Le Chatelier's principle.

Step 1 of 5

Adding a reactant

In 1884 the French chemist Henri Le Chatelier stated a rule that predicts what an equilibrium will do when you disturb it: a system at equilibrium whose conditions change shifts in the direction that reduces the change.

In the Scene, the Push in more NO₂ switch works a syringe that pushes more $\mathrm{NO_2}$⁠ into the settled flask: half as much again as was sealed in at the start. Time now stands at the equilibrium before the push. Predict, then watch the syringe push it in.

Step 2 of 5

The same constant

After the push the flask settles at a new equilibrium. The concentrations differ from before the push. Does the constant?

Step 3 of 5

The moment of the push

Why does the mix move towards $\mathrm{N_2O_4}$⁠ in particular? The answer is in the rates. Push in more NO₂ is on, and Time is fixed at Start: exactly the moment of the push.

Step 4 of 5

Heating it

Pairing $\mathrm{NO_2}$⁠ gives out heat, so splitting takes heat in. When you heat, the “change” is heat coming in, and Le Chatelier’s principle says which way the system will shift. Here Time is fixed at equilibrium.

Step 5 of 5

Half and half

Temperature is the one change that moves $K$⁠ itself: heating lowers the $K$⁠ of a reaction that gives out heat, and cooling raises it. That makes this flask a thermometer: its colour at equilibrium tells the temperature.