1.9 Which ONE of the following compounds in solution will change the colour of bromothymol blue? A \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO} \) B \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \) C \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3} \) D \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOCH}_{3} \)

B \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \) will change the colour of bromothymol blue. As a weak acid, it can donate protons (H⁺) in solution, leading to a pH change that affects the color of this pH indicator. Bromothymol blue shifts from yellow at pH lower than 6.0 to blue at pH higher than 7.6, making it a fantastic tool for detecting acidity changes. Understanding pH indicators like bromothymol blue is key in various scientific fields, from chemistry to biology. In laboratories, both students and professionals use such indicators in titrations to determine the concentration of solutions through visual color changes. This not only helps in learning about acid-base chemistry but also aids in practical applications, such as quality control in food and beverage industries.