To detect the presence of CO2 ​ ________ water is used.

Explanation

To detect the presence of CO2, lime water is used. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the substance used for detection
To detect carbon dioxide (CO2), we need to identify the appropriate reagent. The common reagent used for this purpose is lime water.

### Step 2: Define lime water
Lime water is a dilute solution of calcium hydroxide (Ca(OH)2) in water. It is important to note that lime water is not just any water; it specifically refers to this solution.

### Step 3: Write the reaction
When carbon dioxide is bubbled through lime water, a chemical reaction occurs:
\[ \text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]
In this reaction, calcium carbonate (CaCO3) is formed along with water.

### Step 4: Observe the result
Calcium carbonate is insoluble in water, which results in the formation of a white precipitate. This precipitate gives the lime water a milky appearance, indicating the presence of CO2.

### Step 5: Additional observation
If excess CO2 is passed through the lime water, the milkiness disappears. This happens because calcium carbonate reacts with additional CO2 to form calcium bicarbonate, which is soluble in water:
\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3\text{)}_2 \]

### Final Answer
To detect the presence of CO2, **lime water** is used.

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