When two or more compounds having the same molecular formula differ in physical or chemical properties, the phenomenon is known as ......... and such compounds are called ...

Explanation

To solve the question, we need to identify the phenomenon when two or more compounds have the same molecular formula but differ in their physical or chemical properties, and what we call these compounds.

### Step-by-Step Solution:

1. **Understanding the Question**: The question states that we have two or more compounds with the same molecular formula but differing in physical or chemical properties. We need to find the term that describes this phenomenon and the name for the compounds.

2. **Identifying the Phenomenon**: The phenomenon where compounds have the same molecular formula but differ in their structures (and thus their physical and chemical properties) is known as **isomerism**.

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