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Question Video: Explaining the Cause of Sodium Metal’s Reaction with Water Science

When a piece of sodium metal is placed into a large trough of cold water, a chemical reaction takes place. Which of the following statements best explains why this is the case? [A] Sodium is more active than hydrogen. [B] Sodium is less active than hydrogen. [C] Sodium is more active than water. [D] Sodium is less active than water.

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Video Transcript

When a piece of sodium metal is placed into a large trough of cold water, a chemical reaction takes place. Which of the following statements best explains why this is the case? (A) Sodium is more active than hydrogen. (B) Sodium is less active than hydrogen. (C) Sodium is more active than water. (D) Sodium is less active than water.

In this question, we are told that a chemical reaction occurs when sodium metal is placed into cold water. Sodium metal is a metal found in group one on the periodic table. Group one metals are very active metals. Sodium and potassium metal, which are both group one metals, appear at the very top of the series of chemical activity. Metals found at the top of the series are the most active, and metals found toward the bottom of the series are the least active.

Group one metals can replace hydrogen in substitution reactions because they are more active than hydrogen. In a single-substitution reaction, a more active metal element will replace a less active metal element in a chemical compound. It is also possible for an active metal to replace hydrogen in a compound as long as hydrogen is less active.

In this question, we know that the reactants of the single-substitution reaction are sodium, the group one metal, and water. The products of this type of single-substitution reaction are a group one hydroxide and hydrogen gas. Let’s write a word equation to represent the reaction between sodium and water. We can write sodium plus water before the reaction arrow and sodium hydroxide plus hydrogen gas to represent the products. Now, let’s write a chemical equation for the reaction. The chemical formulas of the reactants are Na followed by the state symbol “s” for sodium solid and H2O followed by the state symbol “l” for liquid water. The product sodium hydroxide is a solution. The chemical formula is NaOH followed by “aq” to represent an aqueous solution. And the chemical formula for hydrogen gas is H2 followed by the state symbol “g.”

Now that we have this basic unbalanced equation, let’s take a closer look at what’s going on in this reaction. We know from the series of chemical activity that sodium is more active than hydrogen. During the reaction, the more active sodium atoms replace the less active hydrogen atoms in the water molecules. And the hydrogen atoms that got replaced combine to form hydrogen gas. We’ve confirmed that a single-substitution reaction takes place because sodium is a more active element than hydrogen. The statement which best explains why sodium metal reacts with water is answer choice (A). Sodium is more active than hydrogen.

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