Problem 7 - Chemistry and Chemical Reactivity

Question

In each case, decide if the underlined property is a physical or chemical property. (a) The color of elemental bromine is orange-red. (b) Iron turns to rust in the presence of air and water. (c) Hydrogen can explode when ignited in air (Figure \(1.16)\) (d) The density of titanium metal is \(4.5 \mathrm{g} / \mathrm{cm}^{3} .\) (e) Tin metal melts at \(505 \mathrm{K}\) (f) Chlorophyll, a plant pigment, is green.

Step-by-Step Solution

Answer

(a) Physical, (b) Chemical, (c) Chemical, (d) Physical, (e) Physical, (f) Physical.

Step 1: Understanding Physical and Chemical Properties

Physical properties can be measured or observed without changing the identity of the substance, such as color, density, and melting point. Chemical properties describe how a substance interacts with other substances, i.e., its ability to undergo a chemical change, such as rusting or explosion.

Step 2: Analyze Bromine Color

The problem states that elemental bromine is orange-red. Color describes a substance's appearance and does not involve a chemical change, thus it is a physical property.

Step 3: Evaluate Iron Rusting

Iron rusts in the presence of air and water, which is a transformation that changes iron to iron oxide. This involves a chemical reaction, making this a chemical property.

Step 4: Consider Hydrogen Explosion

Hydrogen's ability to explode when ignited in air indicates a reaction with oxygen, causing a transformation into new products (water). Thus, it is a chemical property.

Step 5: Determine Titanium Density

Density is a measure of how much mass is contained in a given volume. It can be measured without altering the substance, so this is a physical property.

Step 6: Assess Tin Melting Point

Melting point, the temperature at which a solid becomes a liquid, does not change the chemical identity of the substance. Therefore, it is a physical property.

Step 7: Evaluate Chlorophyll Color

The greenness of chlorophyll refers to its color, a sensory property that does not involve a chemical change, making it a physical property.

Key Concepts

Chemical Change

A chemical change involves a substance's transformation into one or more new substances. This process often involves breaking or forming chemical bonds. In the context of a chemical change, the original substance undergoes a reaction to form a different chemical composition.
For instance, rusting of iron involves iron reacting with oxygen and water, resulting in iron oxide. This showcases a complete change in substance, characterized by different properties such as color and texture.

• Transformation is permanent.
• Involves energy changes (either released or absorbed).
• Results in the formation of completely new substances.

Recognizing a chemical change is crucial in fields like chemistry and material science, where understanding reactions allows for the creation of new materials and the prediction of their behaviors.

Physical Change

Physical changes are alterations in a substance that do not change its chemical identity. These include changes in state, form, or appearance. A classic example is water freezing or melting.
During a physical change, the substance remains the same at a molecular level, meaning its chemical properties remain unchanged. Characteristics such as shape, size, or phase can change, but the actual substance does not.

• No new substances are formed.
• The changes are generally reversible.
• Energy changes are usually minimal or related to physical forces like melting or freezing.

Understanding physical changes is vital for processes that require separation or purification of substances without altering their molecular structures.

Substance Interaction

Substance interaction refers to how different substances influence one another when they come into contact. These interactions can lead to either chemical reactions or physical changes.
For example, when hydrogen ignites in the presence of oxygen, it interacts in a way that results in a chemical change, producing water. This type of reaction often releases energy in the form of heat and light, typical of an explosive reaction.

• Can lead to either chemical or physical changes.
• Driven by the intrinsic properties of the substances involved.
• May involve both visible and non-visible changes.

Recognizing the type of interaction helps in identifying possible outcomes and managing these processes safely in scientific experiments or industrial applications.

Property Analysis

Property analysis involves evaluating the attributes of substances that define their usage and behavior under various conditions.
It is essential to distinguish between physical and chemical properties, as each type dictates different potential applications and handling methods for a substance. Physical properties such as color, density, and melting point influence usage scenarios like storage conditions and transport processes. Chemical properties, including reactivity and flammability, are critical in processes that involve chemical transformations.

• Enables the prediction of substance behavior.
• Assists in selecting materials for specific applications.
• Facilitates safety assessments and quality control in manufacturing.

Effective property analysis is a cornerstone of scientific inquiry and industrial innovation, guiding decisions to improve materials and products.