This section was added per request for being able to complete all achievements along with a playthrough. Here is a list of Chapters and achievements completeable in the chapter. Many achievements can be completed on multiple chapers and where you see an achievement under a certain chapter, that is the earliest point in game where you can complete it.

NOTE: The order of appearance should be accurate for what achievements appear in which chapter, but the order of achievements in a chapter may be wrong. If anyone can point out mistakes please comment.

Throughout the game:

• Rare Specimen (Find the Hat and bring it to Xen.)
  
• The Rarest Achievement (Continue carrying the purple hat through Xen.)
  
• Caffeine Extraction (Get drinks from vending machines.)
  
• Calculated Trajectory (Kill 10 jumping headcrabs mid air.)
  
• Gray Matter Propulsion (Kill 20 enemies with magnum headshots.)
  
• Prophylactic Suggested (Let a headcrab kill you.)
  
• Well-Done (Equip a grenade and hold mouse1 until it explodes in your hand.)
  
• One Step Ahead (Kill 10 enemies with trip mines.)

Anomalous Materials

• Proverbial Thinkerer (Use alarm and laptop.)
  
• Convection Refinement (Use the microwave in the room before the Hazard Suit.)
  
• Brownian Motion (Give toilet paper to scientist near where you find the Hazard Suit.)
  
• Less Than Desirable Consequences (Die to the beam in test chamber.)
  
• Resonance Procrastinator (Don't push the cart to the beam in test chamber.)

Unforeseen Consequences

• Inflammatory Actions (Kill a headcrab with fire.)
  
• Rare Specimen (This is where you find the Hat. Take it through the game to Xen.)
  
• Make sure you find grenades in Unforeseen consequences to be able to complete Mega Herz on the next chapter. (See Mega Hertz for detailed video guide on finding grenades.)

Office Complex

• Permeable Infrastructure (Find the alternate route.)
  
• Mega Hertz. (Kill 5 enemies with one grenade.)
  
• Our Brains And Your Brawn

We've Got Hostiles

• Unified Coupling Theorum (Lure Headcrabs into Barnacles.)
  
• Calculated Trajectory (You'll find shotgun in this chapter. Kill headcrabs mid air.)
  
• Digital Wizardry (Kill 10 enemies with a turret.)
  
• Kinetic Repulsion (Kill a Marine with their own grenade.)

Blast Pit

• Remote Detonation (You will find satchels in this chapter. Kill a barnacle with them.)

Power Up, On a Rail (no specific achievements)

Apprehension (Start saving Magnum ammo for the end of the chapter.)

• Dead Reckoning (Kill an assassin with a magnum)

Residue Processing (no specific achievements)

Questionable Ethics

• Ethically Questionable (Do five experiments.)
  
• Universal Gravitation (You'll finds Snarks in this chapter. Kill yourself with them.)
  
• Futile Resistance (Kill 10 enemies with Snarks.)
  
• Quantum Capacitance (Overcharge Tau Cannon after finding it.)

Surface Tension

• Stealth Technology (Escape the Hangar without alerting the Soldiers.)
  
• Nuclear Fishin'
  
• Centripetal Attraction (Kill 20 enemies with the Hivehand.)

Forget About Freeman (no specific achievements)

Lambda Core

• Premature Expulsion (Use entire clip of Gluon gun.)
  
• Hyper Saturation Conundrum (Drown in the Lambda Core.)

Below you can find all achievements in alphabetical order with detailed information on how to achieve them. Use the Guide index to navigate or search with ctrl + F.

Pass the toilet paper to the scientist in need.

When you exit the train in the beginning get into the facility and follow the green line on the wall to reach the locker room. To the left is the Hazard suit and to the right is the toilets. Take a toilet paper from the sink and drop it to the guy in need.

http://steamproxy.net/sharedfiles/filedetails/?id=442137077

Perform surgical caffeine extraction from 20 machines.

Use vending machines throughout the game to get out total of twenty cans. Each can you consume replenish 1 HP and you can get cans out of a machine until you reach 100 HP or the vending machine runs out of cans. Every can you get counts towards the achievement. Or you can simply shoot a machine to empty it at once, but you have to be below 100hp to drink them. You can also use save/load game trick to get this achievement faster.

http://steamproxy.net/sharedfiles/filedetails/?id=439176537

Kill 10 Airborne Headcrabs using the Shotgun.

When ever you see headcrabs, equip shotgun and get close to them. Wait for the headcrab to jump at you then shoot them mid air.

Kill 20 enemies with the Hivehand.

Hivehand is a weapon found later in the game. It's a thing you wear, I suppose, and it shoots things. Unlimited automatically reloading things.

NOTE: summer 2015 there was a bug where hive hand was not found on the elevator in the big booby trap room. On 30th July developer Chon Kemp said: "It is bugged and will be fixed in the next update."

Ruin the microwave casserole.

In the beginning of the game before you get to wear your hazard suit, there is a little staff room just before the locker rooms on the right hand side of the corridor. Use the microwave to ruin someone's food. There's a scientist and a security guard in the room.

http://steamproxy.net/sharedfiles/filedetails/?id=439724604

Kill an Assassin with the .357 magnum

You'll first encounter assassin a short while after finding the crossbow the first time. Crossbow can be found in the rusty cage hanging over the water. Shortly after you take an elevator ride up and when the elevator door opens a security guards tells Gordon he has a message for him. Assassin will kill the security guard in the middle of the sentence. Retaliate by blowing the assassins head off with a magnum.

If you miss this opportunity (like I did because magnum was empty) you encounter some more assassins later in the game.

Kill 10 enemies with a reprogrammed Sentry.

When you start seeing sentry guns run behind them and pick them up and go find enemies. When you use a sentry to kill 10 enemies you get the achievement. If you avoid the red lasers the sentry guns wont go active and you can easily sneak up on them.

http://steamproxy.net/sharedfiles/filedetails/?id=439815495

IHaveNoMouthAndIMustMeme commented below: "For Digital Wizadry, throwing turrents at enemies, and killing hem, also counts. I threw a turret at a headcrab at the beginning of "We've got Hostiles" and got the acheivement."

Conduct all of the questionable research experiments.

When you reach the chapter Questionable Ethics you'll find experiment labs. You need to do all the researches for this achievement.

1/5 The first room you enter when Questionable Ethics chapter starts you'll find a grunt in the middle of the room. Go inside the control room facing the grunt and hit one of the buttons to kill it.

http://steamproxy.net/sharedfiles/filedetails/?id=439267087

2/5 Exit the grunt room by crouching under the door and up the corridor straight ahead through a door labeled as: A07-Minor Xenos. Spam the launch button in the console until you get the achievement notification.

http://steamproxy.net/sharedfiles/filedetails/?id=439270426

3/5 Proceed the corridor to the next big room with stairs immediately to the right. In the room upstairs you'll find "Door release" button. Press it to release a bullsquid, next hit the big red button "Danger push to fire" to kill it.

http://steamproxy.net/sharedfiles/filedetails/?id=439270561

BEFORE using the experiment gas feed on 4/5 see achievement Futile Resistance

4/5 Now proceed a long way. Go past all the landmarks mentioned next: male locker room, front door of facility(Lobby), B01-Conference room, B02-Aquatics, B04-Cryonics (frozen room with two grunts), B05-Autopsy. After going past all of the mentioned you'll find a door called B08-CES Chamber. Enter the room and pick up a bottle of gas on the counter and put it in the "experiment gas feed".

http://steamproxy.net/sharedfiles/filedetails/?id=439271079

5/5 Exit the B08-Ces Chamber and follow the signs upstairs towards Lab D and find a room D07-Primary Laser. Complete an easy puzzle to power up the big laser gun (activate lasers in multiple rooms nearby). Make sure the shield plug is on the wall and hit the "Warning laser test fire" -switch to get the last piece of the achievement.

http://steamproxy.net/sharedfiles/filedetails/?id=439271385

Kill 10 enemies with Snarks.

Snark is a weapon found in the room B08-CES Chamber (see achievement Ethically Questionable 4/5). This is also a very good spot to get the 10 Snark kills. Save game. Enter the alien habitat area where the headcrabs are and throw all your Snarks in the room and let Snarks do their job. Now load game and do it again (and again if necessary).

Headshot 20 enemies with the .357 magnum.

Headshot your enemies using the .357 magnum. Easiest to do on Zombies and Marines. So go ahead and shoot their heads off until you get the achievement.

Drown in Lambda Core's coolant.

In chapter Lambda Core after getting the Gluon gun take the elevator down to the Lambda Coolant System and find water to drown into. See image below.

http://steamproxy.net/sharedfiles/filedetails/?id=439367546

Kill a Headcrab using fire.

This is easily done in the beginning soon after the accident. You'll encounter plenty of headcrabs and find flares on tables and lying on the ground. Pick up a flare and poke it at a headcrab to light it on fire (you don't have to throw it). Wait for the fire to kill it.

Kill a Marine with their own grenade.

When a marine throws a grenade at you quickly run to the grenade and press E to pick it up then aim at the marine and throw it back. If the grenade kills the marine you'll unlock the achievement.

Kill 5 enemies with one grenade.

Good spot to do it in a screenshot below early in the game in the Office Complex chapter at the cafeteria. When you get close to the room the ceiling will collapse and more headcrabs will fall down. Make em suffer!

http://steamproxy.net/sharedfiles/filedetails/?id=439176464

How to find your first grenades: In Unforeseen Consequenses when you encounter your first Houndeye you have to complete a small puzzle of turning two red valves to proceed. Before you proceed make sure you find a scientist upstairs and lead him across the catwalk to a security room and he'll open it for you. There you'll find grenades and some ammo. Watch video below.

https://www.youtube.com/watch?v=-0c9XAclpOo

Another good spot comes later is at Questionable Ethics chapter when doing the achievement Ethically Questionable: specimen 4 - gas the headcrabs. Before using the gas, make a save game and go in the specimen room and grenade the headcrabs. (Screen shot below. Also read more info from Ethically Questionable achievement.)

https://steamproxy.net/sharedfiles/filedetails/?id=575396203

Kill an Icthysosaur with either the Tau or Cluon.

Once you get outside Black Mesa and you proceed killing every marine on the way you also encounter a helicopter that you need to take out with Tau Cannon. After the chopper fight look at the water on the right side of the dam. The water is very clear so you can see the fish from afar. Use Tau Cannon to easily take down the fish. There are two icthysosaurs in the water. You can sometimes even see the them when standing on the dam, but usually only appear once you dive into water.

http://steamproxy.net/sharedfiles/filedetails/?id=440407468

Use the alternate path in Office Complex.

In the chapter Office Complex in a big office area where many Vortigaunts spawn is a room that leads you to an alternative route. After killing the Vortigaunts find a room with ladders going up behind a sofa. Enter a ventilation system and get out the other end. In the screen shot below I am about to enter the office area with vortigaunts and straight ahead where my crosshair points is the room where you access the ladder.

http://steamproxy.net/sharedfiles/filedetails/?id=439176375

Use an entire clip of Gluon ammo in one continuous shot.

Soon in the chapter Lambda Core you meet scientists that have been waiting for you. You'll find a shooting range where the Gluon gun is waiting. Get your ammo to the max (100) and empty the gun with one long continuous shot.

http://steamproxy.net/sharedfiles/filedetails/?id=440667705

Get overtaken by a Headcrab's ample anterior

Get killed by a headcrab. Simple as that.

Interfere with the alarm and laptop in Anomalous Materials.

In the very beginning of the game when you exit the train and get the door opened by the security guard you'll enter the facility and when you enter the lobby "Anomalous Materials" -title appears on screen. Immediately turn left and use the computer in the corner. Now go behind the info desk and use the alarm button under the counter. After using both the computer and the alarm you'll unlock the achievement.

http://steamproxy.net/sharedfiles/filedetails/?id=442139009

Overcharge it anyway.

When you get your hands on the Tau Cannon use the overcharge (weapon secondary) long enough to hurt yourself. You'll find this weapon near the big laser gun (See 5/5 Ethically Questionable achievement).

http://steamproxy.net/sharedfiles/filedetails/?id=439274417

Send the Hidden Hat to Xen.

Find the hidden hat and carry it all the way through the game and send it to Xen via portal. It is suggested to make a lot of save games throughout cause you tend to lose the hat from time to time.

Where to find it:
When you first exit the train in the beginning and you step out onto the first platform where the guards meets you and opens the door to the facility notice the pipes to the left of the door just outside the platform. After the accident happens you'll backtrack your way up to this platform again and you are now able to jump on the platform railing and on to the pipes. Use flashlight to locate the hat in a dark corner.

http://steamproxy.net/sharedfiles/filedetails/?id=439722636
http://steamproxy.net/sharedfiles/filedetails/?id=439732269

http://youtu.be/LWh-XB7fnxU

Parts you can complete without carrying the hat around:
There are some long puzzles where you can leave Mr. Hat waiting and go alone to finish the puzzle. If you leave the hat in a safe place you can go far away past loading screens and it will be there when you get back. I still suggest to save game before leaving Mr. Hat behind. Also note the level of helpfulness on a scale minor-medium-major.

• Major: In Office Complex when you reach the cafeteria (see Mega Hertz achievement screenshot) look up and you can throw the hat to the upper level. Mr. Hat will be waiting for you before the elevator shaft jump.
  
• Major: In We've got hostiles right in the beginning you can leave Mr. Hat next to the First aid and HEV suit chargers where the scientist goes crazy and the security guard gets pulled into ventilation shaft by a zombie. You'll get back here near the end of the chapter.
  
• Major: Tentacles (aka 3-headed monster): leave Mr. Hat waiting near the security guard. Finish the puzzle: power up the engine and fuel up and return to kill the monter with fire.
  
• Medium: Power up rail way: You can leave Mr. Hat with the wounded security guard in the control room. Then power up the railway and go pick up Mr. Hat for train ride.
  
• Medium: In Questionable Ethics leave it at the main lobby. You'll return here later after finding a group of scientist to open the front door for you.
  
• Minor: In Surface Tension after fighting a tank and an LAV you soon go through heavy double doors and see a mine field on the left. Throw Mr. Hat over the fence near an antennae leaning on the main building.
  
• Minor: In Surface Tension the laser maze room where you find Hivehand you can bounce Mr. Hat off the ceiling into the cage with Hivehand to make the rest much easier. Easiest way to do this is to enter the room through the left most option.
  
• Major: Inside Lambda Core after getting the Gluon gun and you take an elevator ride down to Lambda coolant system you can leave Mr. Hat to the left from the elevator to the corridor to maintenance station then go finish the flooding puzzle before continuing to maintenance station.

Transporting the hat with train
When you get to the sections with the train you can try these two following spots to place the hat to be able to cart it through the tracks (thanks to: FURIOUS SPRITE BRADBERRY)
https://steamproxy.net/sharedfiles/filedetails/?id=1637046351
https://steamproxy.net/sharedfiles/filedetails/?id=1637046152



You'll get separated two times:

• After you get knocked unconscious by marines you find the hat again after climbing off the trash compactor. Mr. Hat will be sitting on the edge watching you struggle. http://steamproxy.net/sharedfiles/filedetails/?id=439994621
  
• When you exit Black Mesa and reach the dam you need to get to the lower area. You'll lose the hat in the process (while riding the tube down the dam). Took me a while to realize the hat will get off the tube fast down stream as the current is strong here. Look for it down stream. (Make an auto save before entering the tube as it took a few tries to locate the hat). http://steamproxy.net/sharedfiles/filedetails/?id=440402747

Dedicated section to the final portal puzzle:
When you're nearly at the end you have the last portal puzzle. Very annoying portal puzzle. You must walk into the portal hat first. Mr. Hat must go into the portals or it wont teleport with you. And everytime you exit a portal you are gonna drop the hat so you'll have to be quick to pick it up again. Also keep in mind that you can always look up to see the exit portal and the platform passing under it. The correct portals to enter are numbered 2, 4 and 7. See the video below.

http://steamproxy.net/sharedfiles/filedetails/?id=440667479
https://youtu.be/WlvwobOenVc

Kill a Barnacle with a Satchel.

After you start encountering marines you'll soon find Satchels too. Throw a satchel into the tongue of a barnacle and wait for the barnacle to pull the satchel up. When the satchel is close enough use secondary attack to blow it up.

http://steamproxy.net/sharedfiles/filedetails/?id=439847119

Refuse to insert the specimen into the Anti-Mass Spectrometer.

In the test chamber when the specimen arrives in a delivery system, don't push the cart into the beam and you'll get the achievement. When the cart is pushable you have to wait about 1min 20sec and the beam goes off and the game starts loading as you failed the experiment.

http://steamproxy.net/sharedfiles/filedetails/?id=439132643

Help the Barnacle and Headcrab find true love.

Lure a headcrab into a barnacle's tongue and the barnacle will eat the headcrab. Now do it again a few times, total of 10 required. Good place for this is in chapter "We've got hostiles" when you get to a part where there are conveyor belts. There is a room with three barnacles and when you go through the conveyor belt to the next room there is plenty of headcrabs. Lure them back to the room with the barnacles.

You can also use the save game/load game repetitions to finish this achievement.

http://steamproxy.net/sharedfiles/filedetails/?id=439816545

Get killed by your own Snarks.

After you find the Snark weapon use it to kill yourself. If Snarks can't find enemies they attack you so just throw a few at your feet. (Snarks can be found during the achievement Ethically Questionable in the room B08-CES Chamber or later near the end)

Cook a Grenade for too long.

When holding a grenade hold mouse1 long enough until the grenade explodes in your hand. Don't throw it by releasing mouse1.

(Might contain errors, still working on this section)

List of Xen Achievements in order of appearance:

Throughout Xen:

• Prolific Popper (Kill organism like floating creatures, they are non-hostile.)
  
• The Plan (Carry a gas canister from the beginning to about half way through the game.)
  
• PhD in Pacifism (Don't kill any Vortigaunts.)

Chapter 15: Xen

• Orbital Trajectory (Secret jump in the very beginning of the chapter.)

Chapter 16: Gonarch's lair

• Gonad glider (First encounter with Gonarch, let him charge you to oblivion.)
  
• Post-Mortem Interval (Find 7 missing research team bodies. All 7 are in Gonarch lair.)
  
• Unethically questionable (Avoid killing Gonarch at the Gas location.)
  
• Doctor's Signature (Deal final blow to Gonarch using crowbar.)

Chapter 17: Interloper

Chapter 18: Nihilanth

• Specimen GG
  
• Late for Work

Chapter 19: End game

• Limitless Potential
  
• Rather an Anticlimax

Get whacked into oblivion in the ledge fight.

When you first encounter Gronarch run close to the edge and let Gronarch charge you into oblivion. This took me plenty of tries to work. Make a save game when Gronarch breaks you from the shackles when you first encounter it. Then just try to get Gronarch charge you out of the island. If the achievement doesn't unlock then try again a few times.

https://steamproxy.net/sharedfiles/filedetails/?id=2211180481

Skip the opening Xen island with a secret jump.​

Very first achievement in the very beginning of Xen:
https://youtu.be/u5oz2HlAlLo

Kill all Protozoans in the Borderworld.

On your playthrough you need to kill total of 65 of these guys. (Screenshot below)
https://steamproxy.net/sharedfiles/filedetails/?id=2211000340
Warning: It is suggested that you try to achieve 3 different achievements on a single playthrough: Protozoans, The Plan and Unquestionably Ethical.

The plan is similar to Rare Specimen. You have to pick up a gas ganister and deliver it to a spot to kill Gonarch. While following the video below to kill Protozoans, remember to pick up gas canister between kills #3 and #4-5. See exact location on "The Plan".

Since they are easily missable it is best done by following this video made by "FreSsh" on Youtube (link below). Also note that you don't have to kill them in a single playthrough, but you have to kill a protozoan in every location in the game. https://youtu.be/9MYOCmdOlL4

Similar to Rare Specimen. Pick up a gas canister early in Xen and deliver it to a device to kill Gonarch with gas.

Tips:

• Save often to avoid losing the canister and have to replay.
  
• Hold the canister in hand while making save games. When loading back you can see a thumbnail of the location you're at and also see the canister. Easy to see if you still have it.
  
• Put the canister in firm spots where it doesn't roll away and go clear the areas ahead. Then return to get the canister back when all is clear.
  
• This achievement really is a lot shorter than Rare Specimen so it shouldn't be too difficult.
  
• After your first encounter with Gonarch you are pretty much done carrying the canister. Only short walk in the caves ahead before the teleporter.

Suggested to attempt this achievement along with 3 other achievements:

• Prolific Popper
  
• Unethically questionable
  
• Post-Mortem Interval

Follow the bulletin points below to finish all four achievements above in a single playthrough!!!

I advice you to follow the Youtube video mentioned at Prolific Popper and follow it while carrying the canister with you. While you follow the video please note the bulletins below to be able to finish all 4 achievements in a single playthrough:

• #3: Photo below where the gas canister is located. It is after #3 and before #4-5 in the video. https://steamproxy.net/sharedfiles/filedetails/?id=2211393130
  
• #41: In the room where you kill #41 is the goal for your gas canister. Set it in the teleporter seen in the photo and the button turns green. This wont unlock The Plan yet, but the hard part is over. (photo coming soon)
  
• #43: Just after #43 take next turn left into a cave to find 1/7 Post-Mortem Interval
  
• #44-46: there are plenty of hostiles in this room. Shortly after you drop down to a room with bright yellow crystal and you'll see 2/7 Post-Mortem Interval (photo coming soon)
  
• #44-46: Proceed forward and you'll come to open area where you have to run away from Gonarch. You'll end up in a hill where you press a detonator. Right next is 3/7 Post-Mortem Interval (photo coming soon)
  
• #48-51: After falling down the pit you will soon come to 4/7 Post-Mortem Interval. You can't miss it.
  
• #48-51: You come to a spot (see image below). Gonarch will appear in front of you and it breaks a tunnel to the left. 5/7 Post-Mortem Interval is inside the new tunnel on the left hand side.
  
• #55-58: Just when you shoot them you will slide down to a pit where you see familiar red light blinking on the left 6/7 Post-Mortem Interval.
  
• At this point you make a save game. Now keep following the path of flames and you'll end up in an arena and final fight against Gonarch. Shoot rockets at the hanging sack until the floor breaks and you drop down. Now make save games and damage Gonarch little by little and get a finishing blow with crowbar to unlock: Doctor's Signature.
  
• #55-58: Right after getting 6/7 Post-Mortem Interval you will have to run along a path of flames. Keep looking a left turn that is only available if you brought the gas canister to the teleporter. Shortly after crossing a bridge the flames burn down a web and you run uphill into a narrow cave. When you see a supply box with red light turn left.
  
• #59: After 59, make a save game. Multiple achievements unlock here, reload the game to unlock them all:
  1. Unlock the Plan: use the gas canister and kill Gonarch with gas.
  2. Unlock Unquestionably Ethical: Do not use the gas and look to the left and jump on the ledges to avoid Gonarch fight completely.
  3. 7/7 Post-Mortem Interval is on the ledges to the left dead on some thorns.

Thank you for stopping by! Thanks everyone for the positive feedback and comments. Was having a blast writing this guide. Hope this helps everyone in their quest for 100% achievements!

Special thanks to:

Totenkopf - For clarifying what happens to Mr. Hat after Gordon gets knocked out unconcious and giving the correct numbers for the portal puzzle.
iSmile02 - For giving more specific information on the location of the scientist in Unforseen Consequenses and also for hinting the spot in Rare Specimen to leave the hat at the beginning of We've got hostiles.
NighT WolF - Pointing out save/load game trick can also be used on Unified Coupling Theorum.
Mc Intire - For breaking down and explaining how the Caffeine Extraction achievement exactly works.
http://steamproxy.net/id/ElmerJFapp - For grammar.
Gondile - For pointing out the antennae and lambda core locations for Rare Specimen.
Snowhunter - For saving us valuable time by shooting vending machines on Caffeine Extraction.
Jordan_1493 - For providing a save game for me to make a guide video to the portal puzzle.
IHaveNoMouthAndIMustMeme - For a great tip with digital wizardry.
FURIOUS SPRITE BRADBERRY - Tips to place the hat when driving the train cart
Cheers to Milloni for notifying me about the Xen release :)


and also for generally being helpful and good guys.

http://steamproxy.net/sharedfiles/filedetails/?id=440794981