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Question Video: Identifying the Statement That Best Describes a Substitution Reaction Science

Which of the following statements best describes a substitution reaction? [A] A more active metal replaces hydrogen in another compound. [B] A more active metal replaces another less active metal in another compound. [C] A less active metal forms a compound with another more active metal from another compound. [D] A less active metal replaces another more active metal in another compound. [E] A more active metal forms a compound with another less active metal from another compound.

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Video Transcript

Which of the following statements best describes a substitution reaction? (A) A more active metal replaces hydrogen in another compound. (B) A more active metal replaces another less active metal in another compound. (C) A less active metal forms a compound with another more active metal from another compound. (D) A less active metal replaces another more active metal in another compound. Or (E) a more active metal forms a compound with another less active metal from another compound.

The word substitution means to swap or replace. Options (A), (B), and (D) all involve replacing one metal with another, whereas options (C) and (E) involve forming a compound from two metals. So as neither option (C) nor option (E) involve replacing or substituting one substance for another, then they cannot be used to describe a substitution reaction and therefore cannot be the answer to this question.

We’ve now narrowed the answers down, so all statements involve substituting one element for another. But we now need to figure out whether (A), (B), or (D) is correct. To do this, we should consider the series of chemical activity. It is a list showing the chemical activity of metals with the nonmetal hydrogen as a reference.

Option (A) states that a more active metal replaces hydrogen in another compound. A metal can replace hydrogen in another compound. For example, an atom of sodium can replace one of the atoms of hydrogen in a molecule of water, producing sodium hydroxide and hydrogen. For a metal to replace hydrogen in another compound, it must be more active than hydrogen, so it must be higher than hydrogen in the series of chemical activity, which sodium is.

But although a more active metal can replace hydrogen in another compound, the metal doesn’t just have to replace hydrogen; it can replace another metal. For example, magnesium can react with lead nitrate to form magnesium nitrate and lead. As the metal is not replacing hydrogen, we can rule out option (A).

The difference between the two remaining answer choices is that one says “A more active metal replaces another less active metal in another compound” and the other states that a less active metal replaces another more active metal in another compound. We can see from this example that magnesium is replacing the lead in a lead compound. Magnesium is higher up in the series of chemical activity than lead is, so magnesium is more active than lead. Therefore, this substitution reaction involves a more active metal replacing another less active metal in another compound. This best matches the statement in (B). Thus, option (B) is the correct answer. And option (D) is incorrect.

So the answer to the question “Which of the following statements best describes a substitution reaction?” is (B). A more active metal replaces another less active metal in another compound.

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