What is meant by reactivity series of metal?

Explanation

### Step-by-Step Solution:

1. **Understanding the Concept**: The reactivity series of metals is a list that ranks metals based on their reactivity. Reactivity refers to how easily a metal can react with other substances, such as acids, water, or oxygen.

2. **Arrangement**: In the reactivity series, metals are arranged in **decreasing order of their reactivity**. This means that the most reactive metals are placed at the top of the series, while the least reactive metals are at the bottom.

3. **Reference Point**: The series often uses **hydrogen** as a reference point. Metals that are more reactive than hydrogen can displace hydrogen from acids, while those that are less reactive cannot.

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