Which of the following statements are incorrect ?

Options

A. `CaH_2` is also called hydrolith
B. `BeCl_2` in vapour phase exist as polymeric
C. `MgSO_4` is less soluble than `CaSO_4` in water
D. `MgCO_3` decomposes at lower temperature than `BaCO_3` .

Answer

To determine which statements are incorrect, let's analyze each statement step by step:

### Step 1: Analyze Statement A
**Statement A:** CaH2 is called Hydrolith.
- **Analysis:** This statement is correct. Calcium hydride (CaH2) is indeed known as Hydrolith in the market. It reacts with water to produce hydrogen gas.

### Step 2: Analyze Statement B
**Statement B:** BeCl2 in vapor phase exists in polymeric form.
- **Analysis:** This statement is incorrect. Beryllium chloride (BeCl2) in the vapor phase does not exist in a polymeric form; it exists as a dimer (Be2Cl4).

### Step 3: Analyze Statement C
**Statement C:** MgSO4 is less soluble than CaSO4 in water.
- **Analysis:** This statement is incorrect. Magnesium sulfate (MgSO4) is more soluble in water than calcium sulfate (CaSO4). The hydration energy of Mg2+ is greater due to its smaller size compared to Ca2+, leading to greater solubility.

### Step 4: Analyze Statement D
**Statement D:** MgCO3 decomposes at lower temperature than BaCO3.
- **Analysis:** This statement is correct. Magnesium carbonate (MgCO3) decomposes at a lower temperature than barium carbonate (BaCO3) due to the greater covalent character of MgCO3, making it less stable.

### Conclusion
Based on the analysis:
- **Correct Statements:** A and D
- **Incorrect Statements:** B and C

Thus, the incorrect statements are B and C.