Question

Which of the following is evidence for a chemical reaction? (a) Heating copper in air produces a black solid. (b) Heating copper in a vacuum gives a red-brown film.

Short answer

Option (a) is evidence of a chemical reaction as it produces a new substance (copper(II) oxide).

Step-by-step solution

Identifying Evidence of Chemical Reactions

Chemical reactions often show evidence such as color change, gas production, temperature change, or formation of a precipitate. In option (a), heating copper in air produces a black solid, indicating the formation of a new substance. In option (b), heating copper in a vacuum results in a red-brown film, which may not indicate a chemical change but possibly a physical change such as oxidation without fundamentally creating a new compound.

Analyzing Option (a)

Consider option (a): heating copper in air produces a black solid. The black solid is copper(II) oxide, formed by the reaction of copper with oxygen in the air. This color change and formation of a new substance indicates a chemical reaction has occurred: \[ 2 ext{Cu} + ext{O}_2 ightarrow 2 ext{CuO} \]

Analyzing Option (b)

Consider option (b): heating copper in a vacuum produces a red-brown film. In a vacuum, there is no atmospheric oxygen to react with copper, so any change to the copper, such as a layer of copper atoms rearranging or becoming smoother, is likely to be physical—not chemical. Thus, this is not considered strong evidence for a chemical reaction.

Final Evaluation

Since a chemical reaction involves the formation of a new substance, the presence of copper(II) oxide as a black solid in option (a) is clear evidence of a chemical reaction, unlike the physical appearance change in option (b).

Key concepts

Evidence of Chemical Reactions

To determine if a chemical reaction has occurred, look for key pieces of evidence. A chemical reaction usually brings about a noticeable change in one or more properties:

• Color Change: New reactions might produce different-colored substances.
• Gas Production: The appearance of bubbles or fumes can signify a reaction.
• Temperature Change: Exothermic or endothermic reactions involve heat release or absorption, respectively.
• Precipitate Formation: When two liquid solutions mix to form a solid.

In the context provided, heating copper in air results in a black solid, copper(II) oxide. This color change is a telltale sign of a chemical reaction as a new substance forms. However, merely observing a color change isn't always conclusive; we need to confirm new substances are indeed created.

Chemical Change vs. Physical Change

Understanding the difference between chemical and physical changes is foundational in chemistry. This distinction hinges on whether new substances form:In a **chemical change**, substances interact to form one or more new compounds. This process involves breaking and forming of bonds, seen in reactions such as copper reacting with oxygen to create copper(II) oxide—captured by the equation \[ 2 \text{Cu} + \text{O}_2 \rightarrow 2 \text{CuO} \].A **physical change** occurs when a substance undergoes a change in state or appearance but doesn't become a different chemical substance. For example, melting ice into water alters the state but doesn’t change the chemical nature. Similarly, heating copper in a vacuum might change its physical appearance, imparting a red-brown tint, but since no new compounds form, it’s a physical change, not chemical.

Copper Oxidation

Copper oxidation is an informative example of a chemical reaction between copper and oxygen. When copper is heated in the presence of air, it reacts with oxygen. This reaction forms a new substance—copper(II) oxide (\[ \text{CuO} \])—resulting in a black solid on the copper surface.This process can be split into several steps:

• **Exposure:** Copper is exposed to air at high temperatures.
• **Reaction Initiation:** Oxygen molecules contact the copper atoms.
• **Formation:** Copper atoms lose electrons to oxygen, forming copper(II) oxide.

The occurrence of these steps highlights a clear chemical transformation, illustrating an oxidation reaction, which is a form of corrosion often observed in everyday environments. The visible change in color underscores the chemical change, distinguishing it from physical transformations that merely alter state or appearance without producing new substances.