This document contains definitions of 157 key chemistry terms provided by Dr. Sajid Ali Talpur. It begins with basic definitions including chemistry, matter, mass, volume and space. It then covers topics like the three states of matter, kinetic theory, gas laws, atomic structure, types of chemical bonds, electronic configuration and more. The definitions are grouped under chapter headings and range from single words to short paragraphs explaining the terms.

1. Chemistry Definitions: 1 Dr. Sajid Ali Talpur

2. Chapter 1
Introduction to Chemistry
(If your small brain can dream it to understand Chemistry then why can your big
body not let it come true..?)
1. Chemistry: - It is the branch of science, which deals with the properties, composition, and
the structure of matter. It also deals with the changes in matter and the principles, which
govern these changes. OR
“THE SCIENTIFIC STUDY OF MATTER IS CALLED CHEMISTRY”
2. Matter: - anything having mass and volume is called matter.
3. Mass: - the quantity of matter contained in a body is called mass.
4. Volume: - volume is the quantity of three dimensional space occupied by the matter. OR
Volume (also called capacity) is quantification of how much space an object occupies.
5. Space: - the free area between two material objects is called space.
6. Area: - the amount of two-dimensional space is called area. (A=W*L).
7. Significant figures: - the reliable digits in a number that are known with certainty.
8. Rounding off data: - it is the procedure of reducing a number to the desired significant
figures by dropping its digits from the extreme right according to certain rules.
9. Logarithm: - logarithm of a positive number is the exponent or power of a given base that is
required to produce that number.
10. Error: - mistake in exercise
11. Atomic mass: - atomic mass is defined as the mass of one atom of the element compared
with the mass of one atom of C12 (the stable light isotope of carbon).
12. Molecular mass: - it is defined as the sum of atomic masses of the atoms of all the elements
present in a molecule shown by its molecular formula.
13. Empirical formula mass/ formula mass: - it is the sum of atomic masses as given in the
simplest (empirical) formula of ionic compound
14. Molecular formula: - the expression, showing types and actual number of atoms present in a
compound, using symbols for component elements and numerals for number of atoms of
each element per molecule, like C6H6 for benzene.
Chemistry Definitions: 2 Dr. Sajid Ali Talpur

3. 15. Empirical formula: - a chemical formula that indicates the relative proportions of the
elements in a compound rather than the actual number of the atoms of the elements.
16. Mole: - one mole is gram atomic mass, gram molecular mass or gram formula mass of any
substance (atoms, ions, molecules), which contains 6.02*1023 elementary entities.
17. Molar mass: - the mass of one mole is called molar mass.
18. Avagadro’s number: - the number of basic quantities as present in 12 grams of C12 i.e
6.02*1023
19. Stoichiometry: - the study of the relationships between the amounts of the reactants and the
products in chemical reactions as given by chemical equation is called Stoichiometry.
20. Limiting reactant: - the reactant or the substance that stoichiometricaly limits or controls
the amount of the product to be formed and is usually consumed first during the chemical
reaction.
Chapter 2
THE THREE STATES OF MATTER GASES, LIQUIDS AND
SOLIDS
(I worry about life more than death because death is not determined by me but
success and failure; heaven or hell is determined by my thoughts, walks, talks
and tasks…!)
21. Kinetic theory: -the theory which deals with the way in which the arrangement of particles
of a substance determines the properties that substance will possess and particularly the state
in which it is likely to be found under a given set of conditions is known as the kinetic
theory.
22. Diffusion: - the distribution or spreading of the gas molecules throughout the vessel is
known as diffusion. OR the mixing and merging of the molecules with each other is called
diffusion.
23. Compression: - lowering the volume.
24. Pressure: -force per unit area.
25. Temperature: - the degree of hotness or coldness of a body is called temperature.
26. Heat: - it is the form of energy that produces sense of warmth and flows from hot body to
cold body.
Chemistry Definitions: 3 Dr. Sajid Ali Talpur

4. 27. Boyle’s law: - According to it
“The volume of a given mass of a gas is inversely proportional to the applied pressure at a
given temperature”
28. Charles’s law: - according to it
“ the volume of a given mass of a gas is directly proportional to the absolute
temperature at constant pressure” OR “ the volume of a given mass of a gas increases or
decreases by 1/273 times of its original volume at 0o C for every degree rise or fall of
temperature at a given pressure”.
29. Absolute zero: - it is the temperature where all the gases molecular motion ceases to exist
and equals to -273K or 0o C or -459F.
30. Avagadro’s law: - “equal volumes of all gases at the same temperature and pressure contain
the same number of molecules.” Thus the volume of a gas is directly proportional to the
number of molecules at a constant temperature and pressure.
31. graham’s law of diffusion: - according to it
“The rate of diffusion of a gas is inversely proportional to the square root of the density
of that gas provided the temperature and the pressure are same for the two gases”.
32. Density: - the mass unit volume is called density. (d=M/V)
33. Dalton’s law of partial pressures: - “the total volume of a mixture of gases is the sum of the
partial pressures of the gases in the mixture”.
34. Ideal gas: -the imaginary gases whose molecules don not occupy any space and don not
attract each other.
35. Viscosity: - it is the resistance to the flow of a fluid, primarily generated by intermolecular
attractions between the molecules within the bulk of a liquid occurring in all the directions.
36. surface tension: - the force per unit length or energy per unit volume
37. Vapour pressure: - it is the pressure exerted by the vapors on the surface of liquid when rate
of condensation exceeds the rate of evaporation.
38. Boiling: - when vapour pressure of liquid equals the atmospheric pressure, the bubbles of
vapour form easily and rise to the surface. The liquid is said to boil.
39. Boiling point: -it is the temperature at which boiling starts.
40. Melting: - conversion of a solid into liquid.
41. Melting point: - it is the temperature at which melting begins to take place.
Chemistry Definitions: 4 Dr. Sajid Ali Talpur

5. 42. Deformity: - dislocation.
43. Sublimation: -the process of conversion of solids directly into gases form without passing
through the liquid state is called sublimation.
44. Crystalline solids: -such these solids have a specific three dimensional geometrical
arrangement of the particles.
45. Amorphous solids: - having random arrangement of particles.
46. Cleavage: -the breakage of a big crystal into smaller crystals of identical shape is called
cleavage.
47. Symmetry: - having posture.
48. Atomic crystals: -the crystals of metals held by metallic bond.
49. Metallic bond: - formed by attraction between electron cloud and positively charged metals.
50. Ionic crystals: - such crystals consist of positively and negatively charged particles, held
together by electrostatic forces of attraction.
51. Covalent crystals: - crystals of atoms held together by covalent bond.
52. Molecular crystals: -consist of molecules, held together by hydrogen bonding or weak
Vander Waal’s forces.
53. Hydrogen bonding: - it is an electrostatic force of attraction between a partially positive
hydrogen atom of one molecule and the electronegative atom of the other molecule.
54. Weak Vander Waal’s forces: -the force of attraction generated between the atomic nuclei
one molecule and electrons of the other molecule.
55. Isomorphism: -the phenomenon of being isomorphous in which different substance have
same crystalline structure.
56. Polymorphism: -the substance which can exist in more than one crystalline form is called
polymorphous and the phenomenon is known as polymorphism.
57. Phenomenon: -an observable event.
58. Unit cell: -the basic structural unit which when repeated in three dimensions generates the
crystal structure, called unit cell.
Chemistry Definitions: 5 Dr. Sajid Ali Talpur

6. 59. Cell dimensions or parameters: -the length and angles of a unit cell are collectively known
as cell dimensions or cell parameters.
60. space or crystal lattice: -if atoms, ions or molecules constituting a crystal are replaced by
points and placed at the same positions as in a unit cell, then three dimensional array of
points is called crystal lattice or space lattice OR the space amongst the particles constituting
crystal.
61. The latent heat of fusion: - heat required during conversion of the state. OR the heat energy
required converting 1g of a solid into liquid at its melting point.
Chapter 3
The Atomic Structure
(Tungsten is not illuminating until given voltage; you are not beautiful until
skilled…!)
62. Atom: -the smallest building block of the matter that takes part in the reaction
OR the smallest component of an element having the chemical properties of the element,
consisting of a nucleus containing the combination of protons and neutrons and some
electrons bound to the nucleus by electrical attraction; the number of protons determines the
identity o the element
63. crook’s discharge tube experiment: - indicates the existence of protons and electrons in the
atom
64. Element: -the simplest matter whose all atoms have same atomic number and can not be
further simplified is called element.
65. Cathode rays: -the rays which consist of electrons and are produced when the current is
passed through gases at low pressure are called cathode rays.
66. properties of cathode rays (electrons):
Discovered by J.J THOSMSON
¸ Carry a negative charge equal to 1.6*10-19C=4.803*10-10
¸ Their e/m ratio is 1.76*108 C/g
¸ Their mass is 9.11*10-31 kg = 0.00055a.m.u
¸ They can be deflected by electromagnetic field.
¸ These don’t depend upon the material of which the electrode is made nor of the gas which is
enclosed inside the tube.
67. the properties of canal rays ( protons):
¸ GOLDSTIEN discovered them.
¸ These rays carry positive charge
¸ They travel in straight line
Chemistry Definitions: 6 Dr. Sajid Ali Talpur

7. ¸ The value of e/m depends upon the gas enclosed
¸ The mass is 1.67*10-27kg=1.0073amu
¸ They are composed of heavy particles than electrons.
68. the properties of neutrons:
¸ Discovered by CHADVICK.
¸ They are neutral, carrying no charge.
¸ The mass is same as that of proton but slightly heavier. i.e. 1.0087 a.m.u.
¸ It is highly energetic particle.
69. Nucleus: -the central part of the atom comprising the mass of the atom conferred by
protons and neutrons, initially discovered by RUTHERFORD.
70. Radioactivity: -the spontaneous emission of radiations by certain elements is called
radioactivity.
71. X-rays: -any of the electromagnetic radiations that have an extremely short wavelength of
less than 100 angstrom and have the properties of penetrating various thicknesses of the
solids, of producing secondary radiations by impinging on material bodies and of acting on
photographic films and plates as light bones.
72. Atomic number (Z): -The number of protons in nucleus is called the atomic number of the
element.
73. Atomic mass: - the total number of protons and neutrons in nucleus constitute its atomic
mass.
74. Plank’s quantum theory: - the energy absorbed or emitted by the atoms is not continuous
rather in the form of packets called QUANTA or PHOTON.
75. Spectrum: -the band of colors formed through the dispersion of light is called spectrum.
76. Dispersion: -the phenomenon of breakage of light into its constituent colors on the passage
from a prism is called dispersion.
77. Continuous spectrum: -in such a spectrum the spectral colors are intermixed and are not
clearly separated by a line.
78. line spectrum: -the spectral colors are separated by discrete sharp lines or there is dark
background
79. Energy level or shell: -according to Bohr these are the circular orbits at certain distance
from the nucleus where electrons exist.
80. Heisenberg’s uncertainty principle: it states that “it is impossible to describe the position
and moment of an electron in an atom simultaneously.
Chemistry Definitions: 7 Dr. Sajid Ali Talpur

8. 81. Electronic configuration: - the arrangement or distribution of electrons in the available
orbitals is called electronic distribution.
82. Orbital: -the certain regions of space around nucleus, where the likelihood of finding
electron is maximum. Such regions around nucleus are called orbitals.
83. S-orbital: - all s orbitals are spherical in shape with the nucleus at the centre.
84. P-orbital: - the p orbitals are dumb bell shaped.
85. Pauli’s exclusion principle: -it states that “In an atom two electrons can never have same
values of all of the four quantum numbers”
86. Aufbau principle: -it states that “the orbitals of lowest energy are filled first, two electrons
in each with a sequence of increasing energy order”
87. n + l rule: -according to it “in building up the electronic configuration of the elements, the
orbitals with the lowest value of (n + l) fills first; when two orbitals have its same value,
the orbital with the lower value of n fills first”
88. Hund’s rule of maximum multiplicity: -according it:
I.Electrons go singly in degenerate orbitals first
II.At the time, their spin is also parallel only it becomes opposite when they become paired.
89. Atomic radius: -the half of the bond length, taken between two homonuclear diatomic
molecules. OR the distance from nucleus to the valence electron is called atomic radius.
90. Ionic radius: -the radius of an ion.
91. ion: -the charged atom is called ion
92. Cation: -the positively charged ion is called cation.
93. Anion: -the negative charged ion is called anion.
94. Ionization potential: -the energy required to make an atom ion is called ionization
potential.
95. Electron affinity: -the amount of energy released when an electron is added to a neutral
gaseous atom to form a negative ion is called electron affinity.
96. Electronegativity: - the force with witch an atom attracts the shared pair towards itself is
called electronegativity.
Chemistry Definitions: 8 Dr. Sajid Ali Talpur

9. Chapter 4
Chemical bonding
(Love is indirectly proportional to the physical closeness)
95. Chemical bond: - the attractive force that holds atoms together in a compound is called
chemical bond.
96. Ionic or electrovalent bond: - it is defined as the electrostatic force of attraction between
positive and negative ions.
97. Lattice energy: -the energy given off when one mole of the oppositely charged ions in
gaseous phase comes closer together to form a crystal lattice of a solid.
98. Covalent bond: - a chemical bond that involves the sharing of electrons is called covalent
bond.
99. Single covalent bond (—): -such a bond is formed when one electron is contributed from
each atom making one shared pair. E.g. Cl2
100. Double covalent bond: -in such a covalent bond each of the two atoms contribute two
electrons as in O2
101. Triple covalent bond: -in triple covalent bond each atom contributes three electrons as
in N2
102. Co-ordinate or dative covalent bond: -a covalent bond in witch one of the bonded
atoms furnishes both of the shared electrons.
103. Hydrogen bond: -- it is an electrostatic force of attraction between a partially positive
hydrogen atom of one molecule and the electronegative atom of the other molecule
104. Dipole moment: - the measure of the degree of the polarity of a polar compound is called
dipole moment.
105. Bond energy: - the energy released during bond formation or the energy required to
break bond.
106. Valence bond theory: -according to this theory covalent bond is formed by the
overlapping (fusion) of half filed orbitals.
107. Sigma bond: - the head to head overlapping of two orbitals results in sigma bond.
Chemistry Definitions: 9 Dr. Sajid Ali Talpur

10. 108. Pi bond: - the covalent bond is formed by the side ways or lateral overlapping of the half
filled atomic orbitals.
109. Hybridization: - it is the process of mixing or blending of pure atomic orbitals of
different energies and shapes to produce the same number of the new orbitals of the same
energies and shapes.
110. Sp3 hybridization: -when one s orbital intermixes with three other p orbitals to produce
four orbitals of equal energy, shape and size, is called sp3 hybridization.
111. Sp2 hybridization: -when one s and two p orbitals intermix to produce three equivalent
orbitals is called sp2 orbitals.
112. Sp orbital: - the blending of one s and one p orbital results in sp hybridization producing
two other equivalent orbitals.
113. Molecular orbital theory: - according to this theory linear overlapping of atomic
orbitals give two kinds of molecular orbitals i.e. bonding molecular orbital and anti
bonding molecular orbital.
114. Bonding molecular orbital: - the molecular orbital, low in energy as compared with the
atomic orbitals from witch it is derived.
115. Anti bonding molecular orbital: - it is an energy rich molecular orbital.
116. Valence shell electron pair repulsion theory: - this theory assumes the shapes of the
molecules depending on repulsion between bond pairs and lone pairs of electrons.
117. Bond pairs electrons: - also called active set of electrons they take part in bond
formation and are incorporated as shared pair.
118. Lone pair of electrons: - also called non bonding pairs, are paired electrons but don’t
take part in bonding.
Chapter 5
ENERGETICS OF CHEMICAL REACTIONS
(There is no way to turn over a new leaf other than understanding the way the
Prophet Muhammad(s.a.w) lived life, because he himself being orphan introduced
the world such a rich GOD that the billions follow him today…!)
119. Thermodynamics: -the study of conversion and conservation of heat and other forms of
energy is called thermodynamics.
Chemistry Definitions: 10 Dr. Sajid Ali Talpur

11. 120. Thermochemistry: - it deals with the measurement or calculation of heat absorbed or
abandoned in chemical reactions.
121. Thermo chemical reactions: - the chemical reactions which are accompanied by energy
changes along with the material changes are generally known as thermochemical
reactions.
122. Exothermic reactions: -the chemical reactions which are accompanied by the liberations
or emission of energy are called exothermic reactions.
123. Endothermic reactions: -the chemical reactions which are accompanied by the
absorption of energy are called endothermic reactions.
124. system: -the collection of matter having certain boundaries is called system
125. Surroundings:- the environment of a system or the thing that affect on a system are
included in surroundings.
126. Macroscopic properties: - the properties of a system in bulk, rather than that of the part
the system which are easily measurable called macroscopic properties.
127. Intensive properties: -such properties of system are amount of material independent.
128. Extensive properties: -it is the property that depends upon the amount of the substance
present in the system.
129. Initial state: -the description of the system before it suffers any change is called initial
state of the system.
130. Final state: -the description of the system after it undergoes a change is known as the
final state of the system.
131. State functions: -the physical properties which depend upon the states of the system
(initial and final) are called state functions.
132. first law of thermodynamics: -according to it:
“the energy in the universe is constant” OR “the energy can neither be created nor
destroyed, although it may be changed from one form to another form but the total energy
of the system and the surroundings remains constant.”
133. Hess’s law of constant heat summation: -
“The heat change in a chemical reaction depends only on the initial and final states of
the system instead of the manner in which it is brought about”
Chemistry Definitions: 11 Dr. Sajid Ali Talpur

12. 134. heat of formation: - the change of enthalpy when one gram mole of a substance is
formed from its elements
135. Standard heat of formation: -the change of enthalpy when one gram mole of a
substance is formed from its elements at 25oC and 1 atm is called standard heat of
formation.
136. Enthalpy: -the total heat content of a system is called enthalpy.
137. Entropy: - the disorder of the system is called entropy.
138. Thermodynamic process: -when a thermodynamic system changes from one state to
another, the operation is called thermodynamic process.
Chapter 6
Chemical equilibrium
(The difficult work of life is to maintain equilibrium in life by planned destination
and hard work in life..!)
139. Chemical equilibrium: - the state of balance where forward and backward reactions
proceed at similar pace.
140. Equilibrium state: -the state of a reversible reaction where there is no net change in the
concentration of reactants and product.
141. Irreversible reactions: -the reactions which go to completion and the reactants again are
not formed from the product.
142. Reversible reaction: -the reactions which never go to completion and the reactants are
again formed from the product.
143. equilibrium law or law of mass action: - “ the rate of a chemical reaction is directly
proportion to the molar concentration (active masses) of the reactants”
144. Homogeneous equilibrium: -the equilibrium that involves only single phase is called
homogeneous equilibrium.
145. Heterogeneous equilibrium: -the equilibrium that involves more than one phase is
called heterogeneous equilibrium.
146. Equilibrium mixture: -the mixture of reactants and products obtained at equilibrium
state is called equilibrium mixture.
Chemistry Definitions: 12 Dr. Sajid Ali Talpur

13. 147. Equilibrium constant: -it is a number that expresses the relationship between the
amount of products and the reactants present at the equilibrium at a given temperature.
148. le Chatelier’s principle: -according to it
“When equilibrium is stimulated or stressed, it will automatically shift itself in the
direction of change so as to minimize the change and regain the equilibrium state”
149. haber’s process: it is used to prepare ammonia
150. Contact process: it is used to prepare sulphuric acid.
151. solubility: -the quality or state of being soluble OR solubility is that property by which
we can determine that up to how much extent a solute is soluble in 100 g of a solvent.
152. Solubility product: -the solubility product is the equilibrium constant for a chemical
reaction in which a solid ionic compound dissolves to yield its ions in the solution.
153. Common ion effect: -the common ion effect is a term used to describe the effect on a
solution of two dissolved solutes that contain a common ion or ions.
Chapter 7
SOLUTIONS AND ELECTROLYTES
(Solving the problems is the solution of problems than increasing the problems by
worrying on the problems…!)
154. Solution: -the homogeneous mixture of two or more things.
155. Molar concentration: -the number of moles of the solute contained in one cubic decimeter
or one liter of the solution.
156. Molal concentration: -the number of moles of solute per kilogram of the solvent.
157. Hydration: -the incorporation of water into other complex molecules is called hydration.
158. Hydrolysis: the breakage of a compound with the addition of water OR the reaction of
cation or anion (or both) with water to change its pH is called hydrolysis.
159. Electrolysis: -the breakage of a solution on the passage of electric current.
160. Electrolyte: -the substance capable of being dissociated into ions.
Chemistry Definitions: 13 Dr. Sajid Ali Talpur

14. 161. Anode: -the positively charged electrode where oxidation occurs.
162. Cathode: -the negatively charged electrode where reduction takes place is called cathode.
163. Electrode potential: -the potential difference between the electrode and the solution.
164. Oxidation: -the measure of degree of oxidation.
165. Indicators: -color showing organic compounds. OR for acid-base titrations, organic
compounds that exhibit different colors in solution of different acidities; used to determine
the point at which the reaction between two solutes is complete.
166. Acid: -a compound having usually sour taste and capable of neutralizing alkalis and
reddening blue litmus paper, containing hydrogen that can be replaced by a metal or any
other electropositive atom to form salt, or containing an atom that can accept electrons
from a base.
167. Base: - a base is a substance that can accept hydrogen ions or more generally, donates
electrons pairs.
168. Salt: -the ionic substance that result from the neutralization of an acid with a base.
169. Titration: -titration is the quantitative measurement of an analyte in solution by
completely reaction with reagent solution.
170. Ph: - a measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral
solutions, increasing with increasing alkalinity and decreasing with increasing acidity.
171. Buffer: -any substance that prevents changes in pH is known as buffer.
172. Neutralization: -when the two substances (acid and base) having opposed properties are
allowed to react, salt and water are produced. The reaction is known as neutralization.
Chapter 8
Introduction to Chemical Kinetics.
(Learn, practice and preach)
173. Chemical kinematics: -the study of rates of chemical processes.
174. Rate = anything or substance etc divided by time.
175. Velocity of reaction: -the rate of a reaction at a specific given time.
176. Activation energy: -the energy difference from the energy the substance possesses to the
threshold energy required.
Chemistry Definitions: 14 Dr. Sajid Ali Talpur

15. 177. Threshold energy: -the minimum energy required to initiate a reaction is called threshold
energy.
178. Calorimetric method: -the physical method of determining rate of a reaction through
change in color intensity.
179. Catalyst: -a substance that without being consumed speeds up the reaction is called a
catalyst.
180. Positive catalyst: -the rate, enhancing catalysts.
181. Negative catalyst: -they reduce the rate of a chemical reaction.
182. Photochemical reactions: -the chemical reactions that are accompanied in the presence of
light, such reactions are known as photochemical reactions
Chemistry Definitions: 15 Dr. Sajid Ali Talpur

16. By the same Author:
¸ Success tips by Dr. Sajid Ali Talpur
¸ Physics definitions by Dr. Sajid Ali Talpur
¸ Definitions and MCQs on branches of Biology
¸ Ninth Class Chemistry MCQs
¸ Scientific Biological Reasons
¸ Definitions, Scientific Reasons and MCQs on Chemistry
¸ Skeletal muscles Physiology BCQs
¸ Biology MCQs test for entry test
¸ Cardiovascular system anatomy quiz
¸ Lower limb anatomy quiz
¸ Lower limb Anatomy MCQs
¸ Spiritual successful life quotations
¸ Islamic poetry and quotations (video)
¸ Love the Prophet Muhammad (s.a.w)
¸ Quotations on the Prophet Muhammad (s.a.w)
Chemistry Definitions: 16 Dr. Sajid Ali Talpur

17. Chemistry Definitions: 17 Dr. Sajid Ali Talpur