Problem 44 - Chemistry The Molecular Science

Question

A finely divided black substance is placed in a glass tube filled with air. When the tube is heated with a Bunsen burner, the black substance turns red- orange. The total mass of the red-orange substance is greater than that of the black substance. (a) Can you conclude that the black substance is an element? Explain briefly. (b) Can you conclude that the red-orange substance is a compound? Explain briefly.

Step-by-Step Solution

Answer

(a) No, the black substance undergoes a reaction, suggesting it's not a pure element. (b) Yes, the red-orange substance is likely a compound formed by a reaction.

Step 1: Analyze the change in substance

The black substance, when heated, turns into a red-orange substance, and the total mass increases. This suggests that the black substance reacts with something in the environment, likely oxygen from the air.

Step 2: Determine if the black substance is an element

Since elements, by definition, cannot be decomposed into simpler substances by ordinary chemical methods, and because the black substance undergoes a transformation indicating a chemical reaction, we cannot conclude that the black substance is an element.

Step 3: Examine the nature of the red-orange substance

The increase in mass of the red-orange substance compared to the black substance suggests that the red-orange substance incorporates another element, presumably from the air (like oxygen), indicating that it could be a compound.

Step 4: Conclusion about the black substance

We cannot definitively say the black substance is an element because its transformation and interaction indicate it might be a compound or a mixture.

Step 5: Conclusion about the red-orange substance

The red-orange substance can be concluded as a compound because it exhibits properties that result from the black substance chemically combining with another substance, shown by the increase in mass.

Key Concepts

Oxidation

In chemical reactions, oxidation refers to the process where a substance combines with oxygen. The scenario described involves a black substance that turns red-orange when heated, with an increase in mass. This change is likely due to oxidation, where the black substance reacts with the oxygen present in the air.

• When a substance oxidizes, it typically gains oxygen atoms, resulting in a new product. This new product, in our example, is the red-orange substance.
• Oxidation can often be visually identified by a change in color or state, as seen here.

It's important to note that oxidation does not just refer to metals; it can apply to a wide range of substances. It is one half of a chemical reaction pair known as redox reactions, where one substance is oxidized and another is reduced.

Mass Change

Mass change is an essential concept in chemical reactions, indicating whether a reaction involves the gain or loss of substances. In the given exercise, the increase in mass when the black substance turns red-orange suggests a chemical reaction with molecular oxygen from the air.

• Mass increase usually implies that the original substance has incorporated additional elements, which is typical in oxidation reactions.
• In this case, the black substance likely a gained mass due to oxygen atoms combining with it, forming a new compound.

According to the law of conservation of mass, in a closed system, mass remains constant. However, in its reaction with an atmospheric feature like air, some open systems can seem to "gain mass" when viewed relative to the original substance alone.

Element vs Compound Differentiation

Understanding the difference between elements and compounds is crucial when analyzing chemical changes. An element is a pure substance consisting of one type of atom, and it cannot be broken down into a simpler form. In contrast, a compound is comprised of two or more elements that are chemically combined.

• In the exercise, we cannot conclude that the black substance is an element because it changes upon heating, indicating it interacts with another substance, likely forming a compound.
• On the other hand, because the red-orange substance has a different mass from the original black substance, it suggests that new elements have been integrated, supporting the theory that it is a compound.

Elements demonstrate a set of unique properties under fixed conditions, whereas compounds may exhibit different properties due to the combination of their constituent elements, leading to phenomena like the observed color change.