Question

You are working with an artist who has been commissioned to make a sculpture for a big city in the eastem United States. The artist is wondering what material to use to make her sculpture, because she has heard that acid rain in the eastern U.S. might destroy it over time. You take samples of granite, marble, bronze, and other materials, and place them outdoors for a long time in the big city. You periodically examine the appearance and measure the mass of the samples. (a) What observations would lead you to conclude that one, or more, of the materials were well-suited for the sculpture? (b) What chemical process (or processes) is (are) the most likely responsible for any observed changes in the materials? [Section \(18.4]\)

Short answer

To determine which materials are suitable for the sculpture, we should look for materials that show minimal to no signs of corrosion, deterioration, or changes in appearance and mass after being exposed to acid rain for a long time. The most likely chemical processes responsible for any observed changes in the materials are acid rain, chemical weathering, and oxidation. By identifying the materials that demonstrate minimal to no changes due to these processes during our experiment, we can recommend the most suitable material(s) for the sculpture.

Step-by-step solution

Determine Suitability Criteria

To determine which materials are suitable for the sculpture, we should look for materials that show minimal to no signs of corrosion, deterioration, or changes in appearance and mass after being exposed to acid rain for a long time. These observations would suggest that the material can withstand acid rain and maintain its intended appearance over time.

Identify Likely Chemical Processes

The most likely processes responsible for any observed changes in the materials are: 1. Acid rain - Acid rain contains several acidic compounds such as sulfuric acid (\(H_2SO_4\)) and nitric acid (\(HNO_3\)) that can corrode the surfaces of different materials. 2. Chemical weathering - Acid rain reacts with the minerals present in the materials, creating new compounds and breaking down the original structure of the material. For example, when acid rain reacts with carbonate minerals like calcite (\(CaCO_3\)) in marble, it forms soluble calcium sulfate (\(CaSO_4\)). 3. Oxidation - Materials like bronze, which contain copper, may oxidize, forming a green patina on its surface. Once we identify the materials that demonstrate minimal to no changes due to these processes during our experiment, we can recommend the most suitable material(s) for the sculpture.

Key concepts

Acid Rain: An Enemy of Sculptures

Acid rain is a form of precipitation with high levels of nitric and sulfuric acids. It often arises from the emissions of power plants and vehicles. When it rains, these acids can wreak havoc on materials, especially in areas like the eastern United States where industrial activity is high. The acids can dissolve some minerals and corrode surfaces, causing sculptures to deteriorate over time. Acid rain can also dull the appearance of a sculpture by changing its color and texture.

When choosing a material for a sculpture, we should observe samples exposed to acid rain for any signs of damage. Minimal changes in appearance and weight suggest that a material is resistant. Thus, it's crucial to select a material that withstands the acidic assault, preserving the sculpture's integrity.

Chemical Weathering and Sculptures

Chemical weathering refers to the process where rocks and minerals are broken down through chemical reactions. In the context of sculptures, this often involves reactions between acid rain and the minerals within the sculpture's material.

For example, marble, a common sculpting material, reacts significantly with acid rain. Marble contains calcite ( CaCO_3) which can decompose into soluble calcium sulfate ( CaSO_4) upon exposure to the acidic elements of acid rain. This reaction changes the structure and appearance of the marble, leading to deterioration over time.

Observing how different materials respond to chemical weathering can guide us in choosing the right material. A resistant material will show little to no chemical breakdown, retaining its mass and appearance even in harsh environments.

Oxidation: A Common Culprit in Material Degradation

Oxidation is a chemical reaction where materials, especially metals, lose electrons and combine with oxygen, often from the air. This process is familiar as rust for iron, but it affects other metals too, including those used in sculptures.

For instance, bronze, an alloy of copper and tin, is prone to oxidation. When bronze oxidizes, it develops a green patina. This layer can both protect and alter the appearance of the sculpture. While some artists and patrons desire this effect for its aesthetic qualities, others may consider it a sign of degradation.

In the evaluation of materials for sculptures, any change due to oxidation should be considered. The ability to resist oxidation or maintain a desired appearance makes a material suitable for sculpture work, especially in environments like the eastern U.S. where acid rain can accelerate these chemical processes.