Problem 15 - 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: Identify the Property Type in (a)

The color of elemental bromine is orange-red. This is an observation of bromine's appearance without changing its composition. Hence, it is a physical property.

Step 2: Identify the Property Type in (b)

Iron turns to rust in the presence of air and water. This describes a change in the substance itself as iron reacts with oxygen to form rust. Therefore, it is a chemical property.

Step 3: Identify the Property Type in (c)

Hydrogen can explode when ignited in air. Ignition leads to a chemical reaction with oxygen, resulting in a change in the substance. Thus, it is a chemical property.

Step 4: Identify the Property Type in (d)

The density of titanium metal is \(4.5 \mathrm{g} / \mathrm{cm}^{3}\). This measures a characteristic that does not involve a change in composition. It is a physical property.

Step 5: Identify the Property Type in (e)

Tin metal melts at \(505 \mathrm{K}\). Melting involves changing states, but the substance remains the same, indicating a physical property.

Step 6: Identify the Property Type in (f)

Chlorophyll, a plant pigment, is green. This color description involves no composition change, classifying it as a physical property.

Key Concepts

Physical Properties

Physical properties are the traits of a substance that can be observed or measured without altering its chemical composition. This means that the substance remains the same before and after the measurement. Generally, physical properties involve characteristics like color, state of matter, melting point, boiling point, density, and solubility. For example, when we say elemental bromine is orange-red, we are observing its color—a certain trait that does not involve changing bromine into something else.
Here are some key examples of physical properties:

• Color: The appearance or hue of a substance, such as the vibrant orange-red of bromine.
• State of matter: Whether a substance is a solid, liquid, or gas.
• Melting Point: The temperature at which a solid becomes a liquid, like tin melting at 505 K.
• Density: The mass of a substance per unit volume, such as titanium's density of 4.5 g/cm³.

Recognizing physical properties is important because it helps in identifying and studying substances without requiring chemical reactions or transformations.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances. These properties are vital for understanding how substances interact with each other chemically. A change in composition always accompanies a chemical property.
Iron rusting is a classic example of a chemical property. When iron is exposed to air and water, it reacts with oxygen to form iron oxide, commonly known as rust. This process is irreversible under normal conditions and changes the substance into a new material, demonstrating that it is a chemical property. Some other noteworthy examples of chemical properties include:

• Reactivity with other chemicals: How a substance reacts with others, like iron rusting with oxygen.
• Flammability: A substance's ability to burn, such as hydrogen exploding when ignited in air.
• Acidity or basicity: Whether a substance will act as an acid or base in reactions.

Identifying chemical properties is crucial for predicting how materials behave in various environments and conditions.

Elemental Bromine

Elemental bromine is a chemical element noted for its distinct orange-red color. It is one of the halogens, sitting in Group 17 of the periodic table alongside elements like chlorine and iodine. Bromine exists naturally as a diatomic molecule, Br₂, and is the only liquid non-metallic element at room temperature. This property makes it particularly interesting! Some other interesting aspects about elemental bromine are:

• Physical State: Unlike many elements, bromine is a liquid at room temperature.
• Toxicity: It is highly corrosive and can be harmful if inhaled or contacted, requiring careful handling.
• Color: Its distinctive orange-red color is easily identifiable and used to characterize it as a physical property.

Bromine's unique properties, such as its liquid state and vibrant color, make it invaluable in various industrial applications, including the manufacture of certain dyes, flame retardants, and pharmaceuticals.

Iron Rusting

Iron rusting is a well-known chemical process where iron reacts with oxygen in the presence of water to form rust, or iron (III) oxide. This reaction is a major example of oxidation, a process that involves the transfer of electrons, leading to a change in the oxidation state of elements involved. What happens during iron rusting?

• Chemical Reaction: When iron (Fe) comes into contact with oxygen (O₂) and moisture, it forms iron oxide (Fe₂O₃).
• Visual Change: The iron’s surface takes on a reddish-brown color as rust forms, indicating chemical transformation.
• Structural Effects: Rust compromises the structural integrity of iron, making it weaker and more brittle.

The rusting of iron is not just a fascinating chemical phenomenon; it's also a significant concern in industries. Understanding this helps prevent damage and deterioration in iron-containing structures by employing techniques such as protective coatings and galvanization.