Problem 48 - General Chemistry

Question

Decide whether each of the following is a physical property or a chemical property of the substance. A. Salt substitute, potassium chloride, dissolves in water. B. Seashells, calcium carbonate, fizz when immersed in vinegar. C. The gas hydrogen sulfide smells like rotten eggs. D. Fine steel wool (Fe) can be burned in air. E. Pure water freezes at \(0^{\circ} \mathrm{C}\).

Step-by-Step Solution

Answer

A: Physical, B: Chemical, C: Physical, D: Chemical, E: Physical

Step 1: Understanding Physical vs. Chemical Properties

Physical properties are characteristics that can be observed without changing the chemical identity of a substance, such as color, odor, melting point, boiling point, density, and solubility. Chemical properties describe a substance's ability to undergo changes that transform it into a different substance, such as reactivity with acids, flammability, and oxidation states.

Step 2: Identify Property of Salt Substitute in Water

A. Analyze the statement: "Salt substitute, potassium chloride, dissolves in water." Dissolving is a physical change, as the chemical composition of potassium chloride does not change. Therefore, this is a physical property.

Step 3: Identify Property of Seashells in Vinegar

B. Analyze the statement: "Seashells, calcium carbonate, fizz when immersed in vinegar." Fizzing indicates a reaction between calcium carbonate and vinegar, producing carbon dioxide gas. This is a chemical property because a new substance is formed.

Step 4: Identify Property of Hydrogen Sulfide's Smell

C. Analyze the statement: "The gas hydrogen sulfide smells like rotten eggs." Smell is a physical characteristic that can be observed without changing the substance's chemical structure. Thus, this is a physical property.

Step 5: Identify Property of Steel Wool Burning in Air

D. Analyze the statement: "Fine steel wool (Fe) can be burned in air." Burning indicates a chemical reaction with air, typically reacting with oxygen to form iron oxide. Burning results in a chemical change, making this a chemical property.

Step 6: Identify Property of Pure Water Freezing

E. Analyze the statement: "Pure water freezes at \(0^{\circ} \mathrm{C}\)." Freezing is a change of state and does not alter the chemical structure of water, which remains H₂O. Therefore, this is a physical property.

Key Concepts

Physical properties

Physical properties are those attributes of a substance that can be observed or measured without altering its chemical composition. These properties help describe the substance's physical state and appearance.
Some common examples of physical properties include:

• Color: The hue or shade visible to our eyes.
• Odor: The smell emitted by a substance.
• Melting and boiling points: The temperatures at which a substance transitions between solid, liquid, and gas phases.
• Density: The mass per unit volume.
• Solubility: How well a substance dissolves in a solvent like water.

When observing these properties, the identity of the substance remains unchanged. For instance, when salt dissolves in water, it is still chemically potassium chloride. Similarly, water freezing involves a change in state from liquid to solid without altering the chemical formula of H₂O.

Chemical properties

Chemical properties describe a substance's potential to undergo transformations into different substances. These properties are not readily observable without the substance undergoing a chemical change.
Typical chemical properties include:

• Reactivity with other chemicals: How a substance interacts with acids, bases, or other chemicals.
• Flammability: The ability of a substance to ignite and sustain a combustion reaction.
• Oxidation states: Details about how atoms in the substance lose or gain electrons.

When chemical properties are examined, the substance's chemical composition changes. For example, when seashells fizz in vinegar, it signifies a chemical reaction where calcium carbonate transforms by releasing carbon dioxide gas. Burning steel wool is another example where iron reacts with oxygen, forming a new compound, iron oxide.

Substance transformation

Substance transformation refers to the changes a substance undergoes to become a different substance. This concept is deeply connected with chemical properties.
Transformations can occur in different forms, such as:

• Chemical reactions: Where chemical bonds are broken or formed to create new substances, like when steel wool is burned to form iron oxide.
• Dissolution: Often a physical change, such as when potassium chloride dissolves in water, retaining its chemical identity.
• Phase changes: Where matter changes its state, like water freezing, without altering its chemical composition.

These transformations can either be physical, where the chemical identity is preserved, or chemical, resulting in a different substance formation.

Chemical identity

Chemical identity is the unique combination of elements and structure that defines a substance. It remains constant unless a chemical change occurs.
This identity is determined by:

• The types of atoms present: Represented by elements like hydrogen or oxygen.
• Bonding arrangements: The specific manner in which atoms are bonded together.
• Molecular composition: The exact number and type of atoms that form a molecule.

Physical transformations, such as dissolving or changing states, do not affect chemical identity. Potassium chloride dissolving in water does not alter its chemical identity—it remains the same compound in solution. However, a chemical reaction like the burning of hydrogen sulfide, changing its makeup, alters its chemical identity.

Observation of properties

Observing properties is crucial in distinguishing between physical and chemical properties. It requires a keen understanding of the changes a substance may undergo.
Observation can involve:

• Direct sensory engagement: Such as seeing, smelling, or touching.
• Measuring changes: Recording temperature changes, measuring volumes, or analyzing mass variations.
• Using indicators: Such as pH strips to detect acidity, which might imply a chemical change.

For instance, noticing the odor of hydrogen sulfide relies on our ability to observe physical properties, while observing the fizzing of seashells in vinegar involves recognizing a chemical property. The ability to subtly identify these transformations is pivotal in various scientific and real-life applications.