# CBSE Class 9 Science NCERT Solutions: Chapter 3, Atoms and Molecules

Any exam preparation is a rigorous process which requires an overall understanding of the concepts and basics discussed in a chapter. This process demands a thorough study of the concepts and a tricky approach to get through the solutions for various questions/problems based on those concepts. Here, significant role is played by the NCERT books as these books explain each topic in detailed  but simple language to help students understand each and every line easily. Also these books offer a number of problems which students can practice to assess their learning and prepare for the examinations. Hence, all the students prefer studying from NCERT books.

Here we are providing NCERT solutions for CBSE Class 9 Science Chapter 3, Atoms and Molecules. Students can refer these NCERT solutions to check the correctness of their own answers and thus assess their level of understanding about each topic. All these model solutions have been prepared by the subject experts and are hundred percent accurate.

Main topics discussed in Class 9 Science chapter- Atoms and Molecules are:

• Laws of chemical combination

Law of conservation of mass

Law of constant proportions

• Atom, symbols of atoms of different elements and atomic mass
• Molecule, molecules of elements and compounds
• What is an ion?
• Writing chemical formulae
• Molecular mass and mole concept

Some of the questions and their solution from NCERT Solutions for Class 9: Atoms and Molecules, are as follows:

Q. Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

Sol.

Following postulate of Dalton's atomic theory is the result of the law of conservation of mass ‘Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

Q. What is meant by the term chemical formula?

Sol.

Chemical formula of a compound (or element) is the symbolic representation of its composition. It represents

(i) The number and kind of atoms present per molecule of the compound,

(ii) One mole of the compound,

(iii) Molar mass of the compound.

Q. What are polyatomic ions? Give examples.

Sol.

The group of atoms which carry a fixed charge (either positive or negative) on them and behave as ions are called polyatomic ions.

Example:

(i) Carbonate ion (CO32−)        (ii) Sulphate ion (SO42−)

(iii) Ammonium ion (NH4+)       (iv) Phosphate ion (PO43−)

Q. Write the chemical formulae of the following.

(i) Magnesium chloride           (ii) Calcium oxide

(iii) Copper nitrate                 (iv) Aluminium chloride

(v) Calcium carbonate

Sol.

(i) Formula = MgCl2 (Magnesium chloride)

(ii) Formula = Ca2O2 or CaO (Calcium oxide)

(iii) Formula = Cu(NO3)2 (Copper nitrate)

(iv) Formula = AlCl3 (Aluminium Chloride)

(v) Formula = CaCO3 (Calcium carbonate)

Q. What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Sol.

(a) Mass of 1 mole O-atoms = 16 g

Mass of 0.2 mole O-atoms = 16 × 0.2 = 3.2 g

(b) Mass of 1 mole of H2O molecules = 18 g

Mass of 0.5 mole of H2O molecules = 18 × 0.5 = 9.0 g

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