 We will look at the equilibrium between NO2 gas and N2O4 gas. NO2 gas is a brown colored gas. N2O4 is colorless. We'll place the tube in an ice bath. We notice that the color in the tube gets lighter. The concentration of NO2 has decreased. It indicates that the equilibrium has shifted in the forward direction towards the N2O4. If we cool a reaction, the equilibrium is exothermic in the forward direction. We'll place the tube in a warm water bath. We notice that the color in the tube gets darker, indicating an increase in the concentration of NO2. As we warm a mixture, it shifts in the endothermic direction. The tube became darker, which indicates that the equilibrium has shifted in the reverse direction.