Why it is hard or impossible to decompose a stable compound??? Such as - water is a stable compound and it doesn't decompose on heating.At very high temperature(2000°C- 3500°C), it decomposes very slightly to form oxygen and hydrogen.Explain it.

The amount of heating required depends on the degree to which the ion is polarized. More polarization requires less heat. The smaller the positive ion is, the higher the charge density, and the greater effect it will have on the negative ion. As the positive ions get larger, they affect on the negative ions near them less. More heat must be supplied to decompose the compound thermally.

I hope you can relate this with H2O