Acertain material has a density of 0.2 g/ml is this material good for building a bridge or for making cushions for a couch

A bridge must be made of metal or concrete or other materials that are strong enough to withstand the forces of the traffic passing over it. I doubt that the material could do that.

If the substance is a metal foam, it would make a rather hard cushion.

If the substance is too soft, it would make a cushion.

If the substance has the right combination of softness and elasticity so that it returns to its original shape when you get up, it might be a good material for a cushion.

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.