Corn starch will produce a firm smooth paste while flour will produce a thick paste. Overheating or overstirring a starch solution will affect the thickening ability. If the starch is cooked before using it will not thicken as well. Always dissolve your starch in a liquid before adding it. If not you will get lumps in a thin sauce.
Every liquid has a specific boiling temperature.
Once it reaches this temperature it will not get any hotter no matter how much heat is applied. Cooking foods in a boiling liquid over a high flame will not cook it faster, the water will just boil away more quickly. Adding salt or sugar will increase the boiling temperature, therefore the heat must be higher. Fats used in cooking should be appropriate for the dish. In salad dressings the taste of the oil is important.