Why should apparatus always be clamped securely on a metal ring stand before starting a chemical reaction?
Any chemical reaction apparatus should be held securely in place before attempting a chemical reaction for the sake of safety and also possibly cost. Many chemicals are harmful to humans either through inhalation or direct contact with the skin or eyes. Also most solvents are highly flammable. So securing the apparatus will help to prevent any accidental spills and thus cut down on exposure to chemicals. Also cost can be a factor. If you are working with an expensive chemical reagent or are working with a material that you have spent a lot of time preparing you really don't want to accidentally spill everything and have to start all over again or have to re-purchase more of the expensive reagent.