Combustor

Combustor is a 2D gas simulation you can draw into. Every cell of the grid holds an amount of each gas, an internal energy, and a velocity. Pressure and temperature aren't stored — they're derived, every tick, from what's actually in the cell. Add moles, add heat, or box it in, and it pushes harder on its neighbours. Nothing here is a particle effect.
Paint walls. Drop a generator. Spark it. Then find out what you built.

Putting fuel next to oxidizer does not start a fire. Combustion needs 700 K and ambient air sits at 300 K, so you need an ignition source  and a mixture rich enough to stay hot once it lights. Thin plume in open air? It flickers and dies. Box it in with walls, or turn the feed rate up, and it holds.

Every reaction is gated on a temperature window, a pressure window, and an induction time. Reactants have to sit together long enough for the timer to fill. Blow them apart halfway and you get nothing the timer unwinds and the reaction never happened. Multiple reactions, from plain Combustion to Aether Condensation, Shock Detonation, Rift Cascade and Vitrium Cracking, each with its own conditions to discover in the in-game Chem Codex.
Multiple gases, each with its own molar mass, heat capacity and diffusivity. Light gases move readily; heavy ones sit where you put them.

Flip between Species, Pressure, Temperature, Velocity and Reactions views. The Reactions view is where the chemistry lives: amber means a cell is serving its induction time, green means it's firing. The readout under your cursor shows exactly how far along every reaction is, right there.

The campaign is the same simulation with most of your tools taken away, and the restriction is the puzzle.
Objectives show what they're measuring against what they want, live "2100 / 2600 P" and holding so a level never just quietly refuses to complete. Everything is counted in simulated seconds, so a three-second hold means the same thing at 1x and 3x.