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Which of the following is a postulate of Dalton’s Atomic Theory that explains the Law of Definite Proportions?

Atoms of different elements combine in fixed whole number ratios to form compounds.

Atoms of different elements have different masses.

Atoms can be created or destroyed.

Which of the following postulates of Dalton’s Atomic Theory explains the Law of Definite Proportions?

Atoms of a given element are identical in mass and chemical properties.

All matter is made of tiny particles called atoms.

Atoms combine in simple whole number ratios to form compounds.

Atoms cannot be created or destroyed in a chemical reaction.

Which postulate of Dalton’s Atomic Theory explains the Law of Definite Proportions?

C. Atoms of different elements combine in fixed ratios to form compounds.

A. Atoms cannot be divided further.

B. Atoms of different elements have different masses.

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Key Concepts in Atoms and Molecules

Laws of chemical combinationDalton's atomic theoryAtomic mass and molecular massWriting chemical formulaeMole concept and Avogadro's number

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Atoms and Molecules — Important Questions with Answers

Practice these Atoms and Molecules questions for Class 9 Science, each with the correct answer and a step-by-step explanation. Sign up free to practice all 101+ questions with adaptive difficulty.

Q1Easy
Which ancient Indian philosopher proposed the idea of smallest indivisible particles called 'Parmanu'?
- A.Maharishi Kanad
- B.Aristotle
- C.Democritus
- D.Leucippus
Answer: Maharishi Kanad
Explanation: Maharishi Kanad, an ancient Indian philosopher, postulated that matter could be divided into smaller particles until reaching the smallest indivisible particles, which he named 'Parmanu'. This concept was foundational to early atomic theories.
Q2Easy
Which ancient Greek philosopher proposed the concept of 'atomos' (indivisible particles)?
- A.Aristotle
- B.Plato
- C.Democritus
- D.Socrates
Answer: Democritus
Explanation: Democritus, along with Leucippus, proposed that matter could be divided into smallest indivisible particles, which he called 'atomos' (meaning indivisible). This idea laid the groundwork for modern atomic theory.
Q3Easy
Which of the following elements is diatomic in nature?
- A.Argon
- B.Oxygen
- C.Phosphorus
- D.Sodium
Answer: Oxygen
Explanation: Oxygen exists as a diatomic molecule (O₂) in its natural state, meaning it consists of two oxygen atoms bonded together. This is a common characteristic of several non-metallic elements.
Q4Easy
What is the atomicity of chlorine?
- A.Monoatomic
- B.Diatomic
- C.Tetra-atomic
- D.Poly-atomic
Answer: Diatomic
Explanation: Chlorine exists in nature as a diatomic molecule (Cl₂), meaning each molecule consists of two chlorine atoms. This is a common property for several non-metallic elements.
Q5Medium
Which law states that mass is neither created nor destroyed during a chemical reaction?
- A.Law of Conservation of Mass
- B.Law of Definite Proportions
- C.Law of Multiple Proportions
- D.Law of Octaves
Answer: Law of Conservation of Mass
Explanation: The Law of Conservation of Mass, established by Antoine Lavoisier, states that the total mass of substances remains constant during a chemical reaction. This was demonstrated through experiments involving chemical reactions in sealed flasks.
Q6Medium
Which of the following is an example of a polyatomic ion?
- A.Sodium ion (Na⁺)
- B.Chloride ion (Cl⁻)
- C.Sulphate ion (SO₄²⁻)
- D.Hydrogen ion (H⁺)
Answer: Sulphate ion (SO₄²⁻)
Explanation: Polyatomic ions consist of multiple atoms bonded together and carrying a net charge. Sulphate (SO₄²⁻) is a common example of a polyatomic ion.
Q7Medium
Which of the following is the correct formula for calcium chloride?
- A.CaCl
- B.CaCl₂
- C.Ca₂Cl
- D.Ca₂Cl₂
Answer: CaCl₂
Explanation: Calcium has a valency of 2, and chlorine has a valency of 1. To balance the charges, two chlorine atoms are required for one calcium atom, resulting in the formula CaCl₂.
Q8Medium
What is the molecular mass of water (H₂O)?
- A.10 u
- B.16 u
- C.18 u
- D.20 u
Answer: 18 u
Explanation: The molecular mass of water is calculated as the sum of the atomic masses of its constituent atoms: 2 atoms of hydrogen (2 × 1 u) + 1 atom of oxygen (16 u) = 18 u.
Q9Hard
Which of the following is a postulate of Dalton’s Atomic Theory that explains the Law of Definite Proportions?
- A.Atoms are indivisible.
- B.Atoms of different elements have different masses.
- C.Atoms of different elements combine in fixed whole number ratios to form compounds.
- D.Atoms can be created or destroyed.
Answer: Atoms of different elements combine in fixed whole number ratios to form compounds.
Explanation: Dalton’s Atomic Theory postulates that atoms combine in fixed whole number ratios to form compounds, which directly explains the Law of Definite Proportions.
Q10Hard
Which of the following postulates of Dalton’s Atomic Theory explains the Law of Definite Proportions?
- A.All matter is made of tiny particles called atoms.
- B.Atoms of a given element are identical in mass and chemical properties.
- C.Atoms combine in simple whole number ratios to form compounds.
- D.Atoms cannot be created or destroyed in a chemical reaction.
Answer: Atoms of a given element are identical in mass and chemical properties.
Explanation: Dalton’s postulate that atoms of a given element are identical explains why compounds always contain the same elements in the same proportions.
Q11Hard
Which postulate of Dalton’s Atomic Theory explains the Law of Definite Proportions?
- A.A. Atoms cannot be divided further.
- B.B. Atoms of different elements have different masses.
- C.C. Atoms of different elements combine in fixed ratios to form compounds.
- D.D. Atoms are indivisible.
Answer: C. Atoms of different elements combine in fixed ratios to form compounds.
Explanation: Dalton’s Atomic Theory postulates that atoms combine in fixed ratios by weight, which explains the Law of Definite Proportions.
Q12Hard
What is the correct formula for sodium phosphate?
- A.A. NaPO₄
- B.B. Na₃PO₄
- C.C. Na₂PO₄
- D.D. Na₄PO₄
Answer: B. Na₃PO₄
Explanation: Sodium (Na) has a valency of 1 and phosphate (PO₄³⁻) has a valency of 3, so the formula for sodium phosphate is Na₃PO₄.

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Atoms and Molecules Important Questions.

Exam-Ready Questions

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Key Concepts in Atoms and Molecules

Atoms and Molecules — Important Questions with Answers

Which ancient Indian philosopher proposed the idea of smallest indivisible particles called 'Parmanu'?

Which ancient Greek philosopher proposed the concept of 'atomos' (indivisible particles)?

Which of the following elements is diatomic in nature?

What is the atomicity of chlorine?

Which law states that mass is neither created nor destroyed during a chemical reaction?

Which of the following is an example of a polyatomic ion?

Which of the following is the correct formula for calcium chloride?

What is the molecular mass of water (H₂O)?

Which of the following is a postulate of Dalton’s Atomic Theory that explains the Law of Definite Proportions?

Which of the following postulates of Dalton’s Atomic Theory explains the Law of Definite Proportions?

Which postulate of Dalton’s Atomic Theory explains the Law of Definite Proportions?

What is the correct formula for sodium phosphate?

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