Why do people say “weight” even though it is more accurate to say“mass”?

People often use "weight" to mean "mass", and vice versa, because Gravity is almost the same everywhere on Earth and we don't notice a difference.

Explanation:but they do not mean the same thing,

and they can have different measurements. An object's mass doesn't change but its weight can change.

that tool would be a compass

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.