Some students investigated the effect of bile on the digestion of fat by lipase.

The students:
1. mixed milk and bile in a beaker
2. put the pH sensor of pH meter into the beaker
3. added lipase solution
4. recorded the pH at 2-minute intervals
5. repeated steps 1 to 4, but used water instead of bile.

Suggest two variables that the students should have controlled in this investigation. ​