Question

State whether the following properties of matter are physical or chemical. (a) An iron nail is attracted to a magnet. (b) A piece of paper spontaneously ignites when its temperature reaches \(451^{\circ} \mathrm{F}\). (c) A bronze statue develops a green coating (patina) over time. (d) A block of wood floats on water.

Short answer

The properties of (a) an iron nail being attracted to a magnet and (d) a block of wood floating on water are physical. The properties of (b) a piece of paper spontaneously igniting when its temperature reaches \(451^{\circ} \mathrm{F}\) and (c) a bronze statue developing a patina over time are chemical.

Step-by-step solution

Property of Iron Nail

Determine whether the property of an iron nail being attracted to a magnet is chemical or physical. It's physical. Iron is known as a ferromagnetic material, which is a substance that possesses magnetic properties in the presence of a magnetic field. This property can be observed without altering the chemical composition of the iron nail. Therefore, the attraction of an iron nail to a magnet is a physical property.

Ignition of Paper

Determine the nature of paper catching fire at a certain temperature. When paper spontaneously ignites, it's undergoing a chemical reaction known as combustion. This alters the paper's composition, turning it into a completely different substance (ash and releasing gas). Thus, this property is chemical.

Patina on Bronze Statue

Classify the transformation that causes a bronze statue to develop a patina. A patina is a green layer that forms on bronze, copper and similar metals due to a long-term exposure to the atmosphere. This change is the result of a chemical reaction - oxidation, where the metal reacts with oxygen in the atmosphere. Therefore, the formation of a patina on a bronze statue is a chemical property.

Floating Wood Block

Identify the nature of the property that makes a block of wood float on water. Wood's ability to float is owing to it having a lower density than water. This can be observed without chemically changing the wood. Thus, the property of wood floating on water is a physical property.

Key concepts

Physical Properties

Physical properties are characteristics of matter that can be observed or measured without changing the material's chemical composition. They include attributes like color, density, melting point, boiling point, and magnetic properties. For example, the attraction of an iron nail to a magnet is a result of its ferromagnetic nature and is considered a physical property. You can measure or observe these properties without altering the identity of the substance. Iron retains its composition while displaying magnetic attraction, making it a purely physical property. Similarly, wood floating on water is due to its density being less than that of the water. This floating capability also falls under the category of physical properties because it doesn't change the wood's composition.

Chemical Properties

Chemical properties describe a substance's ability to undergo changes that transform it into different substances. These include flammability, reactivity, and oxidation states. A classic example of a chemical property is the spontaneous ignition of paper at a temperature of 451°F. This type of ignition involves a combustion reaction, transforming the paper into ash and gaseous by-products, distinctly altering its chemical makeup.
The formation of a patina on a bronze statue is another example. This process is a result of oxidation, a chemical reaction between bronze and oxygen in the air, forming a new substance—the patina. Both examples demonstrate how chemical properties involve substantive changes that convert the original material into distinct compounds.

Ferromagnetism

Ferromagnetism is a specific kind of magnetic property found in certain materials such as iron, cobalt, nickel, and their alloys. It is a physical property characterized by the ability of a material to become magnetically aligned with an external magnetic field and retain this alignment even after the external field is removed.
In the case of the iron nail, ferromagnetism explains why it is attracted to a magnet without any alteration to its chemical makeup. The alignment of tiny magnetic domains within the material facilitates this strong attraction. Ferromagnetism is solely a physical property because it relates to the structure and behavior of materials under magnetic influence without a chemical reaction occurring.

Combustion Reaction

A combustion reaction is a specific type of chemical reaction where a substance combines with oxygen to produce heat and light. This exothermic process often results in the formation of different new substances. When paper ignites at 451°F, it undergoes combustion—a classic chemical reaction. During this process, the cellulose in paper reacts with atmospheric oxygen, releasing energy in the form of heat and forming new substances like carbon dioxide, water vapor, and ash.
Combustion reactions are vital for various applications, including energy generation and propulsion. They are a clear demonstration of a chemical property, as they reorder the molecular structure of the involved molecules, leading to entirely new products.