An alkene reacts with a strong protic acid to form a carbocation. however, an alkene will react with a halogen electrophile to form a cyclic intermediate. draw the curved arrow notation for both reactions.

Reaction mechanisms are shown below.

Explanation:

Alkene gives nucleophilic addition reaction with strong protic acid.

Proton from a strong protic acid ruptures pi-bond in alkene and makes a new C-H sigma bond and thereby produces a carbocation.

Lone pair on halogen atom first adds onto double bond by rupturing the pi-bond and this pi-bond electrons then simultaneously substitutes a halogen atom from diatomic molecule of halogen to produce a cyclic halogenium cation by forming two new C-X sigma bonds. (X=halogen atom)

Reaction mechanism has been shown below.

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.