Chemical equilibrium
Reactions that go only towards the products are irreversible (a precipitate, gas or a poorly dissociating substance forms): NaCl + AgNO₃ → AgCl↓ + NaNO₃. Reactions that, under the same conditions, go both forward and backward are reversible and written with ⇌: SO₂ + H₂O ⇌ H₂SO₃, 2SO₂ + O₂ ⇌ 2SO₃. As time goes on the forward rate falls and the reverse rate rises; when they become equal (v₁ = v₂) chemical equilibrium is set up. This is a dynamic state: the reaction does not stop, but the concentrations no longer change. Equilibrium can be shifted: raising the concentration of a reactant shifts it to the side where that substance is used up; raising the pressure shifts it to the side with fewer gas molecules; lowering the temperature in an exothermic reaction shifts it towards the forward reaction. A catalyst does not shift equilibrium; it only helps it to be reached sooner. The decomposition of CaCO₃ is reversible in a closed vessel, but in an open kiln CO₂ escapes and the reaction goes to completion.
“Dynamic equilibrium game”: label two cups “left” and “right”. Put 10 counters in the left, 0 in the right. At each step move half of the left cup to the right and one third of the right cup to the left. After several steps note that the amounts hardly change. Only paper balls or coins.