Video Transcript
Some iron filings are sprayed with water and secured in the end of a test tube with a piece of cotton wool. After one week, what will have happened to the level of water in the test tube?
Oxidation is a naturally occurring phenomenon that happens when certain elements are exposed to oxygen. In this question, we are given an example of a test tube that contains iron filings as well as molecular oxygen. We’ve also been told that the iron filings have been sprayed with water, which is necessary to facilitate the oxidation process.
When a reaction occurs between the correct ratios of iron, water, and aqueous oxygen, or oxygen that’s dissolved in water from the air, hydrated iron oxide is formed. And this type of oxidation reaction belongs to the classification of reactions that we call corrosion. And the oxidation reaction that turns solid iron into hydrated iron oxide is the process that forms the product that we know as rust. And this process is the same one that takes place causing corrosion of cars, pipes, and bridges made of iron or iron alloy materials.
Now, in order to be able to answer the question and determine what happens to the water level in the test tube, let’s analyze what’s chemically occurring in this reaction. The first step in this process is the formation of positively charged iron two plus ions from iron metal. The process where iron or any other atom loses electrons is also known as oxidation. And when the iron atoms become oxidized and positively charged, they become soluble in water. And this allows them to further react in the next step of the process.
Now, as the oxidation process is occurring, the corresponding reduction process is occurring at the same time. And in this step, oxygen from the air that naturally dissolves in water is being reduced. A reduction reaction occurs when a molecule like molecular oxygen accepts electrons. In this case, oxygen is reduced, accepting four electrons and reacting with hydrogen ions that occur naturally in low concentrations in water. And from this process, two molecules of water are formed. It’s important to note that in this process, oxygen that’s dissolved in the water that we’ve sprayed on the iron filings is consumed. In the next step, dissolved iron(II) ions are further oxidized to iron(III). At the same time, more aqueous molecular oxygen is consumed, and it’s reduced into water.
Now, in both steps so far, oxygen is being consumed in the water, depleting dissolved oxygen around the iron. And so for the reaction to continue, oxygen from the air in the test tube is dissolved into the water on the iron filings. In the third step, the iron(III) ions combine with water to form iron(III) hydroxide. And each reacting water molecule also loses one positively charged hydrogen ion in this process. And now, in the final step, the iron(III) hydroxide is dehydrated and combines with another water molecule to form hydrated iron(II) oxide, or rust. And now, with this information at hand, we should be able to revisit and answer the question.
Some iron filings are sprayed with water and secured in the end of a test tube with a piece of cotton wool. After one week, what will have happened to the level of water in the test tube? At the beginning of the experiment, inside of a test tube, we trapped iron filings, water, and a certain amount of oxygen. This oxygen exists both in the air inside the test tube and dissolved in the water on the iron filings. And as we’ve already discussed, as aqueous O2 is consumed, oxygen gas is dissolved into the water. And so the volume of oxygen gas decreases. And this decrease of gaseous oxygen inside the test tube causes the level of the water inside the test tube to rise.
And so we know that the correct answer to the question “After one week, what will have happened to the level of water in the test tube?” is the level will have risen.
