Warm water is freeze faster than cold water,isn't it?
当陈述句为肯定式时,后面的附加疑问句用否定形式,即肯定的陈述句+否定的附加疑问句.陈述句是主系表结构,所以把系动词is的否定形式isn't和主语warm water（不可数名词）的代词it放到谋面组成isn't it
但是你这句子本身就有问题.把freeze看作动词原形,肯定不能做表语；看做名词,则意思不通.改成：
Warm water freezes faster than cold water,doesn't it?
这样就对了.陈述句是第三人称主谓结构的肯定句,则附加疑问句就要用第三人称的否定形式,所以用doesn't it.