Question

Which of the following is a suitable example for illustrating the law of conservation of mass? (Atomic mass of ${\text{O}} = 16$, ${\text{H}} = 1{\text{ g/mole}}$)
A.$18{\text{ g}}$ of water is formed by the combination of $16{\text{ g}}$ oxygen with ${\text{2 g}}$ of hydrogen
B.$18{\text{ g}}$ of water in liquid state is obtained by heating $18{\text{ g}}$ of ice
C.$18{\text{ g}}$ of water is completely converted into vapour state on heating
D.$18{\text{ g}}$ of water freezes at ${4^{\text{o}}}{\text{C}}$ to give same mass of ice

Answer 1

To answer this question, you must recall the law of conservation of mass. It states that for a closed system, the mass of the system remains constant over time. In other words, the system’s mass cannot change.

Complete step by step solution:
Also, in chemical reactions, the mass of chemical components before the reaction is equal to the mass of chemical components after the reaction. In other words, the mass of reactants is equal to mass of products.
Considering the option A, we are given that $18{\text{ g}}$ of water is formed by the combination of $16{\text{ g}}$ oxygen with ${\text{2 g}}$ of hydrogen.
The total mass of reactants is equal to the sum of the masses of oxygen and hydrogen, which is $16 + 2 = 18{\text{g}}$
The total mass of products is equal to the mass of water which is given to be $18{\text{ g}}$.
We can see that the total mass of reactants is equal to the total mass of the products. Thus, we can conclude that mass is conserved during the reaction.
The law of conservation of mass corresponds to chemical changes. The processes given in the options B, C and D are all physical changes

Thus, the correct option is A.

Note: The law of conservation of mass is true for approximated values and is a part of assumptions belonging to classical mechanics. It needs to be modified in order to be applicable with the laws of quantum mechanics and the principle of mass energy equivalence that defines energy and mass combined together as one conserved quantity. Law of conservation of mass is also not applicable for nuclear reactions.