Bromothymol blue is acid base indicator which has pH intervals 6 - 7.6 and its two colors are yellow in acid and blue in basic. The pH of CaO is about 11.5 to 12.5 this will make the indicator has blue color at the beginning. By bubbling CO₂ to CaO this leads to formation of CaCO₃ which also basic (pH about 9.5 - 10) so the color of indicator remains blue No color change of the indicator will observed (remains blue)
