Here is the full list of all 15 Portal achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Earn bronze medals on all Portal challenges.
Earn silver medals on all Portal challenges.
Earn gold medals on all Portal challenges.
Fall 30,000 feet.
Detach security cameras from the walls.
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Jump 300 feet.
Beat two Portal advanced maps.
Beat four Portal advanced maps.
Acquire the fully powered Aperture Science Handheld Portal Device.
Do whatever it takes to survive.
Make the correct party escort submission position decision.
Complete Portal.
Beat all six Portal advanced maps.
Knock down a turret with another turret.