Which of the following is always a characteristic of a reducing agent? A) It contains a halogen. B) It contains hydrogen. C) It contains oxygen. D) It can be oxidized. E) It is soluble in water.
A reducing agent is simply any chemical that, during a process, reduces something else. Reduction is just the name for process where a specific chemical or a specific part of a chemical undergoes a decrease in its oxidation state or gains electrons. For instance, iron three ions can receive electrons to form elemental iron.
The opposite of reduction is oxidation, where we see an increase in oxidation state or loss of electrons. The oxidation of aluminum, where one aluminum atom gives up three electrons, is a good example of this. Oxidation and reduction processes combine together to make oxidation reduction reactions, sometimes abbreviated to redox reactions.
The question wants us to find, out of the five, what is always a characteristic of a reducing agent? What this means is we’re looking for a statement that is always, always true of a reducing agent. So if we find one case where it’s not true, then that can’t be a correct answer.
Statement A asserts that reducing agents always contain a halogen. Now let’s have a look at the combination of the iron and aluminum reactions we looked at earlier. If we react aluminum solid with iron three oxide, we get aluminum oxide and iron. Here we can see the oxidation state of aluminum increasing from zero to positive three. And at the same time, we can see the oxidation state of iron decrease from positive three to zero. So iron three oxide is undergoing reduction. And aluminum is our reducing agent, which doesn’t contain a halogen. So even though there are reducing agents that contain halogens, it is not a characteristic of reducing agents that they do.
So we can move on to statement B, which says that a defining characteristic of a reducing agent is that it contains hydrogen. We can go back to our original example. Aluminum does not contain any hydrogen. But we know it can act as a reducing agent. So it can’t be a characteristic of a reducing agent that it contain hydrogen.
Statement C says that a reducing agent must always contain oxygen. But aluminum does not contain any oxygen. Iron three oxide does contain oxygen. But iron three oxide is no acting as a reducing agent in this case.
So we can move on to statement D. Statement D says that it’s a defining characteristic of a reducing agent that it can be oxidized. We can see in our example that the aluminum atoms are undergoing oxidation. It is always true that something that can reduce something else will be oxidized itself. So it’s always true that a reducing agent can be oxidized.
So we have our correct answer. But just in case, let’s look at statement E. Statement E says that a reducing agent must always be soluble in water. While aluminum can react with water to produce aluminum ions, aluminum itself is not considered soluble.
So we’ve just proved that a reducing agent does not need to be soluble in water to be a reducing agent, meaning that, of the five characteristics given, the only one that is always true for a reducing agent is that it can be oxidized.