The process due to which some solid substance directly vaporise on heating and solidify on cooling, without becoming a liquid is called ________ .

Explanation

**Step-by-Step Solution:**

1. **Understanding the Process**: The question describes a process where a solid substance can change directly into a vapor when heated and then revert back to a solid when cooled, without passing through the liquid state.

2. **Identifying the Term**: The specific term that describes this direct transition from solid to gas and vice versa is crucial. This process is not common for all substances but is characteristic of certain solids.

3. **Defining the Process**: The process where a solid turns into vapor upon heating and vapor turns back into solid upon cooling is known as **sublimation**.

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