Chemistry : 1. Some basic concepts of chemistry

Chemistry

It is the study of matter , its physical and chemical properties and physical and chemical changes it undergoes under different conditions.

Element

The pure form of substance which cannot be broken down into simpler substances by ordinary chemical changes.

Compound

The pure form of substance which can be broken down into simpler substance by ordinary chemical changes.

Law of conservation of mass

Mass can neither be created nor be destroyed in a chemical reaction.

The total mass of the reactants before the reaction is always equal to the total mass of the products after the reaction.

Law of definite proportion

A given compound always contain exactly the same proportion of element by weight.

Every sample of a specific compound like water H2O will always have hydrogen and oxygen combined in the fixed mass ratio of 1:8.

Law of multiple proportion

When two elements A and B form more than one compound the masses of element B combine with given mass of element A are always in the ratio of small whole number.

In compounds like carbon monoxide CO and carbon dioxide CO2 the masses of oxygen 16 gram and 32 gram that combine with a fixed mass of carbon 12 gram are in the simple whole number ratio of 16:32 or 1:2.

Avogadro's law

The equal volume of all gases at same temperature and pressure contain equal number of molecules.

The law establishes the relationship between the volume and the amount of moles or molecules of a gas.

Gay Lussac law of gaseous volume

When gases combine or produce in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

In the reaction H2 + Cl2 gives formation of 2HCL the volume combine in the ratio 1:1:2

Dalton's atomic theory

1.The smallest indivisible particle of element is called atom.

2. Atom of same element have identical physical and chemical properties.

3. Atom of different element have different physical and chemical properties.

4. In chemical reaction reorganisation of atom take place but in the process atom is neither created nor destroyed.

Atomic mass

1.The mass of one atom of an element is called atomic mass.

The mass of one atom of carbon is 1.992648 × 10(-23) g

2.The mass of an atom is very very small therefore we consider the relative mass of an atom.

According to IUPAC the relative mass of carbon is 12 amu.

3.The relation between relative mass of an atom and actual mass of an atom is given as -

1 amu = Mass of 1 atom of carbon / Relative mass of carbon

= 1.66056 × 10(-24) g

4.1 amu is defined as the mass exactly equal to one twelth of the mass of one atom of carbon 12 atom.

5.Recently amu is replaced by unified mass unit (u) or dalton (Da).

Mole

One mole is the amount of of a substance that contains as many entities or particles as there are atoms in exactly 12 g of C 12 .

One mole of any gas occupies a volume of 22.4 DM cube at STP.

No of moles

Volume of gas at STP ÷ 22.4

No of molecules

n × 6.022 × 10(23)

No of moles

Given mass / molar mass

Matter

Anything which has mass and occupies space.

Pure substance

A material containing only one type of particle and having definite chemical composition and distinct properties.

Mixture

A substance having no definite chemical composition and containing two or more pure substances mixed together in any ratio.

Homogeneous mixture

A mixture whose components completely mix with each other and its composition is uniform throughout. e.g., saltwater and air

Heterogeneous mixture

A mixture where the components do not completely mix with each other and the composition is not uniform throughout. e.g.,soil and water