1. Reactants or products are missing. For example, at the beginning of level 1 the equa on is incomplete: "= 2H2O(l)". 2. A reac on equa on is not balanced, i.e., total number and type of atoms of reactants and products do not match each other. For example, "H2(g) + O2(g) = 2H2O(l)". 3. A reac on equa on is considered incorrect if the same chemical appears on the le and right side of the reac on equa on simultaneously. For example, "H2O(l) + H2(g) + O2(g) = 2H2O(l)". 4. A reac on equa on is considered incorrect if the right part of the reac on equa on is equal to the le part. For example, "2H2O(l) = 2H2O(l)". 5. Gibbs free energy is zero or posi ve.