the＋比较级,the ＋比较级,表“越……,就越……”,组成一个主从复合句.前句为从句,后句为主句.你的句子中The hotter the mixture,the quicker the reaction,在mixture和reaction都省略了IS.
e.g.:
The higher the plane goes,the smaller it becomes．飞机飞得越高,它就变得越小.
The more we read,the more we know．我们读得越多,知道得就越多.
The lighter its colour is,the easier it is to pour the liquid．液体的颜色越淡,它就越容易倒出来.