Chemistry- some basic concepts ch 1

class 11 ncert

Matter

Matter is defined as any thing that occupies space possesses mass and the presence of which can be felt by any one or more of our five senses.

Matter can exist in 3 physical states viz. solid, liquid, gas.

The three states are interconvertible by changing the conditions of temperature and pressure as follows

Solid

a substance is said to be solid if it possesses a definite volume and a definite shape, e.g., sugar, iron, gold, wood etc.

Liquid

A substance is said to be liquid, if it possesses a definite volume but no definite shape. They take up the shape of the vessel in which they are put, e.g., water, milk, oil, mercury, alcohol etc.

Gas

a substance is said to be gaseous if it neither possesses definite volume nor a definite shape. This is because they fill up the whole vessel in which they are put, e.g., hydrogen, oxygen etc.

Mass and Weight

Mass of a substance is the amount of matter present in it while weight is the force exerted by gravity on an object. The mass of a substance is constant whereas its weight may vary from one place to another due to change in gravity. The SI unit of mass is the kilogram (kg). The SI derived unit (unit derived from SI base units) of weight is newton.

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. There are three common scales to measure temperature

— °C (degree celsius), °F (degree fahrenheit) and K (kelvin).

The temperature on two scales is related to each other by the following relationship:

Law of conservation of mass

“In a chemical reaction the mass of reactants consumed and mass of the products formed is same, that is mass is conserved.” This is a direct consequence of law of conservation of atoms. This law was put forth by Antoine Lavoisier in 1789.

Law of Constant proportion

The ratio in which two or more elements combine to form a compound remains fixed and is independent of the source of the compound. This law was given by, a French chemist, Joseph Proust.

Law of multiple proportion

When two elements combine to form two or more compounds then the ratio of masses of one element that combines with a fixed mass of the other element in the two compounds is a simple whole number ratio. This law was proposed by Dalton in 1803.

Gay lussac law of combining volume

This law was given by Gay Lussac in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

Avagadro law

In 1811, Avogadro proposed that equal volumes of gases at the same temperature and pressure should contain equal number of molecules.

Atom

Atom is the smallest part of an element that can participate in a chemical reaction. {Note : This definition holds true only for non-radioactive reactions}

Stochiometry

The study of chemical reactions and calculations related to it is called Stoichiometry. The coefficients used to balance the reaction are called Stoichiometric Coefficients.

Limiting Reagent

If the reactants are not taken in the stoichiometric ratios then the reactant which is less than the required amount determines how much product will be formed and is known as the Limiting Reagent and the reactant present in excess

Precentage yield

As discussed earlier, due to practical reasons the amount of product formed by a chemical reaction is less than the amount predicted by theoretical calculations. The ratio of the amount of product formed to the amount predicted when multiplied by 100 gives the percentage yield.

Dalton law of Atomic theory

In 1808, Dalton published ‘A New System of Chemical Philosophy’ in which he proposed the following:

Matter consists of indivisible atoms.

All the atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass.

Compounds are formed when atoms of different elements combine in a fixed ratio.

Chemical reactions involve reorganization of atoms. These are neither created nor destroyed in a chemical reaction.