During the development of chemistry, many chemists attempted to explain the changes that occur when combustible (capable of burning) materials burn and metals corrode or rust. The following are two proposed theories.

Phlogiston Theory

    According to this theory, combustible materials, such as wood, coal, or metal contain a massless "essence" or presence called phlogiston. When combustion occurs, the phlogiston is released from the combusting object and is absorbed by the air. For example, when a piece of wood is burned, phlogiston is released to the air and the wood is converted to ash. The ash is free of phlogiston and can no longer support combustion. Similarly, if a metal is heated, the phlogiston is lost to the air and the metal is converted into a nonmetallic, powdery substance called ash, or calx. The corrosion (changing of a substance by a chemical reaction) of metals, such as the rusting of iron (Fe), also involves the loss of phlogiston from the metal, but at a slower rate than burning. Rust can be turned back into metal by heating it in air with a substance rich in phlogiston, such as charcoal. A transfer of phlogiston from the charcoal to the rust converts the rust back to metal.

Oxygen Theory

    According to this theory, burning and rusting involve an element called oxygen, which is found in the air. The complete combustion of a piece of wood involves the rapid reaction of the wood with oxygen gas (O₂) to produce carbon dioxide (CO₂), which is a nonflammable gas, and water (H₂O). The rusting of iron involves the slow reaction of iron with oxygen to produce iron oxides such as Fe₂O₃. These iron oxides are known as rust. Heating rust with charcoal produces iron because the charcoal combines with the oxygen in the rust. In these transformations, there is a conservation of mass (the total mass of the reactants must equal the total mass of the products in a chemical reaction). In these reactions matter is neither created nor destroyed, but merely transformed.
 

Set 5: Science 

1. The best answer is B.

The Phlogiston Theory states that all combustible materials contain phlogiston. The passage defines combustible as "capable of burning."

A. All combustible substances combine with phlogiston as they burn.
Incorrect. In the Phlogiston Theory, combustible materials contain phlogiston that is released during combustion, leaving behind all other substances (i.e., ashes). So phlogiston is part of the combustible substance before burning, and separates from the mixture during burning.

B. All substances that burn contain phlogiston.
Correct. The Phlogiston Theory states that all combustible materials contain phlogiston. The passage defines combustible as "capable of burning." This implies that all materials that are combustible contain phlogiston.

C. Metals cannot be broken down by chemical means.
Incorrect. The passage contradicts this foil when it is stated in the Phlogiston Theory that "The corrosion (changing of a substance by a chemical reaction) of metals . . . involves the loss of phlogiston from the metal."

D. The loss of phlogiston by a substance will always result in the production of heat and light.
Incorrect. The release of heat and light is not discussed in the Phlogiston Theory, but is implied by the discussion of combustion, which involves a flame. But corrosion is also a process discussed that involves a loss of phlogiston but does not involve the production of heat and light.

2. The best answer is H.
In the Phlogiston Theory, substances lose phlogiston, a massless substance, upon combustion or corrosion, leaving behind pure materials. In the Oxygen Theory, substances mix with oxygen from the air when burning or corroding.

F. amount of light produced in the burning of a variety of combustible substances.
Incorrect. Measuring the light given off during burning would not discern between the two theories. Both theories discuss what occurs when a substance is burned, but neither theory discusses why the flame intensity during combustion may vary.

G. amount of heat produced in the burning of a variety of combustible substances.
Incorrect. Measuring the heat given off during burning would not discern between the two theories. Both theories discuss what occurs when a substance is burned, but neither theory discusses why the heat produced during combustion may vary.

H. masses of all the reactants and products before and after the reaction.
Correct. The Phlogiston Theory claims that phlogiston is a massless substance, so if one measures the masses of reactants and products before and after a combustion or a corrosion reaction, one could determine whether there was a change in the mass of the reactants or the surrounding atmosphere. This would test a key aspect of the Phlogiston Theory.

J. amount of water produced when a substance burns.
Incorrect. The Phlogiston Theory does not mention the role of water in combustion. The detection and measurement of water would lend support to the Oxygen Theory, and it would be a direct test of the Oxygen Theory, but it would not directly test the Phlogiston Theory.

3. The best answer is B.
The Oxygen Theory claims that carbon dioxide and water are produced during combustion.

A. can support the breathing of a mouse.
Incorrect. Mice breathe oxygen, not carbon dioxide and water, so these gases cannot support the breathing of a mouse.

B. are nonflammable.
Correct. The passage states that carbon dioxide is nonflammable, and the other gas that the Oxygen Theory claims is produced during combustion, water, is also nonflammable.

C. are toxic to growing plants.
Incorrect. Plants take in both carbon dioxide and water from the atmosphere to survive, so the gases produced are not toxic to growing plants.

D. are rich in hydrogen gas.
Incorrect. Hydrogen gas is not mentioned in the Oxygen Theory, and there is nothing in the passage to infer that combustion will produce hydrogen gas.

4. The best answer is H.
The Phlogiston Theory states that when ". . . a metal is heated, the phlogiston is lost to the air and the metal is converted into a nonmetallic, powdery substance called ash, or calx." The Phlogiston Theory, referring to the specific example of corroded iron, later states that "Rust can be turned back into metal by heating it in air with a substance rich in phlogiston, such as charcoal."

F. cannot be converted back to zinc metal.
Incorrect. The Phlogiston Theory suggests that the corroded zinc could be converted back to zinc metal if it is heated in air with charcoal.

G. contains pure phlogiston.
Incorrect. In the Phlogiston Theory, phlogiston is lost during corrosion, so if a sample of zinc metal is completely corroded, it would not contain any phlogiston.

H. contains no phlogiston.
Correct. In the Phlogiston Theory, phlogiston is lost during corrosion, so if a sample of zinc metal is completely corroded, it would not contain any phlogiston.

J. is a combination of the zinc metal and phlogiston.
Incorrect. In the Phlogiston Theory, phlogiston is lost during corrosion, so if a sample of zinc metal is completely corroded, it would not contain any phlogiston.

5. The best answer is B.
The common aspects of the Oxygen Theory's description of combustion (burning) and corrosion (rusting) are the formation of oxygen-containing compounds (CO₂ and metal oxides). The source of oxygen is the air, and the source of the other material in the reaction is the substance that is burning or corroding.

A. converting the elements of the material into gaseous compounds.
Incorrect. The Oxygen Theory's description of corrosion does not involve the formation of gaseous compounds.

B. forming oxygen-containing compounds from the elements in the material.
Correct. The Oxygen Theory's descriptions of both burning and rusting involve the formation of oxygen-containing compounds.

C. removing oxygen from the material and releasing it into the air.
Incorrect. In the case of burning and rusting, the Oxygen Theory specifies the air as being the source of oxygen, not the material being burnt or rusted.

D. producing high temperatures as a result of the chemical reactions.
Incorrect. There is no discussion of temperature, except the logical inference that combustion involves high temperatures. Corrosion, though, does not produce high temperatures, so this is not an aspect of burning and rusting.

6. The best answer is F.
The Phlogiston Theory states that charcoal is ". . . a substance rich in phlogiston. . . . A transfer of phlogiston from the charcoal to the rust converts the rust back to metal." The process of corrosion is the same as the process for converting metal into ash (or calx), so by analogy, the ash of a metal should be converted back to the metal if it is in contact with a source of phlogiston, such as charcoal (or the phlogiston collected from burning charcoal).

F. converting the ash from corroded tin back to tin metal.
Correct. The Phlogiston Theory states that charcoal is ". . . a substance rich in phlogiston. . . . A transfer of phlogiston from the charcoal to the rust converts the rust back to metal." The process of corrosion is the same as the process for converting metal into ash (or calx), so by analogy, the ash of a metal should be converted back to the metal if it is in contact with a source of phlogiston, such as charcoal (or the phlogiston collected from burning charcoal).

G. supporting combustion of another piece of charcoal.
Incorrect. The Phlogiston Theory states that charcoal is rich in phlogiston, so it does not need additional phlogiston for combustion. Also, the source of phlogiston in burning is the material being burnt (the charcoal) and not the air surrounding the material.

H. rusting iron.
Incorrect. The rusting of iron involves a loss of phlogiston, so placing iron in contact with gases rich in phlogiston could only inhibit rusting.

J. converting wood ash into rust.
Incorrect. The Phlogiston Theory would describe both wood ash and rust as materials without phlogiston, so one could not be converted to the other by adding phlogiston. If phlogiston was added to wood ash, wood would result. If phlogiston was added to rust, iron would result.

7. The best answer is D.
The apparatus shown contains a closed system with a limited amount of mercury and air. The pressure of the air in the reservoir is measured by a pool of mercury which is open to the atmosphere, so as the volume (and hence, pressure) of the air in the reservoir decreases, the volume of mercury in the air reservoir will increase (and vice versa). If the Phlogiston Theory is accurate, the mercury should give up phlogiston as the red powder (presumably the ash or calx) is formed, causing the air to increase in volume relative to the atmosphere, making the volume of mercury in the column decrease. If the Oxygen Theory is accurate, the mercury will react with oxygen in the air, forming calx (the red powder). This will cause the air to decrease in volume relative to the atmosphere, causing the volume of mercury in the air reservoir to increase.

A. The Phlogiston Theory, because the red powder resembled an ash
Incorrect. The formation of a new substance when a metal is heated in air is not unique to the Phlogiston Theory, it is also part of the Oxygen Theory, so the formation of a powder resembling ash would not support the Phlogiston Theory.

B. The Phlogiston Theory, because the air in the reservoir could not support combustion and therefore did not contain oxygen
Incorrect. Combustion is not the process being tested by the experiment, and the results show that the metal is transformed in the experiment, so this reasoning is not valid.

C. The Oxygen Theory, because the mercury level dropped in the air reservoir indicating increased oxygen content
Incorrect. The item states that the volume of mercury in the air reservoir, which would be measured by the level (or height) of mercury in the air reservoir, increased, so this reason conflicts with the results.

D. The Oxygen Theory, because the mercury level rose in the air reservoir indicating decreased oxygen content
Correct. If the Oxygen Theory is accurate, the mercury will react with oxygen in the air, forming calx (the red powder). This will cause the air to decrease in volume relative to the atmosphere, making the volume (and, therefore, the height) of mercury in the air reservoir increase.