How does a balanced reaction show law of conservation ?

I will take an example just to make things a little bit easier:

E.g. 2H2 + O2 -> 2H2O

The reaction above is the reaction of hydrogen gas with oxygen gas which creates water.

As you can see, in a balanced reaction, the numbers of moles of each substance is the same before and after the reaction (for example, the hydrogen has the same number of moles before and after the reaction which is 4), therefore, if the numbers of moles are the same, so as the mass of the reactants and the products and this fact can give us an idea about conservation which states that the total mass is kept consistance and can't be destroyed or lost.

*hope it helps you

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.