How many compounds form acidic solution when dissolved in water
`H_(3)PO_(4), H_(3)BO_(3),Na_(2)B_(4)O_(7).10H_(2)O, H_(3)P_(3)O_(9), Ba(OH)_(2) Ca(OH)_(2),PbO,CO`

Explanation

To determine how many compounds from the given list form acidic solutions when dissolved in water, we need to analyze each compound based on its chemical nature and behavior in water.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- H₃PO₄ (Phosphoric acid)
- H₃BO₃ (Boric acid)
- Na₂B₄O₇·10H₂O (Sodium tetraborate decahydrate)
- H₃P₃O₉ (Triphosphoric acid)
- Ba(OH)₂ (Barium hydroxide)
- Ca(OH)₂ (Calcium hydroxide)
- PbO (Lead(II) oxide)
- CO (Carbon monoxide)

2. **Analyze Each Compound**:
- **H₃PO₄**: This is an oxoacid and will dissociate in water to release H⁺ ions, thus forming an acidic solution.
- **H₃BO₃**: This is also an acid (boric acid) and will release H⁺ ions in water, forming an acidic solution.
- **Na₂B₄O₇·10H₂O**: This is a salt that will dissociate in water to release Na⁺ and B₄O₇²⁻ ions. It does not release H⁺ ions, hence it forms a basic solution.
- **H₃P₃O₉**: This is another oxoacid and will dissociate to release H⁺ ions, forming an acidic solution.
- **Ba(OH)₂**: This is a strong base that will dissociate to release OH⁻ ions, thus forming a basic solution.
- **Ca(OH)₂**: Similar to Ba(OH)₂, this is also a strong base and will release OH⁻ ions in solution, forming a basic solution.
- **PbO**: This compound can behave as amphoteric, meaning it can act as both an acid and a base depending on the conditions. However, in water, it does not significantly release H⁺ ions, so it does not form an acidic solution.
- **CO**: Carbon monoxide is neutral and does not dissociate to form H⁺ or OH⁻ ions in water.

3. **Count the Acidic Compounds**: From the analysis:
- H₃PO₄: Acidic
- H₃BO₃: Acidic
- H₃P₃O₉: Acidic
- Na₂B₄O₇·10H₂O: Not acidic
- Ba(OH)₂: Not acidic
- Ca(OH)₂: Not acidic
- PbO: Not acidic
- CO: Not acidic

The acidic compounds are H₃PO₄, H₃BO₃, and H₃P₃O₉.

### Conclusion:
Thus, the total number of compounds that form acidic solutions when dissolved in water is **3**.