When we add FeCl₃ in the first trial, it dissolves, increasing the concentration of Fe ³⁺ ions. According to Le Chatelier's principle, the reaction's equilibrium opposes the increase in the concentration of reactants, increasing the rate of the forward reaction and shifting equilibrium to the right. The colour is already dark red, but it gets even darker as shown below.

In the second trial, KNO₃ dissolves into K ⁺ and NO₃ ^- ions. However, neither of these ions change the existing Fe ³⁺ (aq) + SCN ^- (aq) ⇌ FeSCN ²⁺ (aq) equilibrium, so the position of the equilibrium does not shift.

In the third trial KSCN dissolves, increasing the concentration of SCN ^- ions. Similarly to the first trial, this causes the equilibrium to shift to the right.

When we add Na₃PO₄ in the fourth trial, it dissolves into Na ⁺ and PO₄ ^3- ions. However, PO₄ ^3- ions react with Fe ³⁺ ions to form FePO₄ (s). This decreases the concentration of Fe ³⁺ ions, causing the equilibrium to shift to the left in response.