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C. A one (1) is assumed when there is no coefficient

Explanation:

The following chemical equation is given in the attachment:

2H2 + O2 → 2H2O

In this reaction, Hydrogen (H2) and oxygen (O2) gases are the reactants and they combine to form H2O, which is the product. It is necessary that, in accordance to the law of conservation of matter, the same number of atoms of each elements on the reactant side equates that on the product side. When this happens, the equation is said to be BALANCED.

However, to balance an equation, COEFFICIENTS are used. In this equation above, coefficients 2 is used in front of both H2 and H2O but no coefficient is used in front of O2. This is because 1 mole of oxygen is on the reactant and product side respectively. However, one is assumed when no coefficient is written i.e. O2 means 1 mole of oxygen molecule.

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.