Addition of ______ releases ______ which helps in killing the harmful bacteria present in water.

Explanation

**Step-by-Step Solution:**

1. **Identify the Substance:** The question asks for a substance that, when added to water, releases something that kills harmful bacteria.

2. **Recall Relevant Information:** From the video transcript, we learn that bleaching powder is a chemical that can be added to water.

3. **Chemical Formula:** The chemical formula for bleaching powder is Ca(OCl)₂, which is calcium oxychloride.

4. **Releasing Agent:** When bleaching powder is added to water, it releases chlorine gas (Cl₂).

5. **Function of Chlorine:** Chlorine gas is known for its disinfecting properties, which means it helps in killing harmful bacteria present in water.

6. **Final Answer Construction:** Therefore, the complete answer to the question is: "Addition of bleaching powder releases chlorine gas which helps in killing the harmful bacteria present in water."

**Final Answer:**
Addition of **bleaching powder** releases **chlorine gas** which helps in killing the harmful bacteria present in water.

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