NCERT Solutions for Class 9 Science Chapter 3: Atoms and Molecules

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The NCERT Solutions for Class 9 Science are provided in this article. Some of the important topics in Chapter 3 Atoms and Molecules include:

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NCERT Solutions for Class 9 Science Chapter 3

NCERT Solutions

NCERT Solution

Class 9 Science Chapter 3 Atoms and Molecules – Important Topics

Atom is the smallest constituent unit of matter that possess the properties of the chemical element. Atoms don’t exist independently, instead, they form ions and molecules which further combine in large numbers to form matter that we see, feel and touch.

Dalton’s Atomic Theory

Dalton’s Atomic Theory hypothesized and further claimed that the law of conservation of mass and the law of definite proportions could possibly de elaborated by using the idea of atoms. According to him, all substances are indestructible building units.

Example: What are two postulates of Dalton’s Atomic Theory?

Solution: The two postulates of Dalton’s Atomic Theory are:

The substances around us are composed of matter, which are tiny, indestructible particles called atoms.
Every atom of a particular element are identical according to their mass, size, and several other properties.

Atomic Mass

The complete mass of one atom of a distinct element can be otherwise called as its atomic mass. The unit it has is known as unified atomic mass unit, that is represented by the letter ‘u.’

Example: Calculate the atomic mass of an oxygen isotope that has 9 neutrons.

Solution: To calculate the atomic mass of an oxygen isotope, we need to know the number of protons and neutrons it has.
Number of protons in an oxygen atom = 8
As per the question, the Number of neutrons in oxygen atom = 9
Hence, the Atomic mass of given oxygen atom = number of protons + number of neutrons = 8 + 9 = 17

Mole Concept

Mole concept can be defined as the atomic mass of a mole which is usually measured in grams. Ideally, the gram atomic mass of an element is otherwise referred to as a mole.

Example: What is the formula of Mole Concept?

Solution: The number of units responsible to compose mole can be evaluated by 6.022 ×10²³. The fundamental constant is called the Avogadro's number (NA) or Avogadro constant. It can be represented using an explicit unit termed per mole. Thus, to calculate Mole Concept,
Number of Moles = (Mass of the Sample)/(Molar Mass)

Read Also:

Chapter Related Links
Avogadro's Law	Difference between Molar Mass and Molecular Mass	Atomic Mass Formula
Moles to Grams Formula	Number of Moles Formula	Nucleus of an Atom and Atomic Mass
Atomic Number	Atomic And Molecular Masses	Mole Fraction

Also Check:

Important Science Study Guides
Physics Constants	Named Reactions	Difference between in chemistry
Difference between in biology	Difference between in physics	Periodic Table

CBSE X Related Questions

1.
The gases evolved on heating lead (II) nitrate crystals are :
- NO and \( O_2 \uparrow \)
- \( N_2 \) and \( NO_2 \)
- \( NO_2 \) and \( H_2 \)
- \( NO_2 \) and \( O_2 \)
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2.
The reasons for excessive generation of wastes are:
(i) Use and throw policy.
(ii) Increased availability of packaged food.
(iii) Increased construction wastes.
(iv) Non-sorting of dry and wet wastes
- (i), (iii) and (iv)
- (i), (ii) and (iii)
- (i), (ii), (iii) and (iv)
- (ii), (iii) and (iv)
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3.
Which structure in a leaf is mainly responsible for gaseous exchange?
- Xylem
- Stomata
- Phloem
- Cuticle
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4.
The natural sources of oxalic acid, lactic acid and methanoic acid respectively are:
- tomato, curd, ant-sting
- tomato, orange, nettle-sting
- orange, milk, ant-sting
- orange, sour milk, nettle-sting
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5.
Assertion : Reaction of quick lime with water is an exothermic reaction. Reason (R): A large amount of heat is evolved on the reaction of quick lime and water.
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.
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6.
(i) \( \text{AgNO}_3 + \text{NaCl} \rightarrow \text{NaNO}_3 + \text{AgCl} \)
(ii) \( \text{K}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 + 2\text{KCl} \)
Which of the following options clearly describes both the reactions?
- (i) is double displacement, (ii) is displacement reaction.
- Both, (i) and (ii) are displacement reactions and precipitation reactions.
- Both, (i) and (ii) are double displacement reactions and precipitation reactions.
- (i) is displacement, (ii) is double displacement reaction.
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