**Question:** Which of the following is not a method of chlorination of water:

A. Chlorination with sodium hypochlorite (NaClO)
B. Chlorination with chlorine gas (Cl2)
C. Chlorination with calcium hypochlorite (Ca(ClO)2)
D. Chlorination with sodium dichloroisocyanurate (NaCl2C3N4)

**Correct Answer: C. Chlorination with calcium hypochlorite (Ca(ClO)2)**

**Core Concept:** Chlorination is a process of adding chlorine to water to inactivate microorganisms, such as bacteria and viruses, and improve water quality. The correct chlorination methods involve compounds containing chlorine atoms bonded to a central atom.

**Why the Correct Answer is Right:** Calcium hypochlorite (Ca(ClO)2) is a chlorine-containing compound used for disinfecting water. When dissolved in water, calcium hypochlorite releases chlorine, which is effective in killing microorganisms and improving water quality.

**Why Each Wrong Option is Incorrect:**

**A. Chlorination with sodium hypochlorite (NaClO)**: Sodium hypochlorite (NaOCl) is another chlorine-containing compound used for water disinfection. It is commonly used in household bleach and is safe for chlorination purposes.

**B. Chlorination with chlorine gas (Cl2)**: Chlorine gas (Cl2) is a gaseous form of chlorine widely used for water disinfection. It is released from sodium dichloroisocyanurate (Na2S2O5) or calcium hypochlorite (Ca(ClO)2) when dissolved in water.

**C. Chlorination with calcium hypochlorite (Ca(ClO)2)**, as explained above, is the correct method.

**D. Chlorination with sodium dichloroisocyanurate (Na2S2O5)**: Sodium dichloroisocyanurate is a compound that releases chlorine gas (Cl2) when dissolved in water, making it suitable for water disinfection. Although the correct answer is B, providing an explanation for option C helps to understand the misconception and differentiate between Cl2 released from the compound and Cl2 released from calcium hypochlorite.

**Clinical Pearl:** Calcium hypochlorite (Ca(ClO)2) is the correct method for chlorination of water. When dissolved in water, it releases chlorine gas (Cl2) through the reaction:

Ca(ClO)2 + H2O → Ca(OH)2 + HCl + Cl2

However, option C refers to calcium hypochlorite (Ca(ClO)2), which, as mentioned, releases chlorine gas (Cl2) when dissolved in water. This misconception arises due to the confusion between the compound and the gas itself, which is used for water disinfection.