The role of a positive catalyst in a chemical reaction is to increase the rate of the reaction. A catalyst is a substance that speeds up the rate of a chemical reaction without being consumed itself. In the case of a positive catalyst, it lowers the activation energy required for the reaction, allowing it to proceed at a faster rate.

Option 1, "It increases the rate of reaction," is the correct answer. A positive catalyst provides an alternative pathway for the reaction with lower energy barriers, allowing more reactant molecules to successfully convert into products in a given time. This means the reaction will occur faster with a positive catalyst compared to without one.

Option 2, "It decreases the rate of reaction," is incorrect. A positive catalyst does not decrease the rate of the reaction; rather, it enhances it.

Option 3, "It increases the yield of the products," is not the role of a positive catalyst. The yield of products is determined by the stoichiometry and equilibrium of the reaction, not the presence of a catalyst.

Option 4, "It provides better purity of the products," is also unrelated to the role of a positive catalyst. The purity of the products is determined by the reaction conditions and the separation/purification processes, not the catalyst.

Thus