NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules in PDF form to free download. NCERT Solutions for other subjects are also in same format. For NCERT Solutions Class 9 All Chapter, Click here.
NCERT Solutions for Class 9 Science Chapter 3
Class 9 Science – Atoms and Molecules
Extra Questions with Answers for Exams
- Question 1:
Do atoms is some of the elements have actually fractional mass?
- Answer 1:
No, the atoms do not have fractional mass. Only their average comes out to be in fraction in elements which exist as isotopes.
- Question 2:
In what respect does Dalton’s Atomic theory hold good even today?
- Answer 2:
atom has been found to be made up of sub-atomic particles like electrons, protons and neutrons. But it is still the smallest particle of matter which can take can take part in chemical combination.
- Question 3:
What is basic different between atoms and molecules?
- Answer 3:
Atoms except those of noble or inert gas elements cannot exist of their own. However, all molecules can have independent existence.
- Question 4:
Why is it necessary to balance a chemical equation?
- Answer 4:
A chemical equation has to be balanced in order to satisfy the law of conservation of mass. According to the law, there is no change in mass when the reaction changes into the production. Therefore, the chemical equation has to be balanced.
- Question 5:
Why is a cation so named?
- Answer 5:
When electric current is passed through the solution of a salt like sodium chloride (NaCl), the positive ion (Na+) migrates towards cathode (negative electrode). It is therefore, called cation. Remember this:
(i) Positive ion migrating towards cathode on passing electric current is known as cation.
(ii) Negative ion migrating towards anode on passing electric current is known as anion.
- Question 6:
What is wrong with the statement ‘1 mole of hydrogen’?
- Answer 6:
The statement is not correct. We must always write whether hydrogen is in atomic form or molecular form.
The correct statement is:
1 mole of hydrogen atoms or one mole of hydrogen molecules.
- Question 7:
The atomic mass of an element is in fraction. What does it means?
- Answer 7:
If the atomic mass of an element is in fraction, this means that it is existed in the form of isotopes. The atomic mass is the average atomic mass and it is generally fractional.
- Question 8:
Give one limitation of law of constant composition.
- Answer 8:
The law of constant composition does not hold when different isotopes of an element take part in the formation of a particular compound. For example,
In carbon dioxide (CO2)
By using C-12 isotopes, the ratio of C : O is 12 : 32.
By using C-14 isotopes, the ratio of C : O is 14 : 32.
This means that the two ratios are not the same.
- Question 9:
Out of atoms and molecules which can exist independently?
- Answer 9:
Molecules can exist independently. However, the atoms of noble gases (He, Ne, Ar, Kr, Xe) can also exist independently.
- Question 10:
Why are chemical reaction according to law of conservation of mass?
- Answer 10:
In all chemical reactions, there is only exchange of reactants taking place when products are formed. Since there is no loss or gain of mass, the chemical reactions are according to law of conservation of mass.