NCERT Solutions for Class 9th Science Chapter 9 Atomic Foundations of Matter

About this chapter

Law of Conservation of Mass (Lavoisier, 1789): in a chemical reaction matter is neither created nor destroyed, so the total mass of the reactants equals the total mass of the products. It only looks violated in an open vessel, because a gaseous product walks off the balance pan. Law of Constant Proportions (Proust): a compound always contains the same elements in the same fixed ratio by mass, whatever its source. Pure water from a river, a borewell or the ocean is always H : O = 1 : 8 by mass. Dalton's atomic theory explains both laws with one picture — atoms are indivisible in a chemical reaction and merely rearrange, atoms of one element are identical in mass, and atoms combine in simple whole-number ratios. Atoms combine to reach a stable octet (a duplet for the K-shell). They do it in

• Chapter opener
• Let us investigate a physical change
• Let us investigate a chemical change
• Let us verify the law
• .1 Law of Conservation of Mass
• .2 Law of Constant Proportions
• .3 Dalton's Atomic Theory
• .4.1 A Molecules of elements
• .4.1 B Molecules of compounds
• .4.2 Bonding by electron transfer