The temperature influences in the movement of molecules, the energy they contain.
When there is more energy (hotter) molecules move more therefore there is less surface tension.
When there is less energy (colder) molecules moce less therefore there is more surface tension.
To find the answer to "What is the purpose of this investigation?" you'll have to go through the context and through what you've studied so far. But the answer should be something like this: "To test the effects of temperature variation and surface tension".
Hope it helped,
BioTeacher101