lǎo / shī / yòng / shí / ruǐ / zuò / zhǐ / shì / jì / bāng / zhù / wǒ / men / pàn / duàn / róng / yè / de / suān / jiǎn / xìng

老师用石蕊作指示剂，帮助我们判断溶液的酸碱性。

The teacher used litmus as an indicator to help us determine the acidity or alkalinity of the solution.