Some Basic Concepts of Chemistry

SUMMARY

IMPORTANCE OF CHEMISTRY:

  • Chemistry is the branch of science that deals with the study of composition, properties and interaction of matter.
  • Chemistry plays a central role in science and is often interlinked with other branches of science like physics, biology, geology etc. Chemistry also plays an important role in daily life.
  • Chemistry plays an important role in meeting human needs for food, health care products.
  • Chemical industries manufacturing fertilizers, alkalis, acids, salts, dyes, polymers, drugs, soaps, detergents metals, alloys and other inorganic and organic chemicals, including new materials, contribute in a big way to the national economy.

NATURE OF MATTER:

  • Anything which has mass and occupies space is called matter. Matter can exist in three physical states viz. solid, liquid and gas.
  • . In solids, these particles are held very close to each other in an orderly fashion and there is not much freedom of movement. In liquids, the particles are close to each other but they can move around. However, in gases, the particles are far apart as compared to those present in solid or liquid states and their movement is easy and fast.
  • Solids have definite volume and definite shape.
  • Liquids have definite volume but not the definite shape. They take the shape of the container in which they are placed.
  • Gases have neither definite volume nor definite shape. They completely occupy thecontainer in which they are placed.These three states of matter are

interconvertible by changing the conditions of temperature and pressure.

  • At the macroscopic or bulk level, matter can be classified as mixtures or pure substances. These can be further sub-divided as shown.

Many of the substances present around you are mixtures. For example, sugar solution in water, air, tea etc., all are mixtures. A mixture contains two or more substances present in it (in any ratio) which are called its components.

  • A mixture may be homogeneous or heterogeneous. In a homogeneous mixture, the components completely mix with each other and its composition is uniform throughout.

In heterogeneous mixtures, the composition is not uniform throughout and sometimes the different components can be observed. Eg. Grains and pulses is a  heterogeneous mixture.

  • Pure substances have characteristics different from the mixtures. They have fixed composition, whereas mixtures may contain the components in any ratio.

Pure substances can be further classified into elements and compounds.

An element consists of only one type of particles. These particles may be atoms or molecules.

  • An element is defined as a pure substance that contain only one kind of particles. These are further classified  into three classes; Metals, Non-Metals and Metalloids.
  • When two or more atoms of different elements combine, the molecule of a compound is obtained. The examples of some compounds are water, ammonia, carbon monoxide etc.
  • The properties of a compound are different from those of its constituent elements. For example, hydrogen and oxygen are gases whereas the compound formed by their combination i.e., water is a liquid. It is interesting to note that hydrogen burns with a pop sound and oxygen is a supporter of combustion, but water is used as a fire extinguisher.

Some Important Definitions:

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 kilogram. However, its fraction gram (1 kg = 1000 g), is used in laboratories due to the smaller amounts of chemicals used in chemical reactions

Volume: Volume of a substance is defined as the quantity of three dimensional space enclosed by some closed boundary. SI units of volume has units of m3.

A common unit, litre (L) which is not an SI unit, is used for measurement of volume of liquids.

Density

Density of a substance is its amount of mass per unit volume.

SI unit of density =kg m–3

Temperature: Temperature is a physical quantity of matter that 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). Here, K is the SI unit. The temperatures on two scales are related

to each other by the following relationship:

OF=(9/5)  ( oC) + 32 OF= ( oC) + 32

The kelvin scale is related to celsius scale as follows :

K = °C + 273.15

note that temperature below 0 °C (i.e. negative values) are possible

in Celsius scale but in Kelvin scale, negative temperature is not possible.

Significant Figures

Significant figures are meaningful digits which are known with certainty. There are certain rules for determining the number of significant figures. These are stated below:
1.  All non-zero digits are significant. For example, in 285 cm, there are three significant figures and in 0.25 mL, there are two significant figures.
2.  Zeros preceding to first non-zero digit are not significant. Such zeros indicate the position of decimal point.
For example, 0.03 has one significant figure and 0.0052 has two significant figures.
3.  Zeros between two non-zero digits are significant. Thus, 2.005 have four significant figures.
4.  Zeros at the end or right of a number are significant provided they are on the right side of the decimal point. For example, 0.200 g has three significant figures.
5.  Counting numbers of objects. For example, 2 balls or 20 eggs have infinite significant figures as these are exact numbers and can be represented by writing infinite number of zeros after placing a decimal.
i.e., 2 = 2.000000
or 20 = 20.000000

LAWS OF CHEMICAL COMBINATIONS

The combination of elements to form compounds is governed by the following five basic laws.

Law of Conservation of Mass

It states that matter can neither be created nor destroyed.

Law of Definite Proportions

This law states that 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 Proportions

This law was proposed by Dalton in 1803. According to this law, if two elements can

combine to form more than one compound, themasses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

DALTON’S ATOMIC THEORY

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

the following :

1. Matter consists of indivisible atoms.

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

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

4. Chemical reactions involve reorganization of atoms. These are neither created nor

destroyed in a chemical reaction. Dalton’s theory could explain the laws of chemical combination.

ATOMIC AND MOLECULAR MASSES

Today, we have sophisticated techniques e.g., mass spectrometry for determining the atomic masses fairly accurately. But, in the nineteenth century, scientists could determine mass of one atom relative to another by experimental means, as has been mentioned earlier. Hydrogen, being

lightest atom was arbitrarily assigned a mass of 1 (without any units) and other elements

were assigned masses relative to it.

  • However, the present system of atomic masses is based on carbon – 12 as the standard and has been agreed upon in 1961.
  • One atomic mass unit is defined as a mass exactly equal to one twelth the mass of one  carbon – 12 atom.

And 1 amu = 1.6605610–24 g

Average Atomic Mass

Many naturally occurring elements exist as more than one isotope. When we take into account the existence of these isotopes and their relative abundance (per cent occurrence), then the atomic mass can be calculated by using the below formula.

Average atomic mass = f1M1 + f2M2 +… + fnMn where f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

MOLE CONCEPT

One mole is the amount of a substance that contains as many particles or entities

as there are atoms in exactly 12 g (or 0.012kg) of the 12C isotope.

the mole of a substance always contain the same number of entities, no matter what

the substance may be one mole of carbon weighs 12 g, the number of atoms in it is equal to 6.0221367 ×1023 atoms/mol.

It is known as ‘Avogadro constant’, denoted by NA in honour of Amedeo Avogadro.

therefore, say that 1 mol of hydrogen atoms = 6.022 x1023 atoms

1 mol of water molecules = 6.022 x 1023 water molecules

An empirical formula represents the simplest whole number ratio of various atoms present in a compound whereas the molecular formula shows the exact number of different types of atoms present in a molecule of a compound. If the mass per cent of various elements present in a compound is known, its empirical formula can be determined.

For the further understanding of concepts related to mass percent, Mole Fraction, Molarity and Molality please refer to the pdf in which these are discussed in detail.

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