我讨厌化学课，因为要背太多公式。

I hate ___chem class because there are too many formulas to memorize.

这种化学物质用于清洁泳池。

This ___chem is used for cleaning swimming pools.

医生建议他在手术后立即开始化疗。

The doctor advised him to start ___chemotherapy immediately after the surgery.