英语从句省略
A liquid that might be a poor conductor when pure is often used to make solutions that readily transmit electricity.请问在本句话的从句中（即when后的句子里）为什么形容词pure可以作主语?为什么不写成purity?
如果 when后省略了 it is那整句话是不是应该变成：A liquid that might be a poor conductor when it is pure that is often used to make solutions that readily transmit electricity.但如果这样算起来that就是第二个从句的引导词,为什么也能省略?这样不就变成一个句子中有两个谓语的错误用法了吗?还是应该这样理解：分析英语语法时,句子中的省略成分可以不再考虑范围内?