The Alchemist is a triggered effect, triggering whenever a minion is summoned. This includes all means of minion summoning, such as spells, hero powers, Deathrattles and Battlecries. It also triggers on minions created by transform effect Battlecries (such as Faceless Manipulator and Recombobulator, but not minions created by transform effects in any other way (such as by Hex).
The Alchemist triggers before any Battlecry from the summoned minion, and therefore also before any related Secrets, but after effects that trigger 'whenever you play a minion/card' such as Hobgoblin.
It triggers before Lushwater Scout's effect on a played Murloc, a normally 1/2 Spawnpool Forager for example gets flipped to 2/1 and gets +1 attack to become 3/1.
The Alchemist triggers in the 'Summoning Catchup Step'. For more information, see the corresponding section in the Advanced Rulebook.
The Alchemist triggers on mortally wounded (0 or less Health) minions, which may save them from dying.
Poor Maloriak. Turns out that Nefarian's attempt to fuse the power of a dragonkin with the brilliant mind of an alchemist resulted in a stupid, incompetent, and disappointing minion. Despite his deficiencies (by admittedly unrealistic dragon standards), Maloriak's crazed alchemical concoctions make him a most noxious foe.[1]
World of Warcraft
"There can be no disruptions! Mustn't keep the master waiting, mustn't fail again!"
The human Malory was a bright young alchemist until he was kidnapped by Nefarian and fused with the corpse of a dragonspawn. The result - Maloriak - is a bumbling minion who possesses only a dull of his former intelligence.
As suggested by the above dialogue, defeating Maloriak will earn the player the title "Slayer of Stupid, Incompetent and Disappointing Minions". This is based on an identical title from World of Warcraft, obtained by defeating Maloriak on heroic difficulty. However, the Hearthstone version is somewhat more limited, resulting only in the player's name being replaced by the title for the duration of the Atramedes fight (the next encounter in the adventure).
This section uses content from the Warcraft Wiki.
Faceless Manipulator and 
Recombobulator, but not minions created by transform effects in any other way (such as by 
Lushwater Scout's effect on a played Murloc, a normally 1/2 
Release the Aberrations!
Duplicate
This section uses content from the Warcraft Wiki.
Patch 2.4.0.8311 (2015-03-31):