Question

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 burn 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.

Short answer

a. Physical change b. Chemical change c. Chemical change d. Chemical change

Step-by-step solution

a. Classifying moth ball vaporization

Moth balls gradually vaporize in a closet. This process involves the change in the state of matter from solid to gas, but the chemical composition of the mothballs remains the same. Therefore, this process is a physical change.

b. Classifying Hydrofluoric acid etching on glass

Hydrofluoric acid attacks glass and is used to etch calibration marks on glass laboratory utensils. Since the acid reacts with the glass, changing its composition, this process is a chemical change.

c. Classifying alcohol evaporation in a sauce

A French chef making a sauce with brandy is able to burn off the alcohol from the brandy, leaving just the brandy flavoring. Burning off the alcohol is a chemical change, as the alcohol undergoes combustion to produce water and carbon dioxide.

d. Classifying holes in cotton jeans from acid spills

Chemistry majors sometimes get holes in the cotton jeans they wear to lab because of acid spills. The hole formation in the cotton jeans is a result of the acid reacting with the fabric, changing its chemical composition. Therefore, this process is a chemical change.

Key concepts

State of matter

Understanding the state of matter is crucial when distinguishing between physical and chemical changes. Matter exists in three primary states: solid, liquid, and gas. These states describe the arrangement and behavior of particles within a substance.
Solid particles are closely packed, liquid particles are loosely connected, allowing fluid motion, and gas particles are far apart and move freely. When a substance changes its state, like moth balls vaporizing from a solid to a gas in a closet, this is a physical change. The mothballs don't chemically alter; they simply change form, which is why this process is a physical change. By examining the state of matter, we can better understand whether a process is physical or chemical.

Chemical reaction

Chemical reactions involve the transformation of substances into new substances, with different chemical compositions. This occurs when reactants interact to form products, often involving the breaking and formation of bonds.
Indicators of a chemical reaction include color change, gas formation, temperature change, or the creation of a precipitate. For example, hydrofluoric acid etching glass involves a chemical reaction where the acid reacts with silicon dioxide in the glass. This changes both substances' compositions, illustrating a clear chemical reaction. Understanding these changes helps clarify when a chemical reaction, rather than a physical change, has occurred.

Combustion

Combustion is a specific type of chemical reaction involving the rapid combination of a substance with oxygen, producing heat and light. It's commonly seen in everyday life as burning. In the kitchen, when a French chef burns off the alcohol from brandy in a sauce, combustion occurs. The alcohol reacts with oxygen in the air, resulting in carbon dioxide and water, while releasing heat. This not only changes the alcohol chemically but also imparts a distinct flavor characteristic of the sauce.
Combustion is often irreversible, emphasizing its nature as a chemical change.

Acid etching

Acid etching is a process where acids are used to cut into a material's surface, typically glass or metal, through a chemical reaction. This technique is valuable in various industries for purposes like creating markings or patterns. When hydrofluoric acid is used to etch calibration marks onto glass, it chemically alters the glass surface. The acid reacts with the silicon dioxide in glass, forming silicon fluoride, which gets washed away, leaving etched marks.
This process exemplifies a chemical change, as the original material is permanently altered. Understanding acid etching helps comprehend the broader scope of chemical reactions in practical applications.