Mix the powder with boiling water.
为什么用boiling water,不用boiled water?
boiling water 指刚开的水(很烫),boiled water指开过的水(不一定烫)。比较:
She scalded herself on some boiling water. 她被开水烫伤了。
If you give your baby juice, dilute it well with cooled, boiled water. 喂婴儿果汁要用凉开水充分稀释。
你的句子指的用热开水把粉末调和,故用的是boiling water。