Classify the following as physical or chemical changes. a. Moth balls gradually vaporize in a closet. b. Hydrofluoric acid attacks glass and is used to etch calibration marks on glass laboratory utensils. c. A French chef making a sauce with brandy is able to boil off the alcohol from the brandy, leaving just the brandy flavoring. d. Chemistry majors sometimes get holes in the cotton jeans they wear to lab because of acid spills.

When we talk about the state of matter, we are referring to the distinct forms that different phases of matter take on. The classical states—solid, liquid, and gas—are encountered in everyday experiences. An understanding of these states can explain many natural phenomena and daily activities. For instance, in our exercise example, moth balls vaporizing in a closet represent a transition from solid to gas without altering the substance's chemical composition. Similarly, when the chef boils brandy, the alcohol vaporizes from liquid to gas, once again a transformative physical process that doesn't result in a new substance. This is because only the state changes, not the chemical identity of the substances involved.

Each state of matter has its own unique set of physical properties. When a substance undergoes a state change, like when ice (solid water) melts to form liquid water, or water vapor (gas) condenses into liquid, energy is either absorbed or released. These processes, known as phase changes, are typical examples of physical changes that do not affect the molecular structure of the substance.

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Chemical reactions involve making and breaking of chemical bonds, leading to the change in the composition of one or more substances. In the example where hydrofluoric acid is used to etch glass, it's not just a superficial change. The acid reacts with the silicon dioxide in glass to form other chemical compounds, which include silicon tetrafluoride and water vapor. Here, new substances are created through a chemical reaction, indicating a chemical change.

Chemical reactions can manifest through various indicators such as color change, bubble formation, heat or light emission, and the formation of precipitates. To identify a chemical change, look for signs that a reaction has occurred, like a change in temperature, the appearance of a gas, or a change in color.

Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. These changes are usually reversible, and no new substances are formed. In the example where moth balls vaporize, we saw this type of change. The moth balls turned from solid to gas—a change in state, without turning into a different substance. The same occurs when the chef boils off the alcohol from brandy; the alcohol evaporates, but it remains chemically the same as it was before heating.

Physical changes are all about energy and states of matter. Melting, freezing, vaporizing, condensing, sublimation, and deposition are all processes that are examples of physical changes. These changes are essential in daily life, from cooking to the very cycle of water that sustains ecosystems across the globe.

Chemical changes occur when a substance combines with another to form a new substance, called synthesis, or when a substance decomposes into two or more different substances. These changes are chemical because they involve a rearrangement of the way atoms are bonded to each other. For example, the acid holes in the jeans from the exercise indicate a chemical change. As the acid reacts with the cotton fabric, the molecular structure of the cotton alters, and the fabric disintegrates.

A crucial point about chemical changes is that they are typically irreversible without further chemical reactions. For example, once the cotton in the jeans has been dissolved by the acid, you can't simply reverse the process to repair the jeans. This fundamentally separates chemical changes from physical changes, which often can be reversed more straightforwardly.