When a grey coloured object made of metal A is left exposed to damp air for a considerable time, it gets covered with a red-brown flaky coating by the process called B which eats up the whole object gradually. It is said that the presence of C and D is necessary for this process to take place. If this object is galvanised by metal E, then the process B does not occur.
What is D ?

Answer

To solve the question, we need to identify the components involved in the rusting process, which is described in the scenario. Let's break down the solution step by step.

### Step-by-Step Solution:

1. **Identify Metal A**: The grey-colored object made of metal A is likely iron. This is because iron is known to rust and is commonly grey in color.

**Hint**: Think about common metals that rust and their typical colors.

2. **Understand Process B**: The process B that leads to the red-brown flaky coating is rusting. Rusting is a chemical change that occurs when iron reacts with oxygen and moisture in the air.

**Hint**: Recall what happens when iron is exposed to air and water over time.

3. **Identify the Coating**: The red-brown flaky coating mentioned is rust, which is primarily hydrated iron(III) oxide (Fe2O3·xH2O).

**Hint**: Consider what the flaky substance that forms on iron is called.

4. **Components C and D**: The presence of C and D is necessary for rusting to occur. In the context of rusting, C can be identified as oxygen (O2), and D can be identified as water (H2O). Both are essential for the rusting process.

**Hint**: Think about what substances are required for iron to corrode.

5. **Galvanization with Metal E**: If the object is galvanized with metal E (which is zinc), it prevents rusting. Galvanization is a protective process that coats iron with zinc to prevent it from coming into contact with moisture and oxygen.

**Hint**: Recall what galvanization involves and which metal is commonly used for this purpose.

6. **Conclusion for D**: Since the question asks for D, which is necessary for the rusting process, we conclude that D is water.

**Final Answer**: D is water (H2O).

### Summary:
- Metal A is iron.
- Process B is rusting.
- C is oxygen.
- D is water.
- Metal E is zinc (used for galvanization).