If ammonia is added to pure water, the concentration of a chemical species already present will decrease. The species is

Options

A. `O_(2)^(Theta)`
B. `overset(Theta)OH`
C. `H_(3)O^(o+)`
D. `H_(2)O`

Explanation

To solve the question, we need to analyze the reaction that occurs when ammonia (NH₃) is added to pure water. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Reaction
When ammonia is added to water, it reacts with water molecules. The reaction can be represented as follows:

\[ \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \]

### Step 2: Understand the Role of Ammonia
In this reaction, ammonia (NH₃) acts as a Lewis base. It accepts a proton (H⁺) from water (H₂O), which acts as a Lewis acid. This results in the formation of ammonium ions (NH₄⁺) and hydroxide ions (OH⁻).

### Step 3: Identify the Species Already Present
In pure water, the concentration of hydronium ions (H₃O⁺) is very low, but it is present due to the autoionization of water:

\[ \text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^- \]

### Step 4: Analyze the Effect of Adding Ammonia
When ammonia is added to the water, it increases the concentration of hydroxide ions (OH⁻) in the solution. According to Le Chatelier's principle, the increase in OH⁻ concentration will shift the equilibrium of the water autoionization reaction to the left, leading to a decrease in the concentration of hydronium ions (H₃O⁺).

### Conclusion
Thus, the chemical species whose concentration decreases when ammonia is added to pure water is:

**H₃O⁺ (hydronium ions)**

### Final Answer
The species whose concentration will decrease is **H₃O⁺**.

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