Which of the following are Lewis acids ?
`H_2O , BF_3 ,H^+` and `NH_3`

Explanation

To determine which of the given substances are Lewis acids, we first need to understand the definition of a Lewis acid. A Lewis acid is defined as a substance that can accept an electron pair, while a Lewis base is a substance that can donate an electron pair.

Let's analyze each of the given substances:

1. **H₂O (Water)**:
- Water has two lone pairs of electrons on the oxygen atom. It can donate these lone pairs to form bonds. Therefore, H₂O acts as a Lewis base, not a Lewis acid.

2. **BF₃ (Boron Trifluoride)**:
...