Ligand to form a complex which is used to treat hard water

Options

A. `NH_(3)`
B. ox
C. en
D. EDTA

Answer

To solve the question regarding the ligand that forms a complex used to treat hard water, we will follow these steps:

### Step 1: Understand the Problem
We need to identify a ligand that can form a complex to treat hard water. Hard water contains high concentrations of calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which are responsible for its hardness.

**Hint:** Think about common ligands used in water treatment processes.

### Step 2: Identify the Ligand
The ligand that is commonly used to treat hard water is Ethylenediaminetetraacetic acid (EDTA). EDTA is a chelating agent that can bind to metal ions, forming stable complexes.

**Hint:** Consider ligands that can effectively bind to multiple metal ions.

### Step 3: Reaction Mechanism
When EDTA is added to hard water, it reacts with the calcium and magnesium ions. The reaction can be represented as follows:

\[ \text{Ca}^{2+} + \text{EDTA}^{4-} \rightarrow \text{[Ca(EDTA)]}^{2-} \]

\[ \text{Mg}^{2+} + \text{EDTA}^{4-} \rightarrow \text{[Mg(EDTA)]}^{2-} \]

These reactions show that EDTA forms stable complexes with both calcium and magnesium ions.

**Hint:** Look for the chemical equations that represent the formation of complexes.

### Step 4: Result of the Reaction
By forming these complexes, EDTA effectively removes calcium and magnesium ions from the water, thereby softening it. The water is then considered "soft" because the hardness-causing ions have been sequestered.

**Hint:** Think about the implications of removing hardness-causing ions from water.

### Step 5: Conclusion
The ligand used to treat hard water by forming complexes with calcium and magnesium ions is EDTA (Ethylenediaminetetraacetic acid).

**Hint:** Summarize your findings by stating the ligand and its role in treating hard water.

### Final Answer
The ligand used to form a complex that treats hard water is EDTA (Ethylenediaminetetraacetic acid).