The compound (X) on heating gives a colourless gas. The residue dissolved in water to obtain (Y). Excess CO2 is bubbled through aqueous solution of (Y), (Z), is formed. (Z) on gentle heating gives white (X). The compound (X) is :ACaCO3 BNa2CO3 CCaSO4.2H2O DK2CO3

The compound (X) is CaCO3 (Calcium Carbonate). Here's the step by step explanation:

1. When Calcium Carbonate (CaCO3) is heated, it decomposes to give Calcium Oxide (CaO) and Carbon Dioxide (CO2). This is in line with the problem statement that says compound (X) on heating gives a colorless gas (which is CO2 in this case).

2. The residue, Calcium Oxide (CaO), is then dissolved in water to obtain Calcium Hydroxide (Ca(OH)2), which is compound (Y).

3. When excess CO2 is bubbled through the aqueous solution of Calcium Hydroxide (Ca(OH)2), it forms Calcium Carbonate (CaCO3), which is compound (Z).

4. When Calcium Carbonate (CaCO3) is gently heated, it gives off CO2 and leaves behind white residue of Calcium Oxide (CaO). But since the problem statement says that (Z) on gentle heating gives white (X), it implies that (X) is Calcium Carbonate (CaCO3).

So, the compound (X) is CaCO3 (Calcium Carbonate).