Problem 47 - General Chemistry

Question

The following are properties of substances. Decide whether each is a physical property or a chemical property. A. Chlorine gas liquefies at \(-35^{\circ} \mathrm{C}\) under normal pressure. B. Hydrogen burns in chlorine gas. C. Bromine melts at \(-7.2^{\circ} \mathrm{C}\). D. Lithium is a soft, silvery-colored metal. E. Iron rusts in an atmosphere of moist air.

Step-by-Step Solution

Answer

A, C, D are physical properties; B, E are chemical properties.

Step 1: Analyzing Chlorine Gas Property

The property given is that chlorine gas liquefies at \(-35^{\circ} \mathrm{C}\) under normal pressure. This describes a change in state from gas to liquid without altering the chemical composition of chlorine. This is a physical change, so this is a physical property.

Step 2: Analyzing Hydrogen and Chlorine Reaction

The property describes hydrogen burning in chlorine gas. This involves a chemical reaction where hydrogen and chlorine combine to form hydrochloric acid (HCl). Since a new substance is formed, this is a chemical change, making it a chemical property.

Step 3: Analyzing Bromine Property

The property given is that bromine melts at \(-7.2^{\circ} \mathrm{C}\). Melting is a change of state from solid to liquid that does not change the chemical identity of bromine, making this a physical property.

Step 4: Analyzing Lithium Property

The description of lithium being a soft, silvery-colored metal relates to its appearance and physical state. These are observable characteristics that do not involve a chemical change, so this is a physical property.

Step 5: Analyzing Iron Rusting

Rusting involves iron reacting with oxygen in moist air to form iron oxide, a different substance. Since this process involves a chemical change, it is a chemical property.

Key Concepts

State of Matter

The state of matter refers to the distinct forms that different phases of matter take on. Matter can exist primarily in four states: solid, liquid, gas, and plasma. Each state has unique properties that can change under different conditions, such as temperature and pressure.

For instance, chlorine gas liquefying at \(-35^{\circ} \mathrm{C}\) under normal pressure is an example of such a change where gas becomes a liquid. This type of change is known as a phase transition. Importantly, these transitions do not involve altering the chemical composition, only the physical form.

• Solid: Defined shape and volume, particles closely packed.
• Liquid: Defined volume but can change shape to fit the container, particles are more loosely packed than in a solid.
• Gas: No defined shape or volume, particles are far apart and move freely.
• Plasma: Ionized gas with unique properties affected by electric and magnetic fields.

Understanding these states helps in identifying physical properties as any change in the state of matter typically indicates a physical change.

Chemical Reactions

Chemical reactions are processes where substances interact to form new substances with different molecular configurations. These reactions can often be observed through energy changes, such as light or heat, and the development of new materials.

When hydrogen burns in chlorine gas, a chemical reaction occurs, producing hydrochloric acid (HCl). Here, hydrogen atoms bond with chlorine atoms, showcasing a transformation that results in a new chemical entity.

• Reactants: The starting substances in a chemical reaction.
• Products: The resulting substances after a chemical reaction.
• Energy Change: Chemical reactions often release or absorb energy; for example, burning can produce heat.

Recognizing a chemical reaction is essential for identifying chemical properties, as they involve a change in the substance's chemical nature.

Observable Characteristics

Observable characteristics are attributes of a substance that can be noticed without changing the substance's chemical composition. These features are often linked to sensory perceptions like texture, color, state, and luster.

For example, lithium is described as a soft, silvery-colored metal. These descriptions tell us about its appearance without affecting its chemical structure.

• Color and Luster: Visual properties, such as shininess for metals.
• Texture: How the surface or substance feels to the touch.
• Malleability: The ability to bend or be shaped without breaking.

These characteristics are significant for classifying physical properties, as they focus on perceivable traits not involving a transformation in chemical identity.

Material Properties

Material properties refer to inherent attributes that define the behavior of materials during different conditions, including mechanical, thermal, and optical properties. These properties can be physical or chemical.

When iron rusts in moist air, the material property of iron reacting with oxygen forms iron oxide. This is a chemical property because it describes a material's tendency to undergo a chemical change.

• Physical Properties: Include aspects like density, melting point, and electrical conductivity, observed without altering substance identity.
• Chemical Properties: Involve how a material interacts with other substances, like reactivity with acids or oxidation.

Knowing these properties aids in determining how materials will perform or transform in specific environments, crucial for scientific and industrial applications.