Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 4 | PJ Sir
Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 4 | PJ Sir
Exploration – Grade 9 Science | Chapter 9
Part 4: Properties of Ionic & Covalent Compounds, Molecular Mass & Formula Unit Mass
By Priyank Joshi (PJ Sir)
Chemistry Educator & Mentor | 18+ Years of Teaching Experience
In Part 1, we learnt the Laws of Chemical Combination and Dalton’s Atomic Theory.
In Part 2, we understood Covalent and Ionic Bonding.
In Part 3, we learnt Ions, Valency, Polyatomic Ions and Writing Chemical Formulae.
Now, in the final part of Chapter 9, we will understand:
Properties of Ionic & Covalent Compounds
Solubility
Electrical Conductivity
Melting & Boiling Points
Molecular Mass
Formula Unit Mass
Important Numericals
1. Properties of Ionic and Covalent Compounds
Ionic and covalent compounds are formed differently, so their properties are also different.
The chapter compares substances such as sodium chloride, copper sulfate, sugar, camphor and naphthalene through solubility and electrical-conductivity observations.
2. Solubility of Ionic Compounds
Ionic compounds such as:
Sodium chloride
Copper sulfate
are generally soluble in water but insoluble in solvents such as kerosene and petrol.
PJ Sir Trick
For Class 9 level, remember the general trend:
Ionic → Generally soluble in water
3. Solubility of Covalent Compounds
Most covalent compounds such as:
Camphor
Naphthalene
are generally insoluble in water, but dissolve in solvents such as kerosene and petrol.
However, this is a general trend and not an absolute rule. The chapter itself gives sugar as an example of a covalent compound that is soluble in water.
This is important because students sometimes wrongly memorise:
“Every covalent compound is insoluble in water.”
That statement is not correct.
4. Electrical Conductivity of Ionic Compounds
This is one of the most important concepts of this part.
Do ionic compounds conduct electricity?
The answer depends on their state.
Ionic Compound in Solid State
Ionic compounds do not conduct electricity in the solid state.
Why?
Because their ions are held in fixed positions by strong forces.
The ions cannot move freely.
Therefore, electricity is not conducted.
Ionic Compound Dissolved in Water
When suitable ionic compounds dissolve in water, their ions become free to move.
These mobile ions allow the solution to conduct electricity.
Therefore:
Solid ionic compound → Does not conduct
Ionic compound dissolved in water → Can conduct
The chapter uses sodium chloride and copper sulfate to explain this behaviour.
PJ Sir Trick
Remember:
No movement of ions = No conduction
Free movement of ions = Conduction
This is much better than simply memorising the result.
5. What About Covalent Compounds?
Covalent compounds generally do not provide freely moving ions.
For example, sugar may dissolve in water, but it does not provide ions in the solution.
Therefore, its solution does not conduct electricity.
Camphor and naphthalene also do not conduct electricity in the activity discussed in the chapter.
Important Concept
Do not think:
“If something dissolves in water, it must conduct electricity.”
That is incorrect.
The important question is:
Are mobile ions available?
6. Melting and Boiling Points
Ionic compounds generally have high melting and boiling points because of strong attractions between their ions.
Covalent compounds usually have lower melting and boiling points in comparison. Pasted markdown
PJ Sir Trick
Think:
Ionic → Strong inter-ionic attraction → More energy required → Higher melting/boiling points
7. Quick Comparison: Ionic vs Covalent Compounds
Ionic Compounds
Formed through electron transfer
Contain oppositely charged ions
Generally soluble in water
Generally insoluble in kerosene and petrol
Do not conduct electricity in solid state
Can conduct when ions are free to move in solution
Generally have high melting and boiling points
Covalent Compounds
Formed through electron sharing
Generally do not provide free ions
Many are insoluble in water
Many dissolve in solvents such as kerosene and petrol
Generally do not conduct electricity
Usually have lower melting and boiling points than ionic compounds
8. Molecular Mass of Covalent Compounds
Now let us move to numericals.
The molecular mass of a covalent compound is calculated by adding the atomic masses of all the atoms present in one molecule. Pasted markdown
PJ Sir Formula
Molecular Mass = Sum of atomic masses of all atoms in the molecule
9. Molecular Mass of Water – H₂O
Given:
Atomic mass of H = 1 u
Atomic mass of O = 16 u
H₂O contains:
2 hydrogen atoms
1 oxygen atom
Therefore:
Molecular mass of H₂O
= (2 × 1) + (1 × 16)
= 2 + 16
= 18 u
This is the example used in the chapter. Pasted markdown
10. Molecular Mass of Carbon Dioxide – CO₂
Given:
Atomic mass of C = 12 u
Atomic mass of O = 16 u
CO₂ contains:
1 carbon atom
2 oxygen atoms
Therefore:
Molecular mass of CO₂
= (1 × 12) + (2 × 16)
= 12 + 32
= 44 u Pasted markdown
11. Practice Numerical: Molecular Mass of HNO₃
Given:
H = 1 u
N = 14 u
O = 16 u
HNO₃ contains:
1 H + 1 N + 3 O
Therefore:
Molecular mass
= 1 + 14 + (3 × 16)
= 1 + 14 + 48
= 63 u
12. Practice Numerical: Molecular Mass of CH₄
Given:
C = 12 u
H = 1 u
CH₄ contains:
1 C + 4 H
Therefore:
Molecular mass
= 12 + (4 × 1)
= 16 u
The chapter asks students to calculate the molecular masses of HNO₃ and CH₄ as practice questions. Pasted markdown
13. Formula Unit Mass of Ionic Compounds
Here students often make an important mistake.
Ionic compounds form 3-D crystal structures rather than individual molecules.
Therefore, for ionic compounds we use the term:
Formula Unit Mass
A formula unit represents the simplest whole-number ratio of ions in an ionic compound.
The mass of that formula unit is called its formula unit mass. Pasted markdown
PJ Sir Trick
Remember:
Covalent → Molecule → Molecular Mass
Ionic → Formula Unit → Formula Unit Mass
This one distinction can save you from many mistakes.
14. Formula Unit Mass of Sodium Oxide – Na₂O
Given:
Na = 23 u
O = 16 u
Na₂O contains:
2 Na
1 O
Therefore:
Formula unit mass
= (2 × 23) + (1 × 16)
= 46 + 16
= 62 u
This is an example given in the chapter. Pasted markdown
15. Formula Unit Mass of Calcium Nitrate – Ca(NO₃)₂
Given:
Ca = 40 u
N = 14 u
O = 16 u
First understand the brackets:
Ca(NO₃)₂
means:
1 Ca
2 N
6 O
Therefore:
Formula unit mass
= 40 + (2 × 14) + (6 × 16)
= 40 + 28 + 96
= 164 u
The chapter gives the same result.
PJ Sir Trick for Brackets
Whenever you see:
(NO₃)₂
the outside 2 multiplies everything inside the bracket.
So:
N = 1 × 2 = 2
O = 3 × 2 = 6
16. Practice Numerical: Formula Unit Mass of KCl
Given:
K = 39 u
Cl = 35.5 u
Formula unit mass of KCl
= 39 + 35.5
= 74.5 u
17. Practice Numerical: Formula Unit Mass of Mg(OH)₂
Given:
Mg = 24 u
O = 16 u
H = 1 u
Mg(OH)₂ contains:
1 Mg
2 O
2 H
Therefore:
Formula unit mass
= 24 + (2 × 16) + (2 × 1)
= 24 + 32 + 2
= 58 u
KCl and Mg(OH)₂ are included as practice questions in this section of the chapter.
18. Most Important Difference for Exams
Students often confuse these two terms:
Molecular Mass
Used for molecules/covalent compounds.
Example:
H₂O = 18 u
CO₂ = 44 u
Formula Unit Mass
Used for ionic compounds.
Example:
Na₂O = 62 u
KCl = 74.5 u
PJ Sir One-Line Trick
Molecule hai → Molecular Mass
Ionic lattice hai → Formula Unit Mass
30-Second Revision
Before finishing the chapter, remember:
1. Ionic compounds are generally soluble in water.
2. Many covalent compounds are insoluble in water and dissolve in solvents such as kerosene and petrol.
3. Ionic compounds do not conduct electricity in the solid state because their ions cannot move freely.
4. When suitable ionic compounds dissolve in water, mobile ions allow electrical conduction.
5. Sugar can dissolve in water but does not provide ions, so its solution does not conduct electricity.
6. Ionic compounds generally have high melting and boiling points.
7. Covalent compounds usually have lower melting and boiling points.
8. Molecular mass is obtained by adding the atomic masses of all atoms in a molecule.
9. Ionic compounds use the concept of formula unit mass.
10. Always multiply every atom inside a bracket by the subscript outside it.
Check Yourself – Important Questions
Q1. Why does solid sodium chloride not conduct electricity?
Because its ions are held in fixed positions and cannot move freely.
Q2. Why can sodium chloride solution conduct electricity?
Because its ions become free to move in the solution.
Q3. Sugar dissolves in water. Why does its solution not conduct electricity?
Because sugar does not provide ions in the solution.
Q4. Why do ionic compounds generally have high melting and boiling points?
Because strong inter-ionic attractions require more energy to overcome.
Q5. What is molecular mass?
It is the total mass of a molecule obtained by adding the atomic masses of all the atoms present in it.
Q6. Calculate the molecular mass of H₂O.
(2 × 1) + 16 = 18 u
Q7. Calculate the molecular mass of CO₂.
12 + (2 × 16) = 44 u
Q8. What is a formula unit?
It represents the simplest whole-number ratio of ions in an ionic compound.
Q9. Calculate the formula unit mass of KCl.
39 + 35.5 = 74.5 u
Q10. Calculate the formula unit mass of Mg(OH)₂.
24 + 2(16 + 1)
= 24 + 34
= 58 u
Chapter 9 Complete Revision Map
You have now completed the complete chapter through four parts:
Part 1 → Laws of Chemical Combination + Dalton’s Atomic Theory
Part 2 → Chemical Bonding + Covalent & Ionic Bonds
Part 3 → Ions + Valency + Polyatomic Ions + Chemical Formulae
Part 4 → Properties + Conductivity + Molecular Mass + Formula Unit Mass
The chapter’s own summary brings these ideas together: conservation of mass, fixed proportions, molecules, covalent and ionic bonding, chemical formulae, molecular mass and formula unit mass.
Message from PJ Sir
Chemistry becomes easier when every formula has a reason behind it.
Do not just remember that NaCl conducts in solution or that H₂O has a molecular mass of 18 u.
Ask:
Why?
When you understand the reason, revision becomes faster and questions become easier to solve.
Understand → Practise → Revise → Apply
Read All Parts of Chapter 9
Part 1 – Laws of Chemical Combination & Dalton’s Atomic Theory:
PART 1
Part 2 – Chemical Bonding Made Easy:
PART 2
Part 3 – Chemical Formulae Made Easy:
PART 3
Part 4 – Properties, Molecular Mass & Formula Unit Mass:
You are reading Part 4.
Reference: NCERT Exploration – Grade 9 Science, Chapter 9: Atomic Foundations of Matter.
This page contains independently written explanations, examples and learning support prepared by Priyank Joshi (PJ Sir) for students.
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