Question

State whether each of the following is a physical or a chemical property: (a) Iron oxidizes to rust. (b) Dry ice undergoes sublimation. (c) Lithium metal turns gray in air. (d) Plutonium radioactively decays.

Short answer

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

Step-by-step solution

Identify the Type of Change in Option (a)

When iron oxidizes to form rust, it undergoes a chemical change as a new substance is formed (rust). Thus, it is a chemical property.

Analyze the Change in Option (b)

Dry ice undergoing sublimation involves changing from a solid to a gas without forming a new substance, making it a physical property.

Examine the Behavior in Option (c)

Lithium metal turning gray is due to a reaction with oxygen in the air, forming a new compound, which indicates a chemical property.

Consider the Process in Option (d)

Plutonium radioactively decays to form different elements, hence, it is a chemical property as new substances are formed through nuclear changes.

Key concepts

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances. These properties can only be observed during a chemical reaction.
For example, when iron oxidizes, it reacts with oxygen in the air to form rust, a new compound. Oxidation is a chemical change and is considered a chemical property because new substances are created.
Another example involves lithium metal turning gray when exposed to air. This change also involves a chemical reaction with oxygen, forming lithium oxide, and highlighting its chemical property of reacting with oxygen. Chemical properties are central in understanding how substances interact under different conditions.

• Reactivity with other chemicals
• Flammability
• Acidity or basicity

Physical Properties

Physical properties can be observed or measured without changing the substance's chemical nature. They include attributes such as color, density, volume, mass, and state of matter. Unlike chemical properties, observing these characteristics doesn’t result in the creation of a new substance.
A typical scenario involves dry ice, which is solid carbon dioxide. When dry ice sublimates, it transitions directly from solid to gas without forming a new substance. Sublimation is a physical change, indicative of dry ice’s physical properties. Understanding physical properties helps in identifying substances and predicting behavior in various conditions.

• Boiling and melting points
• Density
• Solubility

Changes of State

Changes of state refer to the transition of substances between solid, liquid, and gaseous phases. These transitions are physical changes as they do not alter the fundamental chemical identity of the substance.
For instance, when dry ice undergoes sublimation, it skips the liquid phase and changes directly from solid to vapor. This phase change is considered physical as no new chemical substances are formed.

• Melting: solid to liquid
• Freezing: liquid to solid
• Evaporation: liquid to gas

Each change of state involves energy transfer, typically in the form of heat, but does not break or form chemical bonds.

Oxidation

Oxidation is a chemical process where a substance combines with oxygen and often involves losing electrons. It's an essential concept in chemistry as it can lead to corrosion, combustion, or the formation of oxides.
Iron rusting is a common example of oxidation. It occurs when iron reacts with atmospheric oxygen, forming iron oxide—a new substance. Similarly, when lithium metal turns gray due to exposure to air, it oxidizes and becomes lithium oxide.

• Corrosion of metals
• Combustion reactions
• Formation of oxides

Understanding oxidation helps in preventing corrosion and managing industrial processes effectively.

Radioactive Decay

Radioactive decay is a process by which an unstable atomic nucleus loses energy by emitting radiation. This chemical change leads to the transformation of elements into different elements or isotopes.
For example, plutonium undergoes radioactive decay, leading to the formation of different, more stable elements. This process changes the fundamental structure of the nucleus, marking it as a chemical property. Radioactive decay is vital for understanding nuclear reactions and radioactive dating.

• Alpha decay: emission of helium nuclei
• Beta decay: emission of electrons or positrons
• Gamma decay: emission of gamma rays

The study of radioactive decay is crucial for applications in medicine, energy production, and dating geological formations.