In this explainer, we will learn how to explain the conditions necessary for rusting and learn how to write balanced equations for the key reactions involved.
Rust is a reddish-brown substance often found on the surface of old or abandoned metal, such as an old car, can, or nail.
Rust is a form of corrosion that builds up over time on iron or iron alloys when they are exposed to oxygen and water. Before learning about the chemical formation of rust, let’s take a look at its physical properties.
|Durability||Durable||Brittle and flaky (will chip away)|
|Density||Dense||Less dense (expands upon formation)|
The formation of rust can reduce the strength and stability of an iron object because of the difference in physical properties between iron and rust. An engineer that uses an iron beam in a structure will want it to remain a strong, durable, and dense beam instead of one that will expand, crack, and chip away.
In order to prevent rust, engineers can use coatings, such as oil, paint, or other metals, to prevent the surface of the metal from making contact with water and oxygen in the surroundings. They can also select rust-proof alloys such as stainless steel.
Example 1: Identifying Methods That Prevent Rusting
Which of the following suggestions is not a viable method of slowing or preventing rusting?
- Plating with tin
- Soaking in salt water
- Covering with plastic
- Coating with grease
Most of these answer options slow or prevent rust from forming. The question is asking us to find the one option that does not slow or prevent rusting.
Rust forms when iron is exposed to oxygen and water. In order to prevent rusting, we need to prevent the exposure of iron to oxygen and water.
Knowing this, one option leaps out as different: soaking in salt water certainly does not limit the exposure to water. Also, since there is oxygen dissolved in water, it does not limit the exposure to oxygen either.
The other four options—painting, plating, covering, and coating—all involve protective layers that prevent iron from contacting water and oxygen.
The correct answer is option C, soaking in salt water.
Rust is a flaky, reddish-brown hydrated iron(III) oxide formed through the oxidation of iron in the presence of oxygen and water. It has the chemical formula .
Corrosion is the gradual destruction or damage caused by a slow, irreversible, and spontaneous redox reaction between the surface of a substance and the environment.
Note that all rust is corrosion, but not all corrosion is rust. Other metals can oxidize or otherwise corrode to form various compounds, but only iron will form the compound we call “rust.”
Chemically, rust is hydrated iron(III) oxide, with the chemical formula . The “” signifies that the number of water molecules in the compound can vary.
The simplified reaction for the formation of rust is:
This overall reaction shows that iron combines with oxygen and water to form a hydrated oxide. However, to understand the chemical process in more detail, let’s look at the intermediate reactions.
The first step is the oxidation of iron to iron(II) ions, as shown by the following half reaction. Oxidation is the loss of electrons, and this formation of ions happens as the solid iron becomes a solution:
In the corresponding half reaction, oxygen is reduced, accepting electrons from the reaction above in the presence of hydrogen ions to form water:
As well as reacting to form water, the hydrogen ions and the dissolved oxygen in the water further oxidize iron(II) ions into iron(III) ions:
The iron(III) ions combine with water to form iron(III) hydroxide:
Finally, the iron(III) hydroxide dehydrates to form hydrated iron(III) oxide with chemical formula .
In summary, iron dissolves in water to form iron(II) ions that are then oxidized into iron(III) ions. Hydrogen ions are absorbed, and water is produced along the way. The iron(III) ions then combine with water to make iron(III) hydroxide, which then forms hydrated iron(III) oxide.
With this process in mind, we can take a look at some of the factors that might increase the rate of rusting of a piece of iron. The simplest way to affect the rate of reaction is to change the exposure to the two main reactants, water and oxygen. For example, if we coat the iron in grease so water and oxygen cannot reach it, no rust will develop. Conversely, if we leave an iron object outside in the rain for many days, it will rust more quickly than if it is kept dry.
The iron in objects near the sea, such as boats and chains, also tends to rust quite quickly, as can be seen in the photo below. Interestingly, exposure to salt water increases the rate of rusting compared to fresh water. The oxidation–reduction reaction at the beginning of the rusting process requires the movement of electrons. The ions present in salt water make it a more effective electrolyte than fresh water, allowing electrons to be transferred more easily and rust to form more quickly.
It is worth noting that even underwater iron can rust as there is oxygen dissolved in the water. The rust can clearly be seen in the following photograph of a propeller from a Japanese ship that was sunk during the second world war.
However, if we took water and boiled it to remove the dissolved oxygen, that water would not cause a piece of iron to rust.
Other reactants in this process are hydrogen ions. Hydrogen ions are absorbed during both the reduction of oxygen and the formation of iron(III) ions, so an increase in the concentration of hydrogen ions will speed up these processes. In addition, the hydrogen ions increase the electrical conductivity of the solution, so the electron transfer in the redox reaction happens more quickly.
Acid rain can also erode protective coatings, allowing the process of rusting to begin on the iron underneath. For these reasons, an acidic environment with a low pH will cause iron to rust more quickly.
Example 2: Describing the Effect of Salt on Rusting Processes
Rusting of iron is an example of a redox reaction. The rate of rusting of iron in water varies with increasing salt concentration.
- Which particles are removed from a metal during an oxidation reaction?
- Oxygen atoms
- Hydrogen atoms
- How and why does the rate of rusting of iron in water vary with increasing salt concentration?
- The rate increases because dissolved ions aid the decay of metal nuclei.
- The rate increases because dissolved ions aid the movement of electrons.
- The rate decreases because dissolved ions aid the ionization of water.
- The rate decreases because dissolved ions react with dissolved oxygen.
- The rate increases because dissolved ions react with the metal atoms.
- Which term best describes the role of the salt solution in the rusting process?
- Oxidizing agent
- Reducing agent
This question is asking about the process of oxidation. Oxidation–reduction reactions involve the transfer of electrons from one compound or element to another. Oxidation involves a loss of electrons, while reduction involves a gain of electrons. During oxidation, iron gives up electrons to form iron 2+ ions. So, the correct answer to this part of the question is “electrons.”
This question is asking how and why salt changes the rate of reaction of rusting. To answer this question, we need to determine whether it increases or decreases the rate of reaction and the mechanism behind that change.
Part of the correct answer is that increasing salt concentration increases the rate of rusting. Iron that is either near salt water, or areas where roads are salted, rusts relatively quickly compared to metals in other environments. We can eliminate options C and D from consideration.
Next, why does salt increase the rate of reaction for rusting? As we mentioned in the first part of this question, the oxidation–reduction reaction that occurs at the beginning of rusting involves the transfer of electrons. The faster those electrons can move, the quicker the reaction will occur. In a salt solution, the electrons can move faster. Looking at the answer options, this fits with option B, the rate increases because dissolved ions aid the movement of electrons.
Option A describes the decay of metal nuclei, but radioactive decay is not involved in the rusting process. Option E suggests that the rate increases because of a reaction between the metal atoms and salt ions, but during rusting, the metal atoms react with the water and the oxygen in the solution, not the salt ions.
So, the correct answer is option B, the rate increases because dissolved ions aid the movement of electrons.
This question is asking us to define the role of salt in the rusting process.
Salt cannot be the oxidizing or reducing agent, as it does not accept or donate electrons in the oxidation–reduction reaction.
While some salt solutions can be acidic or basic, the function of the salt in this case is not as an acid or a base. Any salts will increase the rate, not just those that dissolve into hydrogen ions or hydroxide ions.
In the previous part of this question, we determined that the purpose of the salt is to aid the movement of charged particles through the solution. A substance that allows the movement of charged particles is called an electrolyte. Option E, electrolyte, is the correct answer.
The industry and manufacturers are very concerned about the risk of rusting. This concern is due to the widespread use of steel and the detrimental effects that rust has on the properties of iron. These negative effects impact the properties of the metal much more significantly than corrosion in many other metals.
Rust is the specific name for hydrated iron(III) oxide formed during the corrosion of iron, but there are other metals that corrode to form oxides as well. For example, aluminum corrodes in the presence of oxygen in the following reaction:
Aluminum can corrode in other ways, such as in the presence of a chloride, but this way is the most common. We can compare and contrast rust with aluminum oxide to better understand the negative effects of rusting.
Patches of rust can easily chip away after they have formed, exposing more iron to be rusted; however, aluminum oxide does not chip away easily. The oxide coating on aluminum forms very quickly, resealing the aluminum if the surface is scratched or chipped.
Another negative effect of rusting is the fact that iron expands when it corrodes into rust, while aluminum contracts when forming aluminum oxide. These two physical characteristics make rust a much more disruptive oxide for machines and structures. Aluminum oxide will form a thin, dense layer on the outside of the metal that will not noticeably affect its volume. However, rust expands as freshly exposed metal deeper in the metal begins to rust.
Rust has a much more significant effect on the properties of iron than corrosion in other metals. The fact that rust can chip, cause the object to expand, and penetrate deep into the piece of metal shows the significant negative effects that need to be mitigated. Depending on the use of the piece of iron and the time and severity of the rust, the strength of the piece of iron can be compromised, making it unfit for purpose.
Example 3: Identifying Differences Between the Oxidation of Iron and Aluminum
Why does rusting affect iron more than aluminum?
- Aluminum oxides are less soluble than iron oxides.
- Aluminum is less reactive than iron.
- Aluminum oxides are less stable than iron oxides.
- Aluminum is protected by a surface oxide layer.
- Aluminum binds to water less strongly.
This question is asking us to identify a key difference between the oxidation of iron and aluminum. This oxidation process can happen when the metal is exposed to water and air. One reason why the oxidation of iron causes significant changes is that the rust can chip away. When it chips, more iron is exposed that can then rust as well.
The reason aluminum is not as affected by oxidation is that aluminum oxide does not chip. Instead, it forms a thin coating on the outside of the metal. Aluminum is more able to hold its shape and strength when it oxidizes. The correct answer is option D, aluminum is protected by a surface oxide layer.
To be thorough, we can take a look at the other options as well. Aluminum oxide and rust are equally insoluble, so option A is incorrect. Aluminum is more reactive than iron and its compounds are more stable as a result, so options B and C are incorrect as well. Option E is insignificant, as water molecules do not readily bind with aluminum molecules due to the strong oxide coating.
Different sets of conditions cause rust to form at different speeds. We can use a simple experiment to demonstrate which combinations of conditions cause rusting to happen most quickly.
Demonstration: The Effect Different Conditions Have on the Formation of Rust and the Rate of Rusting
- Place an iron nail into five separate test tubes.
- Set up different conditions for each test tube as shown in the image below.
The iron nail in test tube E will begin to rust first, followed by the iron nails in test tubes C, B, and A. The iron nail in test tube D should be the last to start rusting.
Rust occurs when iron is exposed to both water and oxygen. In test tube D, the iron nail is placed into dry air where no oxygen is present. The anhydrous calcium chloride removes any remaining water that might be present. The iron nails placed into test tubes A and B contain either water or oxygen, but not both. So, here, rusting will be slow to occur. The iron nail in test tubes C and E are exposed to both oxygen and water. However, test tube E contains salt water, and since the presence of ions increases the rate of rusting, then the iron nail in C will rust more slowly than the iron nail in E.
- Rusting occurs quickest when iron is exposed to salt water and oxygen.
- Rusting occurs slowest when iron is protected from water and oxygen.
Example 4: Identifying the Necessary Conditions for the Rusting of Iron
Iron nails are placed into three sealed bottles containing different materials, as shown.
- In which of the three bottles will rusting takes place
- 1 only
- 1, 2, and 3
- 3 only
- 1 and 2
- 1 and 3
- Which term best describes the function of in bottle 3?
Rusting occurs when iron is exposed to both water and oxygen. In order to answer this question, we must identify the different conditions in each of the bottles. All three bottles are sealed; however, there is still air, which contains oxygen, present inside.
In bottle 1, the iron nail is placed into boiled water with air being present. The importance of using boiled water is that boiling will reduce the amount of oxygen gas present in the water. However, oxygen from the air will dissolve into the water, and so the iron nail will likely be exposed to oxygen and water. So, rusting is likely to occur.
In bottle 2, the iron nail is again placed into boiled water. However, the water is covered with a layer of oil that will prevent oxygen from the air from dissolving in the water. Even though the iron nail is in the water, the lack of oxygen present means that rusting is unlikely to occur.
In bottle 3, there is no water present, only air and some calcium chloride. The air might contain both oxygen and water vapor; however, the calcium chloride will remove moisture from the air. As a result, the iron nail in bottle 3 is exposed to oxygen from the air, but not to water. Therefore, rusting is unlikely to occur.
Since rusting is likely to only occur in bottle 1, the correct answer is option A.
The calcium chloride in bottle 3 will remove any moisture that is present in the air. During this process, the anhydrous calcium chloride will form a hydrated salt according to the following equation:
This reaction is not oxidation or reduction, so we can exclude options A and B. The calcium chloride is not dissolving in a solvent and therefore is not acting as an electrolyte, so we can conclude that option E is not correct.
Calcium chloride is not involved in the process of rusting and, as there is no other chemical reaction occurring, it is not acting as a catalyst.
This means that calcium chloride is acting as a desiccant. A desiccant is a substance that can induce a state of dryness, often by absorbing water. The correct answer is option C, desiccant.
- Rust is a reddish-brown substance that forms when iron is exposed to water and oxygen.
- Rust is weaker, more brittle, and less dense than iron, so the formation of rust can negatively impact iron objects and structures.
- The chemical formula for rust is .
- The formation of rust is a multi-step process wherein dissolved iron ions combine with water to make iron(III) hydroxide, which then dehydrates into rust.
- Rusting occurs more quickly when there is increased exposure to oxygen or water. It also occurs more quickly when the iron is exposed to salt water or an acidic solution.
- Rust is particularly harmful when compared to other oxides, such as aluminum oxide, as it will expand and crack more as well as chip away to corrode further.