Identify the correct statement abouut orthoboric acid:

Options

A. It has a layer structure in which planar `BO_(3)` units are joined by hydrogen bonds
B. Orthoboric acid `(H_(3)BO_(3))` is a weak monobasic lewis acid
C. On heating ortho-boric acid form meta-boric acid and on further heating to red hot, forms boric oxide anhydride
D. it is obtained by reaction borax withh dilute HCl using phenolphthalein as an indicator

Answer

To identify the correct statement about orthoboric acid (H3BO3), we can analyze each statement based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Understanding the Structure of Orthoboric Acid**:
- Orthoboric acid has the formula H3BO3 or BOH3.
- It has a layer structure where planar BO3 units are joined by hydrogen bonds.
- The structure is trigonal planar due to sp2 hybridization of boron.

**Hint**: Recall the molecular geometry and hybridization of boron in H3BO3.

2. **Assessing the Acidic Nature**:
- Orthoboric acid is classified as a weak monobasic acid.
- It does not release H+ ions directly; instead, it interacts with water to form BOH4- and releases H+ from water.
- This indicates that it acts as an indirect acid.

**Hint**: Consider how acids typically release H+ ions and how orthoboric acid behaves differently.

3. **Thermal Decomposition**:
- Upon heating, orthoboric acid (H3BO3) can dehydrate to form metaboric acid (H4B2O7) and other byproducts.
- Further heating of metaboric acid can lead to the formation of borax or boric oxide (B2O3).

**Hint**: Think about the thermal stability of acids and their decomposition products.

4. **Reaction with Borax**:
- When borax (Na2B4O7·10H2O) is treated with dilute HCl, orthoboric acid is formed along with other byproducts.
- The indicator used in this reaction is methyl orange, not phenolphthalein as stated in the question.

**Hint**: Remember the specific indicators used in acid-base reactions and their significance.

### Conclusion:
Based on the analysis of the statements:
- **Statement 1**: Correct (layer structure with hydrogen bonds).
- **Statement 2**: Correct (weak monobasic acid).
- **Statement 3**: Correct (decomposes to metaboric acid upon heating).
- **Statement 4**: Incorrect (methyl orange is the correct indicator, not phenolphthalein).

Thus, the correct statements about orthoboric acid are 1, 2, and 3.