Mixing bromothymol blue with calcium chloride results in a change in the color of the solution. Bromothymol blue is a pH indicator, meaning it changes color depending on the acidity or basicity of a solution. At a neutral pH, bromothymol blue appears green. When mixed with calcium chloride, the solution becomes more acidic, causing the bromothymol blue to change color to yellow.

The reaction that occurs when bromothymol blue and calcium chloride are mixed is a metathesis reaction, also known as a double-replacement reaction. In this type of reaction, the positive ions (cations) of one compound switch places with the positive ions of another compound, and the negative ions (anions) of the two compounds also switch places. In this case, the calcium ions (Ca2+) from calcium chloride replace the hydrogen ions (H+) from bromothymol blue, forming calcium bromothymol blue and hydrochloric acid (HCl).

The chemical equation for the reaction is:

CaCl2 + HIn (bromothymol blue) → CaIn2 + 2HCl

where HIn represents the bromothymol blue indicator.

This reaction demonstrates the principle of acid-base chemistry, where an acid (HCl) is formed by the combination of a hydrogen ion (H+) and an anion (Cl-), and a base (calcium bromothymol blue) is formed by the combination of a cation (Ca2+) and an anion (In-).