Quick questions on Chemical equilibrium and Le Chatelier: TCE Chemistry (Tasmania)

You cannot see equilibrium directly, but several observations point to it. Macroscopic properties such as colour, pressure or pH become constant and stay constant in a closed system. The reaction can be approached from either direction and reaches the same final state for given conditions. A classic demonstration uses the brown gas $\text{NO}_2$ in equilibrium with colourless $\text{N}_2\text{O}_4$: warming the tube deepens the brown colour (favouring the endothermic dissociation to $\text{NO}_2$) and cooling lightens it, a visible Le Chatelier shift.