Question

Determine whether each of the following is a physical or chemical change, and explain your reasoning. (a) Copper conducts electricity. (b) Gallium metal melts in your hand. (c) Bread turns brown in a toaster. (d) Wine turns to vinegar. (e) Cement hardens.

Short answer

a) Physical change - copper's structure does not change. b) Physical change - gallium's melting is a phase change. c) Chemical change - bread browning involves a Maillard reaction. d) Chemical change - wine becoming vinegar is a fermentation process. e) Chemical change - cement hardening involves chemical reactions.

Step-by-step solution

Classifying Copper Conducting Electricity

To classify copper conducting electricity as a physical or chemical change, consider that for a chemical change to occur, the substance must undergo a change in its chemical structure. Conducting electricity is the flow of electrons through a substance without altering the substance's chemical structure. Therefore, copper conducting electricity is a physical change.

Classifying Gallium Melting

To classify gallium metal melting in your hand as a physical or chemical change, evaluate whether the chemical composition of gallium changes when it melts. Melting is a phase change from solid to liquid, and it does not involve a change in the chemical structure; therefore, gallium melting is a physical change.

Classifying the Browning of Bread

Determining if the browning of bread in a toaster is a physical or chemical change involves understanding the process involved. The browning of bread is the result of Maillard reaction, a chemical reaction between amino acids and reducing sugars. Hence, the bread turning brown in a toaster is a chemical change because new substances are formed.

Classifying Wine Turning to Vinegar

To classify the process of wine turning into vinegar, consider the components involved. This process, called fermentation, involves the chemical transformation of ethanol in wine into acetic acid due to the activity of bacteria. Since this is a chemical process, where the end product (vinegar) has a different chemical composition than the starting material (wine), wine turning into vinegar is a chemical change.

Classifying Cement Hardening

Determining whether cement hardening is a chemical or physical change requires investigation into the process it undergoes. Cement hardening involves a hydration reaction, where water reacts with the cement to form a hard, stone-like material. This is a chemical change because new compounds are produced during the hardening process.

Key concepts

Conductivity in Metals

Understanding the concept of conductivity in metals is essential for explaining why it is considered a physical change. Conductivity refers to the ability of a material to allow the flow of electric current, which is made possible by the movement of electrons within the metal. Metals like copper have free electrons that move easily through the lattice structure, enabling the flow of electricity without altering the metal's chemical composition. This movement is a physical action, similar to water flowing through a pipe, and does not involve any change in the copper's inherent properties. Therefore, when explaining this in a classroom setting, you can liken the electrons to running water to help students visualize the process.

Phase Changes

When we discuss phase changes, we're referring to the transitions between different states of matter, such as solid, liquid, and gas. A common example to illustrate this concept is ice melting into water or water vaporizing into steam. These changes are purely physical because the chemical identity of the substance doesn’t alter; only its form does. A helpful way to convey this to students is to compare it to a person changing clothes. Just as the person is the same, regardless of what they wear, the substance remains the same despite its phase. The example of gallium melting in one’s hand is a phase change from solid to liquid, and remembering that no new substance is created can solidify a student's understanding.

Maillard Reaction

The Maillard reaction is a prime example of a chemical change because it involves the creation of new molecules. This complex chemical process occurs when proteins and sugars in food react under heat, leading to the browning and the development of new flavors. This reaction is responsible for the delicious crust on a loaf of bread or the seared color on grilled meat. Explaining this to students can be fun by relating it to cooking—a favorite dish may start with certain recognizable ingredients, but when heat is applied, something new and usually more flavorful is created. Emphasizing how this changes the food's color and taste can drive the point home that a chemical change has occurred.

Fermentation Process

The process by which wine turns into vinegar, known as the fermentation process, is fascinating and relates to a chemical change. During fermentation, microorganisms like yeast and bacteria convert sugars into other compounds such as alcohol or acid. Specifically, in the case of wine becoming vinegar, bacteria convert the alcohol present in wine into acetic acid through a biochemical process. It’s a true transformation, as the starting and ending substances have distinct chemical structures and properties. Introducing this in the classroom may include discussing how this process is not only fundamental in food production but also in the biological systems of living organisms.

Hydration Reaction

When talking about the hardening of cement, we're looking at a hydration reaction, which is a type of chemical change. In this reaction, compounds in the cement mix with water to form new products that result in the hard and durable material we know as concrete. An interesting tidbit to share with students is that this isn’t a process of drying, as one might intuitively think, but a process of growing together—cement particles react with water to create an interlocking matrix of new compounds. It's similar to baking a cake: once the ingredients are combined and heated, they transform into a cake, and like the hardened cement, you cannot simply reverse the process.