bleach powder to water, a chemical change. Students will also compare the release of steam from boiling water to the release of carbon dioxide gas from the reaction of baking soda and vinegar, a chemical change. Students will also compare a color change due to the addition of food coloring to water (physical change) with the color change associated with the exposure of the interior of an apple to air (chemical change). Through discussion, students will explore the process scientists use to determine whether a change is physical or chemical in nature.