Question

science chapter 3 class 9th notes ​

Answer 1

➡️ Law of conservation of mass (by Lavoisier in 1744):

This law states that mass can neither be created nor destroyed in a chemical reaction.

➡️Law of constant proportions (by Proust in 1797):

This law states that in a chemical substance the elements are always present in definite proportions by mass.

For example, the ratio of hydrogen and oxygen in pure water is always 1: 8 by weight.

This law is also called law of definite proportions or law of constant proportions.

❤️Dalton’s Atomic Theory

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms

❤️Postulates of Dalton’s atomic theory:

Matter is made up of extremely small indivisible particles called atoms that can neither be created nor destroyed.

Atoms of the same substance are identical in all aspects, i.e., they possess same size, shape, mass, chemical properties etc.

Atoms of different substances are different in all aspects, i.e., they possess different size, shape, mass etc.

Atom is the smallest particle that takes part in a chemical reaction.

Atoms of different elements combine with each other in a simple whole number ratio to form compound.

The relative number and kinds of atoms are constant in a given compound.

❣️Atoms:

The smallest tiny particles of matter which can't be divided further is called atom, i.e., an atom is the smallest building block of matter.

For example: Sodium (Na), Hydrogen (H), Oxygen (O), etc.

❣️Atomic Mass

Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

❣️Molecules:

A group of two or more than two atoms of the same or different elements that are chemically bonded together is called a molecule.

For example: Two atoms of hydrogen (H2) and one atom of oxygen (O2) react with each other and form one molecule of water.

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