CLASS 9TH ATOMS
AND MOLECULES
NOTES . PREPARED
BY SAMARTH
SRIVASTAW
, Atoms & Molecules — Class IX (CBSE NCERT) —
Concise, exam-ready notes prepared by
s.srivastaw(Samarth)
Clear, structured notes covering the topics you listed. At the end: a 1-hour quick
revision plan as requested earlier.
1. Laws of Chemical Combination — short & precise
Law of Conservation of Mass (Lavoisier)
• Statement: Mass is neither created nor destroyed in a chemical reaction.
• Implication: Total mass of reactants = total mass of products.
• Example: 2H₂ + O₂ → 2H₂O
If 4 g H₂ reacts with 32 g O₂ → mass of products = 36 g H₂O.
Law of Definite (Constant) Proportions (Proust)
• Statement: A given chemical compound always contains the same elements in
the same fixed proportion by mass.
• Example: Water always contains hydrogen and oxygen in mass ratio 1 : 8 (H : O)
— i.e., 2 g H combines with 16 g O → 18 g H₂O.
(Optional but useful) Law of Multiple Proportions (Dalton)
AND MOLECULES
NOTES . PREPARED
BY SAMARTH
SRIVASTAW
, Atoms & Molecules — Class IX (CBSE NCERT) —
Concise, exam-ready notes prepared by
s.srivastaw(Samarth)
Clear, structured notes covering the topics you listed. At the end: a 1-hour quick
revision plan as requested earlier.
1. Laws of Chemical Combination — short & precise
Law of Conservation of Mass (Lavoisier)
• Statement: Mass is neither created nor destroyed in a chemical reaction.
• Implication: Total mass of reactants = total mass of products.
• Example: 2H₂ + O₂ → 2H₂O
If 4 g H₂ reacts with 32 g O₂ → mass of products = 36 g H₂O.
Law of Definite (Constant) Proportions (Proust)
• Statement: A given chemical compound always contains the same elements in
the same fixed proportion by mass.
• Example: Water always contains hydrogen and oxygen in mass ratio 1 : 8 (H : O)
— i.e., 2 g H combines with 16 g O → 18 g H₂O.
(Optional but useful) Law of Multiple Proportions (Dalton)
