NEET 2025 MCQ with Answer Explanation : “Laws of chemical combination”

Here below some basic MCQ’s about “Laws of chemical combination” with answer which is explained in details. Let’s check one by one which is given below.

MCQ 1
Which of the following statements is a reflection of the Law of Conservation of Mass?

A) Mass can be created or destroyed in a chemical reaction.
B) The total mass of reactants is equal to the total mass of products.
C) Atoms can be converted into energy.
D) Only certain elements can combine chemically.

Answer: B) The total mass of reactants is equal to the total mass of products.

Explanation: The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. Thus, the mass of the reactants must equal the mass of the products.

MCQ 2
The Law of Definite Proportions states that:

A) Elements combine in fixed ratios to form compounds.
B) The mass of the products is equal to the mass of the reactants.
C) The same compound can have different ratios in different reactions.
D) Chemical reactions involve the rearrangement of atoms.

Answer: A) Elements combine in fixed ratios to form compounds.

Explanation: The Law of Definite Proportions, also known as Proust’s Law, states that a chemical compound always contains the same proportion of elements by mass, regardless of the amount of the compound or its source.

MCQ 3
Which law explains why water (H₂O) always has a composition of 2 hydrogen atoms for every 1 oxygen atom?

A) Law of Multiple Proportions
B) Law of Conservation of Mass
C) Law of Definite Proportions
D) Avogadro’s Law

Answer: C) Law of Definite Proportions.

Explanation: The Law of Definite Proportions states that a chemical compound is composed of the same elements in the same proportions by mass, which is why water always has a consistent composition of 2 hydrogen atoms for every 1 oxygen atom.

MCQ 4
According to the Law of Multiple Proportions, when two elements combine to form different compounds, the masses of one element that combine with a fixed mass of the other element are in:

A) Equal ratios.
B) Whole number ratios.
C) Variable ratios.
D) Proportional to their atomic masses.

Answer: B) Whole number ratios.

Explanation: The Law of Multiple Proportions states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element can be expressed as a ratio of small whole numbers.

MCQ 5
What is the correct statement according to Dalton’s Atomic Theory?

A) Atoms of different elements can combine in any ratio.
B) Atoms are indivisible and indestructible.
C) All atoms of the same element have different masses.
D) Atoms can change from one element to another during chemical reactions.

Answer: B) Atoms are indivisible and indestructible.

Explanation: Dalton’s Atomic Theory posits that atoms are the fundamental building blocks of matter and are indivisible in chemical processes. It also states that atoms of the same element are identical in mass and properties.

MCQ 6
Which law states that when two elements form a compound, they do so in fixed proportions by mass?

A) Law of Definite Proportions
B) Law of Conservation of Mass
C) Law of Multiple Proportions
D) Dalton’s Law

Answer: A) Law of Definite Proportions.

Explanation: This law indicates that a chemical compound always contains its component elements in fixed mass ratios.

MCQ 7
In the reaction of hydrogen and oxygen to form water, which law is illustrated?

A) Law of Conservation of Mass
B) Law of Multiple Proportions
C) Law of Definite Proportions
D) Avogadro’s Law

Answer: A) Law of Conservation of Mass.

Explanation: This law is illustrated as the total mass of hydrogen and oxygen before the reaction equals the mass of water produced.

MCQ 8
Which of the following compounds exemplifies the Law of Multiple Proportions?

A) H₂O
B) CO₂ and CO
C) NaCl
D) CH₄

Answer: B) CO₂ and CO.

Explanation: Carbon can combine with oxygen in different ratios to form CO and CO₂, demonstrating the Law of Multiple Proportions.

MCQ 9
According to Avogadro’s Law, equal volumes of gases at the same temperature and pressure contain:

A) Equal masses.
B) Equal numbers of molecules.
C) Different types of molecules.
D) Different volumes.

Answer: B) Equal numbers of molecules.

Explanation: Avogadro’s Law states that equal volumes of gases, at the same temperature and pressure, contain an equal number of molecules.

MCQ 10
What happens to the mass during a chemical reaction according to the Law of Conservation of Mass?

A) It increases.
B) It decreases.
C) It remains constant.
D) It becomes energy.

Answer: C) It remains constant.

Explanation: The Law of Conservation of Mass states that mass is conserved in a chemical reaction; it neither increases nor decreases.